High-Performance Enclosure Strategies, Part 1: Unvented Roof Systems and Innovative Advanced Framing Strategies
February 12, 2015
Joe Lstiburek, Building Science Corporation
Vladimir Kochkin, Home Innovation Research Labs
Gail: Hello everyone! I am Gail Werren with the National Renewable Energy Laboratory, and I’d like to welcome you to today’s webinar hosted by the Building America program. We are excited to have Joe Lstiburek and Vladimir Kochkin here today to talk about high-performance enclosure strategies for roof and wall systems that optimize energy performance and reduce moisture issues.
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We have an exciting program prepared for you today that will focus on designing and building roof and wall systems that optimize energy performance and reduce moisture issues. Before our panelists begin, I will provide a short overview of the Building America program. Following the presentations, we will have a Question and Answer session and a brief survey.
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Our webinar today will present strategies for designing and building roof and wall systems that optimize energy performance and prevent moisture issues. If you would like detailed information about any of these efforts, or if you are interested in collaborating, please feel free to contact either of our speakers.
Our first speaker is Joe Lstiburek, is a Principal of Building Science Corporation and an adjunct professor of building science at the University of Toronto. His work at BSC includes conducting forensic investigations and building design reviews, overseeing research and development projects, and writing for buildingscience.com. A building science pioneer, particularly in the areas of air and vapor barriers and vented and unvented assemblies, his work has impacted building codes and practices throughout the world. Dr. Lstiburek is also a noted educator and author of the bestselling Builder Guides. Joe's presentation will discuss several advances in unvented roof technology to conditioned attics, including the use of diffusion venting and dehumidification. These approaches allow the use of cellulose and fiberglass insulation systems rather than the current approaches that rely on rigid insulation or spray polyurethane foams.
Next up is Vladimir Kochkin who oversees engineering research programs at Home Innovation Research Laboratory on the energy, environmental, and structural performance of residential building construction. He also manages the company's ANSI process for the National Green Building Standard. Today Vladimir will present design and construction methods that increase energy performance and reduce moisture in high performance walls in climate zones 3 through 5. The presentation is based on the Builders Guide to High Performance Walls, which will be published by Home Innovation Research Labs in 2015.
With that I would like to welcome Joe to start the presentation.
Joe: Hello everybody, let's get going here. I'd like to start off with something really significant to say and that is one of the best roof assemblies that anybody can construct in any climate zone is a vented attic. Unvented roof assemblies are, in essence, a second choice. They're a choice to be made if you can't construct a reasonable vented attic. The keys to the vented attic, of course, are an air tight ceiling, plenty of insulation, a balanced ventilation where you have half of the air coming in at the perimeter - at your soffits, and the other half leaving at the top of your building. If you have the opportunity to construct a ventilated attic, do it.
Why have any other choice? Well, this is why. We put stuff in attics that shouldn't be in attics, like building's mechanical systems. That leads to horrible problems with depressurization. In hot humid climates it leads to hot and humidity problems. In cold climates it leads to ice damming. If radon was valuable this is how we would mine it. This is a really bad idea. Mechanical systems should be located inside. It's not easy to construct an air tight ceiling either, because we perforate the heck of it. So if you can have an airtight ceiling, and you can locate your mechanical system inside, go with a vented roof assembly.
This has a tremendous energy implication as well, back in the 1990s this accounted for almost 30% of the heating and cooling load in buildings, simply because of the duct leakage associated air change. The difference between this and this is 30%. That's extraordinary so remember that penalty for a later discussion. If you are going to locate your ductwork outside of your conditioned space, for gosh sakes, make it air tight. The only technology that appears to work is mastic. Duct tape has 1001 uses but not a single one of them makes sense for duct work.
This was a compromise to - I can't move the mechanical system inside, perhaps I can move the inside to the inside of the roof deck to incorporate the mechanical system. This has about a 15% penalty over a vented attic with ductwork located inside but it's 30% better than the ductwork outside, in the attic, and leaking. Because of the greater surface area heat loss, you're losing more heat. You're gaining more heat, because of the larger roof area. You're losing more heat. You're gaining more heat because of the larger roof area but that isn't as bad as locating the mechanical system outside of the conditioned space in the attic. Again, from an energy perspective this is a compromise. The best solution is locating the mechanical system inside an under vented attic. The air change, believe it or not, shouldn't make any difference but what we're finding is that with the conditioned attics we're getting significantly tighter building enclosures because it's easier to make the roof deck air tight than it is to make the perforated ceiling air tight. An unvented roof compensates for the larger surface area because it's significantly tighter. It's easier to make it significantly tighter than a standard ventilated attic.
One of the big problems with unvented roofs and vented roofs is ice damming. The way to deal with ice damming is that you have to basically compensate for the thermal resistance of the snow. All of the under vented roofs do that. If you are going to be building an unvented roof in a cold climate the key is that, if it has a fairly high slope to it, you're going to have to put a vented over roof over the top of the unvented under roof to compensate for the thermal resistance of the snow to control ice damming. People who thought let's just put a membrane roof down to deal with the ice damming, that doesn't do the job. That handles part of the leakage of the ice dam but doesn't solve the ice dam in the first place.
This is the thermal resistance of snow. Snow is about R-1 per inch to R-2 per inch of thickness so 10 inches of snow gives you between R-10 and R-20. On the cold climates, that's enough to be able to get you a temperature above freezing at the roof deck, even with R-60 insulation. So the idea of building an air tight unvented roof that's got R-60 or more to control ice damming is just fallacy. That's not the way it works. You end up having to put a vented over roof over the top of an unvented under roof to compensate for ice damming.
The first technologies to do unvented roofs were basically to take compact flat roofs from the commercial industry and just sloping them. I mean the physics is the same. So this is probably, when they were first doing this in the 1970s and 1980s, this was the technology. It's hard to beat this, a fully adhered membrane on top of wood decking, two or three layers of rigid insulation and another layer of plywood or OSB screwed together at angles. This is pretty much bullet proof and foolproof, except in cold climates with snow, ice and snow accumulation, you'd have to put a vented over roof on top of the unvented under roof to make the assembly work. Don't forget about ice damming and snow accumulation.
You end up having to introduce the air, basically, at the fascia and let it off at the ridge. This is a map of ground snow loads. Anywhere the ground snow load exceeds 50 pounds per square foot you have to have a vented roof over the top of an unvented under roof or you're going to get yourself into trouble.
