High Performance Space Conditioning Systems: Part II
November 18, 2014
William Zoeller, Stephen Winter Associates
Dave Mallay, Home Innovation Research Labs
Jordan Dentz, The Levy Partnership
Francis Conlin, High Performance Building Solutions
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 Bill Zoeller, Dave Mallay, Jordan Dentz, and Francis Conlin joining us today to continue our series on strategies to improve the performance of heating, ventilating, and air conditioning systems for low-load homes and home performance retrofits.
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We have an exciting agenda prepared for you today that will focus on strategies to improve the performance of HVAC systems for low-load homes and home performance retrofits.
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Our webinar today will focus on strategies to optimize the performance of HVAC systems for low-load new homes and home performance retrofits. If you would like more detailed information about any of these efforts, or if you are interested in collaborating, please feel free to contact any of our presenters.
Our first speaker today is William Zoeller, a Senior Architect and researcher at Steven Winter Associates. Bill has 30 years of experience in building design, new construction and retrofit, post-construction evaluations, tech transfer and training, and building materials development.
Today, Bill will provide an overview of several design options for locating ducts in conditioned space, including those ducts in unvented attics, dropped soffits, integrated into modified trusses and floor trusses, and sealed crawlspaces, as well as buried and encapsulated ducts.
Next up is Dave Mallay. Dave is an Engineer at Home Innovation Research Labs where he focuses on evaluating residential HVAC and building enclosure systems and designs.
Dave will discuss buried ducts and design considerations, the compact duct concept, results of field testing and monitoring, and alternative solutions for air sealing and insulating the ducts.
Our next presenters are Jordan Dentz and Francis Conlin. Jordan is a vice president at the Levy Partnership, where he develops and manages building science research projects and consults for building owners and operators. Francis is a mechanical engineer and construction consultant at High Performance Building Solutions, Inc., for energy efficient construction, air barrier system analysis, building envelope commissioning, HVAC systems, indoor air quality, and moisture management.
Jordan and Francis will discuss a project with two retrofit duct sealing techniques—manually-applied sealants and injected spray sealant—that were implemented in several low-rise multi-unit buildings. They will also compare the results of the two methods in terms of reduction in duct leakage to the outside, cost, energy savings and payback.
With that, I would like to welcome Bill to start the presentation.
Bill: Thank you Gail and hello everybody. As Gail mentioned I am Bill Zoeller. I am a registered architect with Steven Winter Associates. We also manage CARB, which is the Consortium for Advanced Residential Buildings through the Building America Program. I am going to be doing a presentation on design options for locating ducts within conditioned space. I do have a lot of slides I will be going through, some of them pretty quickly. A lot of this is an overview and since there are a lot of options that are worth considering we need to make sure all of them are given a little bit of presentation time and a little bit of description. Next slide please.
The primary question is why are we concerned about putting ducts in conditioned space and if we look at that what the typical option would be without putting ducts in conditioned space, basically ducts located in unconditioned attic spaces. We look at attic spaces in the south for instance, and we could have temperatures of 160 degrees and in the north say 0 degrees. When we are putting cooling air through ducts at say 55 degrees in 160 degree space obviously there are thermal problems that occur, thermal losses. Similarly with 150 degree, 140 degree heating air through a space that is 55 degrees that is a huge delta T and tiny little leaks and losses can result in huge thermal losses. We've seen losses range up to 45% or more. It can be very very significant in terms of losses. There are also problems with indoor air quality if we have leaky ducts in an attic space. We've got pressure imbalances in a house that result, moisture problems, comfort problems and ultimately durability problems. All that stuff is something we'd like to avoid and we've come up with some options to do just that. Next slide please.
What we've got is a series of options that give us alternative places to put our HVAC systems within the house that basically limit those thermal losses. Basically we are trying to thermally protect our duct systems. We will be looking at actually 6 different systems. There are 4 located on this slide right here: ducts in unvented attics, dropped ceilings, modified trusses, ducts in floor trusses. We will be looking at basement applications, crawl spaces, as well as buried ducts. The reason we have so many options is that the one that makes the most sense for any given circumstance has to do with how the house is designed, configured, and how it is located. Next slide
So the first one we are going to look at is placing ducts in unvented attics. We are starting to see more application of unvented attics really throughout the country. It is prescriptively allowed in the code now. In a lot of circumstances it makes sense for what we are trying to accomplish. Basically what we are doing in this instance is taking the thermal boundary that used to be almost universally at the building ceiling and putting it up at the roof plane. When we do that we are really just enlarging the thermal enclosure and capturing a large volume of space. Within that thermal enclosure that gives us the opportunity to place the duct work. Next slide please.
When we place our ducts in an unvented attic space, again by placing the insulation up at the roof duct we are utilizing a volume that already exists. What this does for us is it allows us a lot of flexibility in terms of where the ducts can be located. You've got the entire ceiling plane of the house to operate with basically. We can have supplies to where you want them. We can have returns where you want them. You can run the ducts around obstacles and other protrusions within the attic space so in a lot of instances it can be a pretty practical approach. It also does give us a place to place an air handler, which a lot of these other options do not provide for us. There are some limitations however. One of which is that storage is typically not allowed in the space with 4 inch foam at the roof deck without the use of thermal and ignition barriers and some other safety conversations that are also enumerated in the code. Next slide
Another consideration when looking at the unvented attic approach is the type of insulation and the configuration of the insulation that we're going to use. One of the things that we need to keep in mind that when we do put insulation at the roof deck, particularly in colder climates, we've got a concern with condensation control at that roof deck. The code recognizes it and has a minimal amount of air and permeable insulation that needs to be installed directly against the roof deck either underneath the roof deck in the form of spray foam insulation, for example, or on top of the roof deck in the form of rigid insulation such as PolyISO or SPX or a similar product.
That red box is indicating in zone 4 the level of air impermeable insulation would need to be R15 in order to meet this code requirement. Basically what that's trying to do is keep the first condensation plane, the lower part of the insulation, basically above the dew point so that it doesn't condense at that point.
There is one other consideration here too that if we are using open cell insulation, which is air impermeable, we would need to provide a vapor retarder at the bottom surface of it. That is also indicated in the code and that relates to unvented attic spaces in the insulation requirements. Next slide
Here you can see a mockup of an assembly of an unvented attic space with the air impermeable insulation installed up against the bottom of the roof decking. That would be a closed cell foam insulation and in the climate zone 4 that we just looked at would be an R15 approximately. In that instance the rest of the insulation could be just a dense pack cellulose, dense pack fiberglass, a netted product and we have a code compliant effective system. Next slide
So if we're looking at installing ducts in an unvented attic space for thermal protection of those ducts, or any other system for that matter, we are looking at the advantages or disadvantages or limitations of that particular system. In this case the ducts in the unvented attic do give us a place for the air handler, which is important. We're not just protecting the ducts but we don't have to worry about handlers in this instance as well. It's not as plan dependent as some other options, because as I mentioned you can put the ducts pretty much anywhere you want on the ceiling plane. The registers can go anywhere and it's viable for retrofits. If I have an existing house and I have duct work in the attic already, I can go ahead and do spray foam and similar applications at the roof deck and it can work quite well.
Limitations, typically, this is probably the highest cost option for providing volume to place the ducts in. Roof deck insulation typically costs more and there is more of it than say ceiling insulation. There are code limitations in terms of the vapor retarders and the condensation control. By putting my insulation at the roof deck instead of the ceiling I've in essence increased the size of my thermal boundary, which really does increase the heating and cooling load on the house. It depends on how much you are increasing it for that cost-benefit analysis to work out. Next slide
The second option we are going to sort of look at briefly is ducts in dropped soffits. In this instance we basically take the ducts and put them below the ceiling plane inside the floor plan and sort of integrate them so they become part of the floor plan themselves. The architectural integration obviously in the functionality, the aesthetics, are really critical considerations. Far more so than the vented attic approach. Next slide
In this plan you can see sort of an example of how the drop soffit would work in one application. Here we've got the air handler sort of in the middle of the floor plan. That is that little x inside the large blue plenum box. We've got the secondary bedroom wing on the bottom of the plan. There with a 9 foot ceiling in this house we were able to drop the ceiling in the hall, bedroom hallway, about a foot and that just gives us a place to run our takeoffs to those various spaces. On the upper part of the slide we've got the master bedroom. We put a dropped soffit cornice ring around the space and again that gives us a place to put the duct. So the architectural integration of this application really is one of the drivers. The other thing we've got to look at when we are doing this sort of application is the length of air throw. If I throw the air more than say 12 or 14 feet I really don't get good mixing so we have to be careful about how far the register is located from the outside perimeter of the house when we are working this sort of a plan out. Next slide please.
The actual application of that soffit is critical. Basically it requires a separate air plane and air sealing application. The slide on the left shows it done with drywall that is tape and mudded prior to the primary drywall getting installed. The slide on the right is sort of a diagrammatic section of another approach where we just use a laminated fiber product, like a thermal ply, applied directly to the framing air seal, as necessary, and then build the soffit around it later on. This way we don't have the drywall coming in for a second stopover and the framer actually does the installation of the thermal ply. If we do it that way we can sort of skip a step. We don't have out of sequence drywall applications and things of that nature. Air sealing is critical because again this is separating the duct space from the attic. Next slide please.
Again, advantages and disadvantages - it is very low cost in simple plans. It is pretty easy to explain and implement. If you explain it to a contractor, once they get it, there are virtually no code restrictions.
Limitations - heavily plan dependent. It doesn't work in all plans. Advanced planning is absolutely critical. I mentioned that throw distance is also critical. There is no provision for an air handler in the soffit unless you get it into one of the skinny hotel type situations. Next slide please.
The next option we will look at is a modified truss. This is basically the inverse of the modified soffit we just looked at. Instead of creating space below the ceiling plane, we are creating space above the ceiling plane. To do that we are sort of reconfiguring a standard roof truss to create that volume. You can see the image on the bottom shows what that space looks like actually installed in the house and then a diagram above shows what it looks like in section with the truss plenum space and the insulation showing, sort of the relationship of the two. Next slide please.