Can we get into hybrids? Well, it's very expensive to put all of that rigid insulation on the top of the deck and so the idea is, why not split it? Why not have enough insulation on the top of the roof deck to control the temperature of the condensing surface and fiberglass or cellulose on the underside to provide a balance of resistance? This is no different than insulating sheeting on the exterior wall.
This is a wall of insulating sheathing on the outside lying on its side. The colder the climate the greater the thermal resistance on the top of the roof deck. There are some pretty simple elegant calculations. This is all now in the code, thankfully, because it was supported by the Building America program. Codes were changed nationally that allowed this to occur. This is, in essence, the perfect illustration of a wall with insulating sheeting morphing into a cathedral ceiling with insulating sheeting. The key here is to not have a vapor barrier on the interior so that the cavity insulation has the ability to dry to the interior. Once again, the amount of thermal resistance on the top of the roof deck is determined by the climate.
In hot dry climates, with a ventilated roof system such as roofing tile on batten over roofing felt, we in essence did not need to know rigid insulation law. This is, in essence, fiberglass or cellulose that's allowed to be installed as the roof deck passes moisture by vapor diffusion through the roof deck itself, into the space, the air space created by the roofing tile. This is probably one of the most economical ways to build unvented conditioned roofs and there are several hundred thousand of them now in Arizona and Nevada and some parts of California. If we were to put shingles on this, that wouldn't work because the shingles are, in essence, a vapor barrier on the wrong side of the assembly. This could work in Florida if we had a roofing felt that was vapor permeable. In Florida the roofing tile is vapor impermeable. They're fully adhered. What's kind of neat is in the last five years we have fully adhered membranes that are now fully vapor permeable. This technology could move to south Florida with a tile roof. Instead of having to use rigid insulation on the top or, as you will see in a moment, spray foam underneath, you could basically put down a vapor permeable, fully adhered, membrane in Orlando or Fort Meyers, put on tile - a tile roof with battens, and do cellulose and fiberglass just like they've done the last 20 years in Las Vegas. This is a Las Vegas technology that could move to Florida with a fully adhered vapor permeable membrane.
We found that with flat roofs in hot dry climates that were black in color we had enough heat driven down to basically prevent condensation. When the membranes changed from black to white we ended up with roof decay and condensation so we had to elevate the temperature of that condensing surface by putting insulation on the top. The white membrane saved energy but they caused a durability issue that was solved by adding more insulation on the top of the roof deck. When we were first dealing with this in the mid-1980s folks said, well, why can't we put spray insulation - air impermeable insulation, in the underside of the roof deck rather than putting insulation on the top of the roof deck? The physics worked and so this was the beginning of using spray foam insulation on the underside of the roof deck to control the temperature at the condensing surface.
The original insulations that were used were open cell, low density spray foams. The reason for that was at the time we were developing the approach no closed cell spray foam manufacturers were interested in pursuing the technology. Fairly high density insulation foams have problems working this way. Codes were amended to allow that technology and basically for climates 1-4 open cell low density foam or high density foam could be used. In climate zones 5, 6, and 7, only high density foam could be used or you had to apply an integral vapor barrier on the underside, or a vapor retarder on the underside of the low density foam, because of the difference between the wetting and drying potentials in colder climates relative to hotter climates.
This technology is now very mature and is almost ubiquitous. There are a few things that we've learned over the last 15 or 20 years and that is that with low density foams in hot humid climates there needs to be some mechanism moving moisture from attic spaces. The attics have to be conditioned, not merely unvented. There has to be a supply and return in the attic space to remove that moisture.
There are issues with respect to durability of shingles. What we found was that the color of the shingles was much more important to its durability than whether this assembly was vented or unvented. All of this is on the Building America website.
What we found was that the greatest difference between shingle temperature and whether it was a natural shingle or tile shingle. The bigger impact was the temperature of the roof deck itself rather than the shingles. The shingles temperature would be affected 2 to 3 degrees but the underside of the roof deck was affected 20 to 30 degrees. So we don't have much of an issue with shingle durability but we have more of a concern with sheathing durability. It's a really bad idea to have a black shingle on OSD on an unvented roof in Las Vegas, but it's not a problem to have a light colored shingle anywhere in the country.
The durability effect of shingles is roughly the same as a radiant barrier. A radiant barrier increases the temperature of your shingle approximately the same factor as an unvented roof. We're talking about if you ran this equation about a 10 percent reduction in durability. This is a classic indication that, if you really really really want to increase the durability of your shingle, don't use shingles. Use tiles. Use white tiles. People who live in hot dry climates have figured this out and in hot humid climates.
Again, this is a graph showing that defect. It's more pronounced on the plywood with the OSB, rather than the shingle itself. Every now and then you get very lucky and we got very lucky with a massive roof failure in Juno, Alaska. How can you say that you got lucky with a roof failure well, you can often learn more from a failure than you can from a success, but 200 structures were laid with panel roofs began to rot and decay in Juno, Alaska about 15 years ago. The issues were mushrooms growing out of the shingles. We call this the “Wolfgang Puck” effect. It was localized at joints and at ridges. What's interesting here is that virtually all of the moisture accumulates basically at the ridge. This gave us an ah-ha moment. We said maybe if we can remove the moisture once it arrives at the ridge, we don't really care how it got there in the first place. The pattern is quite impressive. If you get at the damage within the first three to five years, we're only talking about moisture accumulating two or three inches from the ridge line. If you were to put a vent over this, just at that location, you would not have any decay and the moisture would end up leaving at that location.
This was a retrofit technology and we said, well why can't we build in that concept in new construction? The idea was that with strips, why not put basically a vapor diffusion vent over the critical area and not have to reconstruct the entire roof assembly. This actually worked out extremely well. It turns out that our ridge vent here is roughly twice as wide as it needed to be. We didn't know that it is twice as wide as it needed to be until we figured it out. With things like this you have to do field work to figure it out.
We then looked at solar drive through various roofing systems and even though the physics said that moisture will pass through, the quantity predicted versus moisture driven through asphalt shingles from the top side into the roof deck but that’s actually not correct. In the early 2000s when we were, or I was, writing code language, we did not know that the inward drive was insignificant so we built into the language the requirement for a roof top vapor barrier to prevent the inward drive. Five years later we found that that was completely unnecessary. Once in the code, it is almost impossible I'm here to tell you it shouldn't be a code. I apologize. We got it in there because we didn't know. We were being careful. We did the research after the fact and it turned out not to be a major drive.