When we install this truss we basically have the same steps we do when we have the drop soffit. So basically the truss gets installed. We've got our extra volume to find where the ducts are going to go and then we put in our air sealing materials and steps. Again, we found that using a laminated fiber board product like thermal ply really works the best. It is really cheap. Framers know how to use it. It's basically dimensionless so you can lap drywall over it so it doesn't get in the way. It is pretty simple, easy, and cheap. Simple, easy, and cheap is usually pretty good. Next slide.
This plan image shows in that house we were just looking at where the plenum space aligns over the second floor plan. What we try to do here is put the plenum space and the truss space just large enough so that it overlaps each of the rooms we are trying to air to. Then by using ceiling supply registers at the inside perimeter of those spaces we can throw air toward the outside perimeter and get good air mixing. It works pretty well. We just have to make sure that that space laps over the spaces we are trying to condition far enough that we can get a ceiling register into the ceiling of the space that wants the conditioned air. Next slide.
If we need to go with a slightly larger plenum space depending on floor plan and other considerations you can get a wider space by using a modified scissor truss, which we've done in multiple applications. In this case the wider plenum space can be achieved and span larger portions of the floor plan. There is less bend in the air seal product so it's a little bit simpler to put on, a little bit plainer, easier to seal and tape and mask it. In both cases, both with the scissor truss and with the square space that we saw prior the ceiling members, the horizontal ceiling members, are actually not structural or added later on after the space is there and sometimes after the ductwork is installed as well. Next slide.
Here we see a sample floor plan with the scissor truss plenum space. In this configuration the wider duct space is created giving us a little bit more flexibility in terms of where we want to put our supply registers. The other thing that we could do with this plenum space, with the scissor truss or the modified truss that we saw prior, is that the volume created actually can be used as a return air path. If we do use it as a return air path, however, you can't put plumbing, you can't put romex wiring, it basically just becomes a plenum space. So it does have that option but you have to be careful when you do it that way. Next slide please.
Advantages - low cost and simple plans, not as plan dependent as a drop soffit because you are basically creating space in the middle of the building, and you can put ducts pretty much wherever you want in that middle space, minimal code restrictions. It does work best in linear plans. That is true for both the scissor truss and the conventional truss. There is an additional air barrier and unique air sealing approach and it does require custom trusses but otherwise it can be pretty simple and can be pretty effective. Next slide please.
Probably the simplest approach we have used is using open web flooring trusses for the integration of the ducts. In concept it is actually very simple. The space exists if you are using trusses that are deep enough, so 12 inches or what have you, and you simply run your ducts within that space. The vertical space is between the bottom cords and the floor deck. Then you general floor registers up or ceiling registers down to get the conditioned air into the space. Next slide.
The real trick to using this approach is the design integration. I've got a structural floor plan that would be worked out by a truss manufacturer for example, and then the HVAC needs to be carefully integrated into that to get the supplies where I want them in the rooms and to work with the truss configurations. The truss can't be chopped up like a dimensional would be with headers anywhere you want. They are specifically designed so the plan integration is important. You also want to make sure the plumber doesn't show up first and try to run his waste line because invariably that is where the HVAC guy wants to run his duct. So coordination is absolutely key. Next slide
As I mentioned, typically ceiling registers blowing down and floor registers going up. You can use high wall registers. Typically we have done them on second floors and heating, excuse me, cooling dominated zones for better cooling performance but floor registers can work there as well. Next slide please.
Very low cost and simple plans. Basically you are using an existing space and not using any existing additional materials. Flexible locations for registers. Minimal code restrictions but really it is the coordination that takes the effort. Really advanced planning is as important with this approach as it is with the drop soffit approach. Next slide
Another approach would be to use a conditioned basement space or an unvented crawl space. These can work quite well. Basically what we are doing is replacing the insulation at the wall of the crawl space or the basement rather than as a floor. It actually improves the building envelope performance as well, so it's a good idea to do it. In this case we do have a place to put the air handling equipment as well. Next slide please.
Again advantages and limitations - it does improve the enclosure performance because we are putting the insulation where it makes the most sense, in this case the walls and not the floor of the crawl space. We do have flexible register locations. We can put them pretty much anywhere we want, same thing with ducts as well. There are very little limitations in terms of placement and all the equipment is testable for service, which can be an advantage. There are code thermal insulation requirements and there are code mechanical ventilation requirements. The mechanical ventilation requirements are important to pay attention to and they can be a little bit strange. We are doing some research on those right now but as it stands right now basically it is 1 CFM of air per 50 square feet, either continuous exhaust or from the conditioned air system within the house. Next slide
I have about 2 minutes left so I will quickly go over the last option here and I know Dave is going to give some more detail on the buried duct option. Basically, what the buried duct option is take the existing duct system and lay it on the attic floor, air seal it to the best you can and then replace the insulation over it. Next slide
We started doing research on buried ducts in Phoenix probably around 1998 or so when we had reluctant builders who would not do our drop soffits we wanted. This is just a real quick proof of concept study. You can see the difference between the slide on the right and the slide on the left. The charts underneath each are showing the conductive heat gains to the duct systems at each location. You can see the chart on the right, essentially there were no conducted heat gains. The chart on the left the heat gains are mimicking the attic temperatures so there is a clear correlation between the temperature in the attic and the conducted heat gains in the ductwork. Next slide
So using that as concept we started working in California around 2000 and came up with a methodology and metric for buried ducts that you can see here, which indicates that deeply buried ducts, fully buried ducts, and partially buried ducts which indicates specific levels of buried insulation. This work ended up being for the California Energy Commission and by 2005 was included as an alternate compliance method for California's code title 24. Next slide please.
Using that information as an approach we wanted to apply this to hot and humid climates and we started that research in about 2003 or so. We started working in Atlanta and Florida and came up with the methodology for condensation control on these buried duct systems because if I've got cold air going through a duct in a hot humid attic, sort of like that glass of ice tea in the summer, we are going to get condensation on it. Our research came up with a method of inhibiting that, preventing that from happening. Next slide please.
Then, eventually working up a table that shows what the equivalent R values for different burial types, different thicknesses in insulation and how to avoid that condensation. This approach now has been included prescriptively in the 2009 IRC. You can look right in the code about how to do it. It's also a Department of Energy Zero Ready Energy Home alternative method for conditioned space ductwork.
Gail I'm going to ask you to skip ahead two slides to the advantages/disadvantages one. One more. I know Dave is going to give some of the details.
The advantages/disadvantages can be very low cost and simple plans, the buried ducts alone are basically zero cost, easy to execute, very little plan coordination. The ducts and registers can be anywhere in the attic plan. Compliant with code, as I mentioned. It does require HVAC/design coordination and there is no provision for AHU. The last thing I will mention before I hand it off is that this process, the buried and/or encapsulated ducts, was awarded a 2013 Building America Top Innovation Award, which we are very proud of. With that I will hand it off to Dave.
Dave: Well thank you Bill. It is great to be here to share some of our research on compact buried ducts. Next slide please.
Bill presented a lot of good options for duct installations in conditioned space. I'm going to focus on a compact buried duct approach and present an overview of the features, benefits and issues of compact ducts and buried ducts. I had some results from two of our projects in a mixed-humid climate and details from our current project in a hot-humid climate. Next slide please
I am going to dive right in with a little background. The benefits of installing heating-cooling-air distribution ducts in conditioned space are well known. The biggest benefit is reduced heating and cooling energy losses due to conduction and duct conduction leakage, compared to ducts in vented attics, crawl spaces, and garage. Locating the ducts and equipment in conditioned spaces is important to help meet whole house energy saving goals but can be a challenge for some house designs and for some builders, particularly slab on grade houses and two story houses with open floor plans or complicated framing. The photo in the upper right shows an open floor plan for a two story house where running ducts up from the basement up to the second floor was very challenging. Ducts can be installed within bulkheads, drop ceilings, or floors but that can complicated at times and require custom designs. The additional framing and air sealing may not be considered cost effective by builders and bulkheads may not be acceptable to buyers in some cases.
Building an unvented sealed attic is an option that can increase whole house energy use because there is more volume to condition and they may have less insulation and those can be expensive to build.
Conventional attic ducts, that is, attic ducts above the attic insulation, are a simple solution. Ducts can be installed anywhere but are not energy efficient, especially with ducts near the roof deck. Next slide please.
Buried ducts can be a practical and energy efficient alternative to ducts in conditioned space, also an alternative to unvented attics or conventional attic ducts. Buried ducts are ducts that are insulated, installed close to the ceiling, in a vented attic, and covered with attic insulation to reduce energy losses. Locating the ducts in the attic is simple and allows for a simple duct layout approach for different house designs. Energy savings will vary by house design and climate. The heating and cooling energy penalty for conventional attic ducts, compared to ducts in conditioned space, can be 15-25%. That is the conventional attics can be that much worse. There is a penalty for buried ducts that can be 5-10% or less, which is roughly the same as building an unvented attic depending on the insulation level. The primary concern with buried ducts is the potential for condensation at the outer jacket of the duct insulation during the cooling season in humid climates. Condensation can occur when the temperature of the duct surface reaches the dew point. As you know, I think the temperature at which water vapor within the air condenses into a liquid on a surface. That's why buried ducts are more common in dry climates. Also, duct leakage, in addition to energy loss, can aggravate the condensation potential if cold air is allowed to blow onto a nearby surface. Current best practices for buried ducts those rely on encapsulated ducts, closed-cell spray foam, to control condensation and duct leakage in humid climates. The DOE Building America Zero Energy Ready Home program allows an exception in humid climates reduction of vented attic that requires R-8 duct insulation encapsulated with at least 1.5 inches of closed-cell spray foam and covered with at least 2 inches of blown in insulation. They tested to a total duct leakage rate of 3 CFM at 25 Pascal’s per 100 square feet of floor area. Next slide please
Compact ducts - a compact duct is shorter duct runs, fewer fittings, and less duct area compared to conventional layouts. This reduces duct pressure losses, improves air flow performance, and reduces fan power. Less duct area also reduces conduction and leakage losses and of course those losses are to the outdoors where ducts are not in conditioned space. Less duct area can translate into lower installed costs as well.