Something that we didn't anticipate was that with open cell foam moisture will accumulate in the underside of the roof deck and then in the old days, when we were doing unvented roofs, we found that the duct leakage itself provided conditioning. So we had air change between the attic and the occupied space simply because of the duct leakage. No good deed shall go unpunished. What happened was that the industry has gotten very good at, guess what, constructing tight ducts. Now, with an unvented roof with low density foam in a hot humid climate there is so little air exchange between the attic and the house that we have to provide a formal air exchange. We have to provide the supply air to the duct and a return path so that the attic is a conditioned space just like any other location. With low density open cell foam, we have to provide mechanical air change between the attic space and the occupied part of the house. It's just like a bedroom. Who knew that we would end up with tight ducts, even with duct work in unvented conditioned attics?
Let's go back and look at this technology. Can we vent moisture through a vapor permeable roof deck? And the answer is yes. Are we limited to tile only? The answer is no. There are some real possibilities. I'm kind of excited to tell you where we're at.
We thought, well, maybe we can do asphalt shingles on top of a polypropylene mesh. It turned out to be unnecessary. We found because of the Juno, Alaska failure that all of the moisture accumulates at the ridge and so what you can do is just put vent openings at the ridge area, cover it with something like a Tyvek or a Typar, put a polypropylene mesh on the top of that and a cap shingle. You now have a ridge diffusion vent. Now, this is with tile. This is with a slot that we have test houses in Orlando, Houston, and I'm here to tell you that after over 12 months of data, it works exceptionally well and so for climate zones 1, 2, and 3, I believe that you can construct an unvented roof with asphalt shingles, with a vapor diffusion vent, with netted cellulose or fiberglass. It's pretty exciting stuff. It means that we're able to take, in essence, Las Vegas technology and adapt it to asphalt shingles and Las Vegas technology and adapt it to Florida and to Texas. So, there you go. I'm ready for questions or I guess I'm done and we wait for questions later.
Back to you Gail.
Gail: Thanks Joe, and Vladimir, you're up next.
Vladimir: Thank you Joe, thank you Gail. So, on the second half of the webinar we're going to come down from the rafters, both literally and figuratively, and talk about walls and talk about walls from the standpoint of implementation. We'll cover topics that a lot of building scientists already know but we'll take the angle of, well the builder is implementing these walls in their construction process and what are some of the things that they need to pay attention to. What are some of the options that they have? What are some of the unintended consequences that they should be aware of? I'm trying to change my slide. There we go.
Overall we know that the market penetration for energy efficiency in walls has been low and I'll show you some statistics in a minute. One of the ways we want to tackle it, we're in the process of developing what we call a Construction Guide to Energy Efficient Durable Walls that also can be constructed in the field. We're looking at the implementation considerations. Right now the version that will come out in 2015 is going to be limited to climate zones 3 through 5 and basically just two wall assemblies.
The goal of the guide is to help builders figure out how to combine the tried and true practices that they have been using for a long time. They know that they work, but add the new energy efficient component to the recipe and understand all the pros and cons of the decisions that they're making and also address the potential conflicts or unintended consequences that can otherwise occur if not addressed properly. All of them can be addressed, obviously, as we'll talk through that. One of the reasons that research is being done by Building America and other stakeholders, people like Joe, over the years have provided information that we need to help builders make these decisions to build the walls that are energy efficient and durable. At the end of the day, we want to minimize the risks to builders as they're transitioning these more energy efficient wall systems.
The guide that we're developing, the construction guide, is really going to have two parts to it this year. The first one is just looking at 2x6 walls. That's all that we're doing - looking at 2x6 walls. That part of the document, the draft of it, it's a stakeholder review draft the way that we call it at this point, has been posted now on our website homeinnovation.com/wallguide. Feel free to go download it. If you have any questions you can submit questions to me as well. Later in the year we're going to have a 2x4 with 1 to 1½ inches of foam. That's going to be the second half of that guide. As far as what I'll be talking about today, we'll touch a little bit on the market in terms of what kind of walls builders are building today, what kind of materials they're using, and then we'll talk about those two wall systems that I already mentioned - 2x6, and look at the implementation considerations that builders should pay attention to and 2x4 walls with 1 to 1½ inches of exterior foam. We're going to kind of partition the discussion along the five primary bullet points. We'll talk about the framing. We'll talk about the sheathing. We'll talk about the interior vapor retarders and the drainage plane and the cladding. Those kind of go hand in hand. Those are kind of the key areas that we will want to address for each wall system.
Let's quickly talk about the code here. In 2012/2015 there was significant change that happened to the wall, minimum prescriptive wall R-values. I'm specifically talking about climate zones 3 through 5, and 3 and 4 particularly where the change happened from the minimum prescriptive requirement went up from 13 to 20 or 13+5. As far as, again those are the 2x6 walls or your 2x4 plus 1 inch of foam. Those are the examples of the typical wall construction and practices that builders can employ to meet code. The code does give you a way out in terms of to show total UA alternative, but many builders will gravitate to this prescriptive option, particularly in the higher climate zones like 4 and 5 and particularly if they are building above code. If they are doing Energy Star or National Building Standard or other above code programs that are available to them. We've already seen this map during Joe's presentation.
Just to give you an idea of climate zones 3 through 5, it's a big part of the country. A lot of the construction actually happening in those 3, 4, and 5 are in those climate zones so it's having a significant impact on the new codes and construction practices around the country. Again, just talking in general terms as far as builders changing technologies and going from the standard 2x4 walls with 16 inches at the center, which is still the dominant wall in the marketplace to something else, a couple of words of advice here. You know don't just take the new and add it to the old. You have to really look at it as a system from the standpoint of finding efficiency in the system, offsetting the cost increases, and finding the sweet spot of where and how you want to combine those tried and true practices that you know that work and add the new energy efficient component to it and find a system that works for you and making it work for you given the cladding system that you have or the cavity insulation that you want to use. You do want to look at the potential unintended consequences that we'll cover today, just to make sure that the process goes smoothly as you go through the implementation. If you address all those you'll have a wall that has higher performance and at the end of the day you're going to have better value for you company, for your own business, and for the homeowner.
So let's switch gears a little and briefly talk about the statistics that we have, that we collect here at Home Innovation. I've shown these statistics at other venues so some of you have probably seen it already but I have new data for 2013 that I have not really shown much yet and the picture hasn't change, honestly, that much. We can see that the 2x6 framing is gaining some market share but the key takeaway here that it's really giving the 2x6 walls. People are still sticking to 16 inches on center and there is definitely the resistance to going to 24 inch on center. From the wall sheathing standpoint, the OSB, plywood, and products like ZIP, which is basically we're talking about a wood structural panel product - continuous sheath now. It's 80% of the wall area. It's definitely the predominant material used on the outside of walls.
Another trend that you can see, the foam is used as a primary here, a primary layer of sheathing layer, dropped over the years. There are two reasons. One is that people went to over sheathing - having two layers, and also some of the construction, actual volume, moved down south where these practices are less common.
Here we see the statistics for the over sheathing. You can see that there is some pick up that did happen where people are using foam as the second layer, rather than the primary layer, and house wrap is really being used very widely now. Some, basically 3/4, of homes are being built using some type of house wrap on the outside. With the cavity insulation, fiberglass remains to be the predominant product. It lost a little bit of market share but it has kind of leveled off and it hasn't changed over the last couple years.
Spray foam gained some market share but also kind of leveled off and hasn't increased in the last couple years. That's basically what we see. The builders are moving toward some more energy efficient practices but not at a pace that we would like them to.
For the next few slides I will talk about 2x6 walls and why it can be a good option for many builders when they're switching from the 2x4 construction. You know, it's probably the most straightforward transition that you can think of going from standard walls, 2x4. You know there were some changes that needed to happen but at the end of the day it's not that many changes. Technology has been used for a while. In colder climate zones, with maybe some mixed successes here and there, for the most part they have performed very well and we do have a track record for this wall system. When you are switching from 2x4 to 2x6, really the studs that you have now are much much stronger. You need to take advantage of that. That's why going to 24 inch on center is really the preferred method because that's really how you're gaining your energy efficiency, both through having deeper cavity, having more insulation because the cavity is deeper, and also by the way adding in R-2 on the outside because the R value for wood is 1. So if you are going from a 3½ stud to 5½, adding an R-2 that helps as well.
Overall what you end up having happen is fewer studs that you have to install and it helps both with the resource efficiency, it helps with number of studs that you can have with the openings, because the studs are so much stronger that you don't need to have as many studs. Then with advanced framing, 2x6 is really more conducive to advanced framing. We've been promoting advanced framing for a while and it's definitely something that works for many builders. Some of the practices are challenging to implement with 2x4 construction because of the limitations. I'll talk about a couple of them in a few minutes here, but 2x6 construction is really meant for house framing and we will talk about what we call advanced framing options. Another benefit is for reduced labor. One of the builders that we work with with the Building America program, they got back to us and they told us that they really felt...they have actually implemented the system, the 2x6 with 24 inches on center throughout the company. They really felt that the savings that they got from using fewer lumber pieces, having to insulate fewer cavities, having to drill through fewer studs. They felt an impact. It was real to them.
Again, the guide, the draft of the guide - the stakeholder review version, is available at homeinnovation.com/wallguide. So, let's talk about some of those transition considerations that, as you go from 2x4 to 2x6, you can keep in mind. Again, 24 inches on center at 2x6 you can go as high as a two story building with an inhabitable attic. Now, if you have a three story building you'd have to go 16 inches on center just because of the structural load that you have at the support. For the majority of homes, you can go 24 inches on center. Also, a key difference here is that if you're using 2x6 walls, with a double top plate, you have to worry about aligning your framing. Your framing between story to story, floor members to studs, does not have to be aligned as long as you have a double 2x6 plate capping the wall. It's different for 2x4s. That's one of the considerations that people had to pay attention to. If you are using 2x4 lumber, with 24 inches on center, you have to think about aligning those members and it can be...that can be a hassle from the practical standpoint. Not everybody can really follow through on that and make all that happen.
Also in the guide we'll address multiple corner options that are available to builders. What you can see in all these options, if you're switching to 2x6 you simply have more space in the corners to install insulation, regardless of what method you want to use. You will have a better corner. You will have an opportunity to have a better, from the thermal standpoint a better, corner simply because you have a 2x6 member with 5½ inches wide. You have the room to put in extra insulation. Even if you need more nailers there for your trim or interior finishes, even if you've put in extra lumber - as I've shown in this slide, you can still find ways to insulate that space and have a much better performing corner.
Let's talk about openings. There are multiple options available for headers. One of the technologies that Joe definitely supported getting the technology into the code is the single header. With 2x6 construction that's even much more advantageous to have a single header because now you have 4 inches of space where you can insulate and if you have an XPS of R-20, basically at that point in the header the performance of the wall is the same as the cavity. From the practical standpoint, it's the technology that is more applicable to single story homes or the top story of multi-story construction simply because of the load characteristics and the capacity that you need to have to carry a load from a two story house.
For a single story or top story we're talking about, you can do openings up to 6 feet or up to maybe 7 feet if you go with structural composite lumber. Often, particularly for top stories of buildings, you have many wide openings anyway so that can be a great solution. Go to IRC and they have prescriptive spans that you can rely on for single headers.
The second option is going double header, but not insulated double header. Again, because it's 2x6 you have the space to insulate. Depending on the foam that you're using, depending on the thickness of the structural members that you're using like, for example, composite lumber is typically a little bit thinner than dimensional lumber. You can get between an R-13 and R-19 in that space. Again, you're going to have a pretty well insulated space there and you will, you know, for a double header you can pretty much cover most doors and windows in a house with maybe some few exceptions of very large openings.
Another new technology, relatively new technology, we will introduce now is an IRC, a 2015 IRC referenced specifically. It's called a rim header. In that case what you see here is the load is being carried by members located in the rim area of the floor. Then you don't have any headers at all. You can just insulate the space above the wall fully and then from a thermal standpoint it's a very good solution. So you do obviously have to install joist hangers to carry that load around the window. Really that technology works much better with a 2x6 package. You can make it work with a 2x4 but you're going to lose some of the advantages and one of them is you only have one type of studs here. You have the studs they call king studs. You need to have a jack stud because you don't have a header right above the window. You can reduce the number of studs that you have at the opening, overall reducing the framing factor. These are just a couple of photos of rim headers actually being built in the field.