For energy - incorporating a compact duct layout with a buried duct layout can reduce heating and cooling energy losses by up to 50%. It's simply based on less duct in the attic.
Design considerations for compact ducts - a compact central return, that is where the return grill is close to the air handler, is the best opportunity to reduce duct area. Ideally the entire return trunk is in conditioned space, serves 1 or 2 return grills per level, and eliminates branch runs as shown in the photo. The lower right corner photo shows a central return within a duct chase that serves the second floor of a house. For the short returns the designer needs to pay attention to air velocity and turbulence to control noise. For example, there should be at least two elbows and 8-10 feet of duct between the grill and air handler to reduce line of sight noise. You may need to consider long radius elbows or turning vanes and even duct liner. The Air Conditioner Contractors of America Manual D provides good guidance for air velocity limits and noise control. Transfer grilles can be installed instead of a ducted return to provide a return air path to bedrooms or other rooms with doors. Those should be baffled to minimize sound and light transfer and an example of those is shown in the photo on the lower left corner. Supply registers can be located to minimize branch ducts, for example, near ceilings in interior walls and away from the eaves, as shown in the photo in the upper right. It is important to select the right register type and location to provide sufficient air mixing. That is to minimize stratifications in stagnant zones of a room and to avoid drafts and excessive noise. Next slide please
Our first buried duct project was this single story slab on grade house in central New Jersey near the coast in IECC climate zone 4a. This was a building America 40% test house and the project goals included testing and monitoring of a buried duct system. It consisted of a compact central return in conditioned space, all below the ceiling plane along with the furnace and bedroom transfer grills. The ducts in the attic were insulated to R8. That was a metal trunk with R-8 insulation, flexible ducts, branches and metal insulated boots, and with limited encapsulation. The trunk was encapsulated and the boots but not the branches, not the R-8 flexible branches. You'll notice a unique design. On the upper right photo there is a duct chase within the trusses. We initially thought we might try to seal that space to make it a conditioned area but decided that it would be simpler to seal the duct instead and not try to seal that chase. We did install temperature and relative humidity sensors in various duct insulation surfaces as well.
The duct leakage test after rough-in for this project measured a tight 1 CFM 25 per 100 square feet. That was for the attic supply ducts only, before spray foam. The duct blaster was attached at the open supply plenum before the furnace was installed. The final duct leakage was a little high but 0 to the outdoors. The leakage was attributed mostly to a leaky furnace cabinet.
Monitoring - no condensation was measured or observed over two summers, notably at the R-8 supply branches without the spray foam. For this house the compact design eliminated 70% of the return duct compared to builder’s standard and nearly 30% for supply duct and the octopus layout near the roof deck. Before I leave this slide I think it is worth noting that the compact central return alone reduced a total equivalent feet of duct from 390 to 263 compared to standard builder design. That is a pretty big deal that HVAC designers will appreciate to improve the friction design rate from 0.05 to 0.08. Next slide
The second buried duct project was a two story house with a basement in Maryland. That is climate zone 4a. It has two independent systems, both furnaces in the basement, conventional first floor duct layout and a conventional chase that houses the central return feeding the upstairs, and a supply trunk to the attic. The return is below the ceiling in the conditioned space. The project goals included testing this revised design for thermal performance and monitoring it for condensation. This design up in the attic had a compact supply duct layout as well. The house is three rooms deep so the supplies are located as closely to the interior walls as they could be, has R-16 branch, which is a double R-8 branch duct - one inside the other, and standard metal boots with R-8 duct wrap. In this case the metal trunk was encapsulated but not the branches or boots. The duct leakage test at rough-in was fair but was the same before and after the spray foam, indicating that the spray foam in this case did not help with the leakage control. The final duct leakage was higher than expected. I suspect a combination of leakage around the boots and sealing using tape and not masking and a leaky gas furnace cabinet, and interior ducts that appear to communicate with the outdoors through the top of the chase or perhaps through the rim. Despite that no condensation was measured or observed over two summers, notably at the supply and register boots. Next slide please
This graph is a section of a two week period where the conditions were closest to condensation at that Maryland test house. It plots the difference between the temperature and dew point for 15 sensor locations. The closest was only two degrees away from condensation. That was at the top of the encapsulated trunk but the average temperatures were much greater during the season and in fact even during this worst week or two the R-8 boots averaged 8-12 degrees away from condensation. Next slide
So the results of these mixed-humid projects gave us the confidence to proceed with our current project. We'll be installing a compact buried duct system in a test house in Beaufort County South Carolina, in the southeast corner of the state. That's climate zone 3a, but just over the river from zone 2a where the average summer dew point is about 72 degrees. The purpose of this project is to build upon previous Building American research to develop a buried duct system that performs effectively as ducts in conditioned space in humid climates. That is one that is durable, energy efficient, and practical to install. One goal is to develop a duct design that does not rely on encapsulation because we work with some builders that prefer to avoid spray foam and instead rely on more common duct materials and products. Other goals are to incorporate a compact duct layout and identify needed inputs for accurate manual J load calculations in energy model programs. Next slide please.