There is one potential option for an energy efficient solution. It would be to have single headers on the top floor, because they have rim headers only applicable to lower stories, and then have rim headers at the lower stories and this way you really have an option to insulate that space above the window very well throughout the entire house.
Now let's talk quickly about rim joists. Again, with 2x6 you have more bearing area so you open yourself up for more options that otherwise would not be available to you with 2x4s. You can use prefabricated insulated rim where you have continuous insulation there. You're still going to have enough bearing area for the joists to rest on. Another option that, if you have an engineer that can evaluate this for you, where you can have continuous insulation where you have trusses, for example, you can have a ribbon board that provides the stability that you need there for the truss. Then you have an option for continuous rigid insulation. Again, you have still enough bearing area there for the trusses to rest on and it's only possible because it's 2x6, a 2x6 wall.
I just briefly want to touch on this configuration that we've just tested in the lab. It’s not available in the code yet but it’s going to involve rim joists plus where we've tested share wall with an insulation being installed on the outside of the rim and the rim being recessed by an inch. The question becomes, does it have any adverse effect on the shear performance of the walls because you have a cantilever bottom plate now. For the wall capacities that we've tested, which are lower capacity walls, the typical walls that you see in the IRC. This would not be applicable to high seismic areas or something like that but for the IRC level walls will actually didn't see any reduction in capacity. That's something that we want to research further and understand more before we can actually fully recommend that but it's a promising option where maybe a solution between a recessed rim, by let's say an inch, and an insulated with an inch of XPS or similar product on the outside and have a better insulated rim on that area.
Again, if you are switching to 2x6 with 24 inches on center with structural panels, sheathing, what would you have to pay attention to? You can do this at 16 and 24 but pay attention to your sheathing thickness and the nail sizes. That may vary based on where you're located, what the wind speed is and what the exposure category is. You may need to, for example, a 3/8 structural panel would not be permitted at 24 inches on center anywhere in the country if you look at it on the site of wind exposure B, sorry C. If you're in B you're fine but if you're in C you cannot use 3/8. Those are the types of things you have to pay attention to. Again, there is a table in the IRC that will kind of specify that for you and also in the guide, obviously, in terms of the panel thickness and the nail size that you need to use. It's one of the considerations that you've got to pay attention to. Similar to gypsum, you can obviously attach 16 or 24 inches on center, but again the code allows you to use 3/8 gypsum. It's not a typical product but, you know, it's in the code for 16 inches on center. You cannot use that product with 24 inches on center. If that's something that you've been using in the past then you may need to make a change there. You can use nails or screws. You can use adhesive or no adhesive, but also what we're hearing from some builders if you are switching to 24 inches on center you may want to use adhesives because at least some people seem to think that if you don't have adhesive and you go to 24 inches on center you may see more cracking than usual. I don't have any statistical information on that but anecdotally we did here that from at least a couple of builders.
Again, as far as changing your paths and your schedules from 16 to 24, if you're using screws you've got to go to an increased number of nails a little bit going from 16 to 24 inches on center.
Let's talk about interior vapor retarders. That's something that's definitely important. Those recommendations vary by climate zone, by what insulation you're going to have in your cavity, by what cladding system you're going to use. That will make a difference as well. You have to look at the entire assembly and you have to think about where this is located to make the decision. For climate zones 3 and 5, your vapor retarder, if you're using one, if you're using one, it's always going to be on the inside. The goal here is to prevent an interior moisture load, that moisture that exists inside the house, from getting into the cavity. That's the purpose of the interior vapor retarder.
Let's talk about some specific recommendations that we give in the guide and they're kind of based on the code and sometimes we give additional recommendations where the code, in our view, may be lacking or unclear. So, it's a climate zone 3. Vapor retarder is not required, but also we're really discouraging people from using class one. You just don't need it there. You just don't. Our recommendation is just don't do it. Climate zone four, again, vapor retarder is not required but based on some of the monitoring and testing that we've done we would recommend a class II, like a Kraft paper or a class III at least - an actual class III, perm 10, on dthe interior of the wall. Now, climate zones 4 and 5, a vapor retarder is required and it's either I or II. Now we probably recommend the class II vapor retarder in that case. Again, Kraft paper can work very well or something similar. If you are using non-vented cladding on the outside, we're definitely discouraging using class I. Air sealing is always a good option. It will always reduce moisture load if you air seal well.
The last topic for 2x6 walls is to talk about claddings and how they'll attach them. There are four considerations that you really have to think about. What type of cladding system do you have? Is it vented, partially vented, non-vented; where your drainage plane is located and how it's integrated with the cladding system; how you're attaching your cladding and from that you'll also figure out if you're compliant with the wind with provisions of the code. It's always a balancing act. We're trying to improve energy efficiency but you don't forget that there are other requirements in the code that you have to comply with, and structural provisions, and wind load provisions are obviously part of the code as well.
Let's look at vinyl siding as an example. It's one of the easier claddings to work with, where if the air gap is integral to the system you can use almost any WRB that's available to builders. What you have to pay attention to here is that when you're going from 16 to 24 inches on center you better make sure you comply with the wind code. You can either get vinyl products that are rated specifically for 24 inches on center or you can use an attachment that's, now it's in the code that, allows you to attach vinyl siding to OSB only or plywood only. Those you typically have to use different fasteners, different shank fasteners, or have more nails as well. That's another option but you have to pay attention. Some product manufacturers are going to have specific recommendations for their particular product that you can follow and install at 24 inches on center and maybe install one fastener in the middle.
Now with anchor veneer, just as an example here, you've got again an air gap integrated. You can use a wide range of WRB products in the system. Now in the seismic area, what you have to pay attention to again here is attachment. How many fasteners do you need? If you are in a high seismic area you need more fasteners and those clips that you need to use on metal angles to attach the cladding. You have 24 inches on center so your spacing on the stud will have to change. You just have to pay attention to that. Products like engineered wood siding, that gets a little bit, we provide more information on that. We don't have a lot of time to get into that here but again 24 inches on center, you definitely need to read the manufacturer’s instructions and it will tell you that you need to use a thicker product that's rated for the application. They've made an alternative fastener schedule that you have to follow as well. So just pay attention to that.