One of our objectives for this project is to determine the lowest level of duct insulation that will prevent condensation. Modeling was performed to try to predict at what point condensation would occur but to better understand moisture conditions within the attic we installed sensors in an existing house, a builder model home, in a nearby community. That included a sensor tree shown in the photo, in the attic, with sensors at 1 inch, 3.5 inches, 6 inches and 8.5 inches above the ceiling. Those were installed and then covered back up with the attic insulation. We also installed sensors on two ducts at 3.5 inches above the ceiling as well. Those are conventional ducts so we tried to capture just a portion of the duct that was buried in insulation. Next slide please.
This is actual data from those sensors. Again, we are collecting temperature, relative humidity, and dew point data. This graph plots the dew points during a typical late August couple of days. You can see the outdoor dew point is pretty steady at 65-70 degrees. The indoor dew point is steady at or below 55 and, as expected for 75 degree indoor air at 50% relative humidity. Look at the attic dew points. Those vary from below 50 degrees at night to up to the mid-80s during the day. The insulation tree sensors in the middle, below the attic curves, show the dew point gradient that rises and falls with the attic dew point but at a lower amplitude. The take away here is dew points in the attic cycle greatly during the day. Next slide.
We compared the data sets to try to determine if the modeling accurately represents the conditions around the duct. This graph compares the model to the points of the actual dew points at the sensor tree locations. You can see the points were lower during the day, except at the lower sensor. The model temperatures did not match exactly but those were close. Comparing model temperatures and dew points, R-8 round duct appeared to be very close but not quite at condensation. By the way, round duct was used for this modeling. The effective R value of round duct is less than flat duct insulations due to the duct's geometry. So what works for round duct should work at least as well or better for rectangular ducts. Based on the modeling comparison using R-8 in South Carolina it appeared borderline or even questionable. Next slide please.
Let's take a closer look at the measure data from the model home. This graph shows the dew points of four sensors on one tree. You can see there as they progress in the day and the duct surface temperature in blue. The duct temperature dips below the 3.5 inch sensor. I have an arrow pointing to that sensor tree dew point. One would reasonably expect condensation at that point but the duct dew point, indicated in yellow on that graph, also at 3.5 inches, just like the sensor, is well below the sensor dew points and duct temperature. Sure enough I had put my hand on that duct about a week later and it was dry and no sign of moisture so we know that duct surface temperatures change with cooling but the dew point also appears to change due to HVAC operation or conditions in the attic or both. Next slide.
This is a summary of the compact buried duct design of the South Carolina test house that we will be installing, or planned for, this December. So it's a central return, central compact return with jump ducts and transfer grilles. Most of the supply registers are near the interior walls. The buried duct insulation will have an R-8.7 duct-board trunk, R-8 flexible branches and metal boots with R-8 insulation. We're going to have one R-12 branch run and supplemental at flex connections. What I mean by that is where the, what I consider the weak link where the flexible duct is compressed with take-offs or boots, we're going to monitor that, the effects of that compression but also have some supplemental insulation at those points and see what that tells us.
The total duct leakage rate for this project is 3 CFM25 for 100 square feet. This rate meets the current requirements for DOE Zero Energy Ready Home Program and exceeds Energy Star 2012 IECC requirements. The target duct leakage rate for the attic ducts only is 1. Isolating the attic ducts is important for this project because leakage here of course represents energy loss outside the building enclosure and may cause or contribute to condensation. Testing at this point, at this phase, will also allow for resealing if necessary.
The attic insulation we are going to mount R-30. It is fiberglass loose insulation over the buried ducts. We are going to install markers with depth gauges for the attic insulation for quality assurance. Next slide please.
So to summarize, some builders successfully install ducts and equipment in conditioned space for all of their house designs and if that's the case I would say stick with that. For those facing any of the design challenges we discussed earlier for the entire house or just portions of the house the compact buried duct approach could be a practical solution. It could also be a good approach for a ducted mini split in a retrofit application. Compact buried ducts can be insulated to prevent condensation in humid climates. Encapsulation is a proven approach. Standard products are also available for above R-8 insulation, for example R-13 duct wrap. I don't recommend the R-8 ducts just yet in humid climates at least until we evaluate the test house results, which we will be monitoring that this coming summer. Compact buried ducts can be sealed to within acceptable standards. Again, encapsulation is a proven method. It can be limited, for example, at boots and take offs. Conventional sealing methods can work also, either way we recommend testing at the rough-in stage to check the duct tightness. Also, compact buried ducts can provide energy savings and comfort and compact duct layouts alone can benefit any duct system and our next steps are to evaluate the compact duct layouts and monitor potential condensation at the test house during this upcoming cooling season. So that's the end of my presentation.
Gail: Thank you Dave and our next presenters are Jordan and Francis.