So now we're going to switch gears and kind of go through the same implementation steps that, you know, builders need to go through if they're using a 2x4 with 1 to 1½ inches of foam. Again, it's a very easy wall to transition to wood with minimal amount of foam on the outside. The cladding can still be attached using the typical construction practices. So again, it should be a pretty easy lift for builders. There are a variety of products available for exterior cladding.
Here you're minimizing your thermal losses by providing a thermal break. You have a continuous insulation. As far as framing practices, there is not as much you can do with 2x4 as you can do with 2x6. I'm not going to get into it. The goal here is you deal with your thermal break by having an exterior foam. The moisture dynamics are a couple of differences but we'll talk about that in a second.
There are different types of exterior insulation products available. Just pay attention to what thickness you have to use because there are values of those products varied, particularly EPS even if it is the same EPS insulation products. The actual R-value per inch varies based on the type of EPS you are using so just pay attention to that because on the code, if you follow a prescriptive method, it's R-13+5. You can get your R-5 on the outside there.
One of the key implementation considerations for when you install exterior insulation is to figure out where your drainage plane is and how the drainage plane is integrated with your penetrations. So they basically have to options available to you. The outboard, that's an option. The outboard on the drainage plane where the drainage plane is behind the cladding, it's outside of the foam, and it can be done in two ways. You can either tape in the foam and your outside of your foam will become the drainage plane or you can install house wrap over the foam.
Again, in the guide we're going to have these specific details on how this is done. We're either going to use graphics similar to what you see here where we're basically showing how it's properly layered, how it's integrated with the flashing, and how the caulking and layer ceiling is done. These are just some photos of houses. On the left you see there is the drainage plane on the outside with the foam tape. On the right it's again the drainage plane on the outside with a house wrap installed.
The second option is to have a drainage inboard. So basically you install your house wrap as you would install on your standard 2x4 construction wall. Then you install your foam product or insulation foam product, in this case probably, over that. So the benefit of this, again, as far as integration with your windows and flashing is pretty much stays standard because it doesn't change. Then you have a layer of insulation over that. Maybe people would also recommend that you use a drainable wrap. It's a recommendation, not a requirement, but maybe a best practice to use a drainable wrap to provide better drainage between the foam and the house wrap.
When you install the cladding, that's another significant consideration that you've got to pay attention to. Installing claddings over the studs and wall systems there are four things you really have to pay attention to. What is the fastener size that you're using? What is the framing spacing that you're using? What is the thickness of the insulation and the weight of the cladding? We'll go through a couple of examples here just to give everybody a flavor of what's possible, what's practical. Keep in mind that the fasteners have to penetrate into the framing by at least 1¼ of an inch. Keep in mind also that you can count the thickness of the wood structural panel towards that. That's solid wood product so that counts as your minimum penetration. This is fresh 2015 IRC. Let's say if you're using 1 inch of rigid foam insulation outside and the nail side that you're using is going to be a 6 penny or 80 pneumatic type nail diameter on .113, the options on the right there - the claddings that are available to you, are based on the nailing spacing that you have is very solid with vinyl siding and fiber cement. That's driven by the weight of the cladding product. If you want to use fiber cement, you have to use it with this nail, with this thickness. You have to think of them as a package. You have to have framing at 16, which is going to be probably standard framing for 2x4 anyway and then your fastener spacing cannot be more vertically than 8 inches. Now let's switch, the same exact assembly, but the only change we made was with the 1½ of foam and all of a sudden, you know, in that case if you're sticking with the same a small nail, fiber cement is no longer an option for you.
So what you can do, next slide, you can increase the nail size and then the fiber cement for 1½ inch rigid foam becomes available to you as well. Then a more extreme option if you are using a bigger nail, such as a 16D nail, you obviously can install heavier claddings over the foam and even stucco. There is an option that you can have stucco installed there as well. These are just some examples of cladding materials installed over foam walls.
Probably the last topic I will talk about today is 2x4 walls and interior insulation. What vapor retarder should we be using? What I talked about with 2x6 walls, it's going to be thrown out the window. This is a different wall system. You have to think about it differently. You have to analyze it differently. Now the sheathing is going to be warmer because they have the foam on the outside. It's a good thing because what it does is reduces the relative humidity inside the cavity and basically in simple terms what it does is uses the moisture load on the materials there. That aside, it also reduces the drying rate to the outside. Based on some of the research that we've done, the walls are still drying out, this wall it gets, as long as you don't have a double vapor barrier. You have foam on the outside and if the wood based products get moisture then we see that they are drying out with insulation on the outside.
In climate zones 3 and 5 you're allowed to have for this wall system class 3 vapor retarder and that can be latex. Latex paint is an example. Keep in mind that, you know, it's really specific to this wall system and this climate zone. If you make even a small change like, going okay, I'm going to go to 2x6 wall rather than a 2x4 wall in climate zone 5, for you to comply with code now you have to increase the R value on the outside of the wall. Or, if you take a 2x4 wall and put it in climate zone 6, again, to comply with this vapor retarder requirement in class 3, you have to have more foam on the outside. So again the point here is to keep in mind that the provisions are very specific to - this is the wall assembly and this is the climate zone you're using it in.
So as far as, again, our recommendations again, we're really discouraging people from using class I vapor retarder when you have a permeable, a less permeable exterior. Less permeable should be careful using terminology. Terminology I use here. Insulation on the outside if you build a perfect wall and nothing leaks this wall may work. It doesn't means this wall will never work. It may work. It may work in many situations but if any leaks occur it will be hard for the moisture to get out. Air sealing, again, is always a good idea. Also we've discovered from the research that we've done, paint is not a reliable class III vapor retarder. Depending on what paint you use, you could be, it could be very permeable. It could be 40 perms or higher.
We would also recommend that for climates zones 4 and 5, if you have foam on the outside still use something like Kraft facing or smart vapor retarder. You want to stay away from using double vapor barrier by having some protection from the moisture load may be a good idea in those slightly colder climate zones.
Just one image here of how not to do flashing here. This is not the write way of doing it. Just a little reminder.
The summary of the presentation here - both of the wall systems I talked about today, 2x6 walls - 16 or 24 inches on center, we recommend 24 inch on center, or 2x4 walls with exterior insulation. I mean they're really ready to go. There may be a few things that the builder has to think through as they're implementing them. For the most part they're ready to go. You can use them. The things you have to think about are what framing package you are going to use, what your drainage plane is going to be located, make sure you are attaching your cladding correctly, or do I need a vapor retarder, integration details and some other considerations but the systems are ready for use. We'll have the guide available by the end of the year. Hopefully it will help people to transition and that concludes my part of the presentation.
Gail: Okay, thank you for those outstanding presentations. We have time now for a few questions. We have some great questions from the audience and you may submit additional questions through the questions pane on your screen. Our speakers will answer as many as time allows.
I have a few questions for Joe. I'll start with those. The first one is - are the unvented over roofs required in all cold climates or just in cold climates with high snow loads? My company is looking to spray foam insulate the underside of an attic roof deck and turn a vented attic into an unvented finished upper floor space.
Joe: You need to put a vented over roof over an unvented under roof only if you are in a high snow area to control ice dams. If you're in Syracuse, New York, which by the way gets more snow than anywhere else in the lower 48 states. People from Syracuse go to Buffalo to get away from snow. An unvented roof without having a vented over roof is a bad idea. In most other places where you don't have many feet of snow an unvented roof with spray foam directly on the underside works just fine.
Gail: Great, thank you. The next question - does a cathedral ceiling with unvented assembly eventually lose its ability to dry to the inside with additional layers of latex paint?
Joe: Not with latex paint but if you did epoxy paint or alkyd or oil based paint the answer is yes.
Gail: Okay. Another question - what is the performance impact of a properly detailed but under ventilated attic? Is there an energy penalty if the attic is over ventilated?
Joe: It's a difficult question to answer and so if the attic ceiling is very leaky, and you have a lot of attic ventilation, and the vents are unbalanced - meaning more vents up high than down low, you're actually ventilating your house through your attic and you're increasing your air change of the house. But, if the attic ceiling is air tight over ventilated...I'm getting some cross talk. Somebody should be muted. Anyway, if the ceiling is air tight, whether the attic is over ventilated or not is not going to affect the air change. A way of thinking of this is back in the day there used to be these turbine vents that when the wind would blow they would suck air out of the attic. A lot of people in cold climates would put garbage bags over them in the winter time and the reason was that when the wind blew they sucked air out of the house, not just the attic and the attic ceiling. If the attic ceiling was air tight, that would not be a factor so if you have an air tight ceiling over ventilating the attic isn't a penalty. If you have a leaky attic ceiling, over ventilating the attic is, in fact, an energy penalty.
Gail: Okay, thank you. The next question is - I have a hot roof with open cell in Michigan. Will vapor diffusion fix the condensation problem or what can I do now?
Joe: I would apply a liquid applied vapor retarder directly on the underside of the open cell foam if you have a problem. The way to determine if you have a problem is to go and cut holes in it and see what the condition of the roof deck is. If you don't have a high interior moisture load, open cell foam will work in Michigan if the relative humidity in the winter time is in the 20s. If the relative humidities inside are in the 30s and 40s, you are going to need a vapor retarder on that open cell foam. The reason that the code wants a vapor retarder in a zone like Michigan for open cell foam is we have to assume that people would have humidities that are in the high 30s and low 40s. So for code compliance reasons we put in that safety factor. If the moisture load is low it's not going to be an issue. You should go and have a look at the actual conditions. There are many many houses with open cell foam without vapor retarders in climate zone 5 without any problems because the moisture load is low. It is the same thing with dense packed cellulose roofs. We have thousands of roofs that are dense packed with cellulose that don't have a problem in cold climates because the interior moisture level is low but when the levels go above into the high 30s during December, January, and February they get into trouble.
Gail: Okay, here's a question about the marine climate. Do you recommend installing the ridge plot vent, like you mentioned in systems 1, 2, and 3? Do you need it at all?
Joe: Oh man, difficult question. You might not need it but I'm not sure. Why am I not sure? Well, because we have not got any test structures out there to say one way or another. There are a bunch of pretty detailed computer simulations that are arguing that it is not necessary. I'm a coward at this point and I'm going to say go with the conservative approach at this point with your diffusion vent, but it might prove in the next two or three years to be unnecessary. The short answer is to say we don't know. I don't know. Somebody might be smarter than me and has the field data might know but I haven't had anyone share that data with me yet.
Gail: Okay, the next question is from someone in South Jersey. They say do you come across any problems for shingle roof under the solar panels, such as weather solar panels, that will increase the rate of failure of shingles?
Joe: (laughter) I'm laughing, again, because the laws of unintended consequences. Solar panels are great. They're going to save the planet. There we go. Kill the shingles. There's an awful lot of heat generated by those solar panels and the hotter the shingle the lower its life and the answer is, yeah, there are some issues with shingle life deterioration under PV assemblies on roofs.
Joe: It wasn't a dumb question.
Gail: (laughter) Okay, here's a scenario for an existing roof in the tropics - metal roof, AC ducting in the attic. Will the details you showed for Orlando work on the equator? Any modifications? Would rock wool work?
Joe: That will work in a cold climate, also in a hot humid climate like the equator. The answer is yes. It will work.
Joe: The details will work. I would like to be invited though to investigate that in January.
Gail: Okay, and another question. What are the major construction issues for building roofs with up to 4 inch exterior foam? Builders are hesitant to build them due to inexperience with this method. I'm in a cold dry climate.
Joe: Well, one of the problems is you're going to have long screws that basically are going to be experiencing some degree of sheer and there are not very many construction engineers that are comfortable with providing calculations to show that that actually works. There is a structural issue with respect to unsloping roof with respect to sheer. I don't believe it's a real issue. It's a perceived issue and sometimes perceived issues stand in the way of you being able to do that. We've done roofs from the late 1970s that have 8 inches of rigid insulation on the top with fairly long screw width - 12 inch long screws. We've not had any sheer issues but getting somebody to do the calculation or to seal it is not going to be easy or is not always easy.
Gail: Okay, thanks Joe. Now we have some questions for Vladimir. The first one is - can 2x6 at 24 inches on center support two stories without a habitable attic.
Vladimir: No, the answer is no if you have more than two stories you always have to go to 16 inches on center on 2x6s.
Gail: Okay, the next one is what is the basis for the vapor retarder recommendations?
Vladimir: Over the last two or three years we've done an extensive monitoring study where we have over 20 houses around the country with various types of energy efficient wall systems instrumented with sensors for us to get a pretty good idea of what's going on inside of the walls. We're also monitoring the inside of the houses to understand what the moisture load is from the inside of the house and overall it's a pretty comprehensive monitoring program in addition to some of the structures that we have here in climate zone four in Maryland. We've tested from a moisture standpoint a range of those assemblies and we also work with people who do roofing simulation. In compilation of all of the data provided the recommendations that I made during the presentation.