Jordan: Thanks. This is Jordan Dentz with the Aries Building America team. The first presentation Bill presented a good case for ducts, leakage being a bad thing, and why it's a bad thing and this presentation will cover retrofits and how to combat problems in retrofits. We have taken two technologies, two approaches, to sealing ducts in 40 buildings and compared the two. We will go over the results of that and talk about how those strategies were implemented. So, we have already talked about duct sealing. It is important to reduce duct leakage, important from an energy standpoint, as well as other reasons that Bill mentioned earlier.
Duct sealing can be difficult, costly, and disruptive to deal with a retrofit situation. So we compared two techniques - manually applied sealants, which is basically masking and tape and injected Aeroseal aerosol. We will get into details about what those are in minutes.
The purpose of this project was to answer a couple research questions and broadly we wanted to understand what the cost and effectiveness was of these two approaches, relatively, as well as what logistical and technical issues might affect large scale home implementation of these approaches in the type of housing that we implemented this in, which is low-rise multi-unit residential public housing complexes.
This is what we are going to go over. We have some modeling results, building analysis, to understand what the implications were in terms of cost effectiveness and payback as well.
So the field study - we had two housing developments in North Carolina. One with two story buildings and with central air conditioning and natural gas heating and we divided them up into two pools. Half of them would get the hand sealed approach and half would get this injected Aeroseal aerosol sealant approach.
The ducts were a variety of types. They were both flex ducts and metal ducts, both attic ducts and floor ducts, and both in conditioned and out of conditioned space. The air handlers were in conditioned space, so a variety of configurations.
So for the 20 units that we did the hand sealant on, the items that were sealed were the boots, obviously, to the floor or to the ceiling such as the example here shown with foil tape.
The return plenums were sealed inside with masking and the air handlers were sealed from the outside with masking. Finally, where accessible and that's a key point - where accessible, the ridges trunk ducts and the trunk flex connections were sealed inside the attic. That's difficult work and often where we'll see later the Aeroseal has better benefits.
I am going to turn this over to Francis to discuss the Aeroseal system. Francis, are you there?
Francis: Yep, I'm here. Thanks Jordan and what I'd like to do is start by giving a quick but maybe oversimplified description of the Aeroseal technology. Aeroseal was created from work done at Lawrence Berkley Laboratories in 1994 and they began doing business as a company in 1997. Some describe the system as similar to being fix-a-flat, which many people recognize as used to plug leaks inflate flat automobile tires however there is one big difference. The Aeroseal sealant particles are partially dried before they enter the duct system so they will not stick to the duct walls. That is pretty important to make this a practical solution for ducts. The sealant is a vinyl material that is suspended in a water solution. The Aeroseal process atomizes the sealant and then the nozzle directs the sealant particles into the ducts and they are pushed forward into the holes with the high pressure and deposited onto the holes. Next slide.
The set-up is straight forward but it is a little bit time consuming. The major task are first to seal the supply registers to contain the sealant inside the duct to prevent it from dispersing into the building. We typically use compressible foam plugs to do this. Another task is to seal either side of the coil and the other HVAC equipment to protect the equipment from any of the sealant material. Even though it doesn't tend to stick to the ducts we don't want it to find leaks in the cabinet that we don't want the sealant to adhere to that. Finally we attach all this equipment together using a plastic tunnel, which is a duct. It's used to give the sealant time enough to dry before entering the ducts. You will see a picture of that next. If we can get to the next page?
When the sealant rise I like to think of it as forming a skin around the beads of sealant. Another bad analogy perhaps is think about water balloons. You have a dry surface of the sealant and there are still some particles inside that are not quite dry. What you see in these two photos in the first one is a very back piece of equipment it's a fan that is like a supped up duct testing fan. That creates pressure in the system. That next little box in that first picture is an injection nozzle where the sealant is put into the air stream and this coupled with the heater will act to dry the particles as they run through the tunnel. Next slide.
This system is controlled with a computer. It monitors several things - flow rate, the air stream, the temperature of the air stream, and the humidity levels. It indicates adjustments that need to be made to the system so that when the sealant is delivered it is neither too dry nor too wet. Either of those will result in the sealant not working correctly. The computer also monitors the sealant progress on the time scale. We are set to shut the system off at a certain time. Next slide.
As Jordan said, we hand sealed several things that the Aeroseal system could not seal. The return ducts were just simply too small to use the Aeroseal system so you can see we are sealing the return plenum using hand sealant foam process. In many cases the Aeroseal can be used to seal return systems. We often use a wide connector in the duct systems to split the air stream towards the return and the supply and just doing both of those at once.
The other thing that was sealed was a plenum air handler and where the supply register is connected to the ceiling and the floor. Next slide.
I want to talk quickly about some results. The Aeroseal units improved air flow more than the units that were sealed totally by hand. The return and supply flow was both increased in the retrofit units. There was one unit that had a decreased flow afterwards, after we did the sealant. One explanation that was presumed happened was since this was a sealing that was done in an attic, most of our attics were fairly tight construction, that more damage was done trying to seal that particular duct system by either compressing or retrofitting the ducts than resulted by the hand seal.