Gail: Thanks, the next question is - what do you mean by the framing must be capped? Would a half piece of plywood suffice for the capping or does it have to be 2x6?
Vladimir: So, there is no exact answer here. I'm not sure if that type of assembly has been tested. I don't think plywood though would be sufficient but it's a judgment call on my end. I think I would want to see a 2x6 there to really build a box structure to provide enough stiffness there.
Gail: Okay, the next question. Is there any issue of exterior heating panel warping with 24 inch spacing? Also, why use the foam board insulation at the box joists as it would still need to be sealed? Why not just seal and insulate it one time with foam after framing?
Vladimir: Let's take it one at a time. The first question with regard to buckling, I don't think the OSB buckling is really a function of the framing spacing. There are three factors that really impact the floor, the buckling issue, that's the panel themselves not always being created equal. Some of them are going to expand more than others. The second one, you're providing a gap so you definitely want to have a gap between panels and the OSB manufacturer's recommended lineage. Two is also the moisture content difference between the panel when it was installed and the panel that's actually in service. If all three of those collide and all three of them are unfavorable, you may see buckling. Another buckling issue that I heard about is that in assemblies where those panels are used in very high sheer capacity applications and they have to put a lot of nails on the perimeter, when the panel had just been installed and came out of a stack that was dry and then you put a lot of nails and then the panel absorbs moisture and is trying to expand and the nailing is so tight that it doesn't let the panel expand. People see buckling in that situation sometimes as well, but as far as just going from 16 to 24 I don't believe that's a driver for OSB buckling. Could you repeat the second question?
Gail: Yes. Why use the foam board insulation at the bottom joist as it would still need to be sealed. Why not just seal and insulate it one time with foam after framing?
Vladimir: I mean it's just an option. You can do it either way. It's just a matter of preference, you know, what's given everything else you're using in the house for your air barrier and moisture barrier and all that. Again, that's just another option that's available to you.
Gail: Okay, another question is - even with exterior insulation sheathing, can there be condensation in the stud cavity with that or spray foam insulation?
Vladimir: Okay, again let's break it up. With the exterior insulation a way for you to control condensation potential is to have sufficient amount of foam with sufficient amount of R value on the outside and the colder the climate zone, technically, the more R value you need to have on the outside. It also, furthermore, can be a ratio of the insulation you have in the cavity to the insulation that you have on the outside. In theory if you don't have enough foam on the outside, there can be condensation but if you follow the recommendations that we have in the presentation, if you follow the code, you should not have problems. Now, if you are talking about a wall system where you have exterior foam on the outside then I think the way I understand the question was spray foam in the cavity. If it's closed cell spray foam, the closed cell spray foam will also act as a vapor retarder from the inside and in theory such system can work. The only kind of issue here is that if there is moisture that gets between those two layers it will take a while for moisture to dry out. If it's just like a, in my opinion, if it's just a rare event that happens and something got all wet there, it will dry out eventually. It's not going to be a huge deal but if it's a continuous, some kind of, source of moisture that exists there, that water is going to stay there.
Gail: Okay, and another question is - what are the merits of installing the house wrap in front of and behind the exterior insulation?
Vladimir: I'll try to answer it, and Joe feel free to jump in. I think it's a matter of preference and if you are installing your house wrap behind the foam then you almost don't have to change your current practice because that's where your house wrap is not right. When you're building a standard 2x4 wall you have your house wrap and the cladding. All you're doing is throwing another layer of insulation between the cladding and the house wrap and you basically you're done. You go home. Now, somebody may ask a question - well, if the water gets behind the foam is it going to be able to escape because the foam is going to be directly against the house wrap? As I mentioned already, the better practice may be to use a drainable wrap. I'm not aware of any issues either way and the code doesn't require that. Again, it's a matter of preference. If you have the house wrap on the outside so you know that that's where your drainage plane is located, it's right behind the cladding, that's a typical way where you're probably going to have, where you want to have, your drainage plane. Then the integration of that drainage plane with a window, you have to change your practices. It's still doable. You can do it. We'll have those details but you have to change your practice there a little bit.
Joe: I completely agree with Vlad on this.
Gail: Okay, here's another. Here's a question that may be for both of you. Could you please address retrofitting insulation in the attic knee wall and roof of a 1½ story cape? Cape cod.
Joe: Well, I think Vlad should answer that. (laughter)
Vladimir: Oh, no go ahead.
Joe: Alright, historical that's been a very very difficult thing to do because you need a plane of airtightness and it's very difficult to connect that knee wall assembly through to the ceiling of the floor below. So a lot of times we like to not insulate the knee wall. We like to run the insulation down the rafter, in essence, and then put some kind of sheathing or gypsum board along that rafter behind that knee wall so that the air control layer and the thermal control layer follow the slope of the rafter and the knee wall is completely inside of the conditioned space. That's a very very difficult detail to do. The first house that I lived in in Boston had that kind of a knee wall configuration and I gave up trying to make that air tight. I followed the slope of the roof. Good luck with that.
Gail: Okay, here's a final question for Vladimir. If you were building a structural concrete insulated panel wall system with conventional wood frame roof, are there any concerns with thermal or vapor transmission into the wall through the solid concrete areas? The wall system is a foam center with wire and concrete on the outer and inner surfaces.
Vladimir: I'm trying to visualize the system. I'm not sure I fully visualize it. Joe, can you answer that?
Joe: I don't think an additional vapor retarder is required. I think the main issue is water control on the outside. If you have a water control area and paint on the concrete I think you have a very nice flow through assembly. I don't think you'll have a problem with that. I think, Vlad, it's much like an ICF block wall.
Vladimir: Okay, ICF. Okay.
Joe: Except it's an inverse ICF, right? Concrete, foam, concrete?
Vladimir: Oh, concrete, foam, concrete? Actually, I lived in a house like that in Russia when I was a kid. It worked perfectly.
Joe: Well, there you go.
Gail: (laughter) Okay, that's all the time we have for questions. Thank you everyone and panelists before we take a quick survey do you have any additional or closing remarks that you'd like to make before we close the webinar?
Vladimir: Thank you.
Joe: Happy President's Day.
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