If you look at the next slide I would like to just focus on the middle column. I think that is the most significant. Here we see improvement in the supply air flow using an Aeroseal system compared by the sealing by hand. We also see the pace near the bottom, the next to the last one in the middle column, where the hand sealing actually resulted in a worse condition after than it did before. Next slide.
Looking at the last column again shows the average hand sealing resulted in almost 60% reduction in duct leakage to the outside. Whereas the Aeroseal system showed a 91% reduction in duct leakage to the outside. This is 32% better than just using hand sealing alone. Next.
We also measured just the flows not just air leakage but the return and supply flows, which were increased. In this photo we are showing a measurement of the airflow in the fan powered flow meter. This is considered a lot more accurate than typical flow meters used commercially. Our return flow for example increased by 7%. Again, these were all hand sealed but these are how the measurements were made for supply and return. Next slide.
The Aeroseal system shows duct leakage in real time as the ducts are being sealed. This graph is a screen shot of the Aeroseal computer. It shows leakage on the y-axis, duct leakage on the y-axis, and the time is shown on the x-axis. I think I will bring this back to Jordan who will take you through some of the modeling, some of the economics, of the work we do.
Jordan: Thanks Francis. One additional point to make on this slide is that while the Aeroseal seals the ducts to a much tighter level than hand sealing about 70% of the leakage reduction in the Aeroseal units was due to the hand sealing - the returns, the registers, and the locations that Francis mentioned. So about 30% being the rest of the duct system that the Aeroseal addressed directly.
So just briefly, we modeled sample unit and plugged in the leakage results that we got from the testing and this was whole house source energy savings ranging from 3-7% for the various units. It was fairly higher for the Aeroseal units because it was based on a higher duct leakage level. In terms of cost, these were, as I mentioned in the beginning, relatively small homes - about 1,000 square feet each. They each had one duct system. For the hand sealant there was a difference in cost between these single story units because in those units the workers had to climb into the attics to address the leakage there and in the two story units there were no ducts in the attics. They were just between the floors so those were inaccessible. The sealing there was just strictly at the registers and air handler and at the return. With Aeroseal there was a flat rate because it is basically all in the set-up of $700 per unit, so slightly more expensive for the Aeroseal application.
This is a billing analysis looking at the cost effectiveness. Again if you look at the annual savings cost in the second to the last column that is the annualized energy expense. That accounts for all the expenses, the retrofit and energy expense annualized over a 30 year period. The annualized savings ranged, from our modeling, from about $30-$70, slightly higher again for the Aeroseal than the hand sealant but interestingly both are positive. So both are cost effective from a modeling standpoint.
We also looked at utility bills and we had a year’s worth of utility bills before and after for most of the units. These were with the same residents looking at the units before and afterward. We normalized the savings of these utility bills for weather conditions, we compared those units we sealed by hand versus those sealed by Aeroseal. Interestingly enough not a huge difference here, although the Aeroseal units were tighter, an average of 90% reduction in leakage versus 50% reduction in leakage. They all had about a 15% reduction in heating and cooling energy use. Again, in terms of payback the hand sealing was slightly cheaper with a shorter simple payback. A little over two years for the hand sealing and 4.7 years for the Aeroseal.
So to summarize some of the lessons that we've learned from this little experiment, there are a number of advantages to the Aeroseal system. It allows us to seal inaccessible ducts. It avoids some of the hassles of manual sealing, which involves removing duct insulation, cleaning ducts, applying masking, waiting for it to dry, reapplying insulation, kicking ducts loose, which Francis was eluding to what they suspected in one of the houses, and again avoid some of the quality control issues of hand sealing. It does have some challenges. These were small units, which were a challenge to set up the equipment in. It does require this long tunnel and clearance was an issue around the air handlers as well. We were also working in a humid environment and that created some difficulties in keeping those globules of sealant dry on the outside as Francis described and caused some clogging issues. We were able to do all the jobs but it did take a little longer because of some of these humidity issues.
If it were to be sort of ramped up on a production scale there are some improvements that could be made in terms of the Aeroseal process. A lot of time was spent on set up and clean up and most of the time, 70% of the time, equipment was not actually sealing ducts. It was being moved or set up. Using a Y connector to do multiple units at a time would be a time saver as well as well as using a scaled down system for small units would have been helpful.
Just some conclusions, both methods the Aeroseal and the hand manual sealing do stop leakage. The duct leakage was greater for the Aeroseal but in both cases the manual sealing areas registers the air handlers accounted for most of the leakage. The utility bills were similar as well as were the duct modeling results for both methods. Both showing that they were cost effective with relatively short paybacks. I think I mentioned these points already. There is an opportunity to streamline the Aeroseal system with these types of units.
Finally, there is a full technical report on this project as well as a case study available on the Building America website. That is all I have. Thank you very much. I will turn this back over to Gail.
Gail: Thank you Jordan and Francis. We have some time now for a few questions. We already have some great questions from the audience and you may submit additional questions through the questions pane on your screen. Please indicate to whom you are addressing your questions and the speakers will answer as many as time allows. The first, we have a handful of questions for Bill.
The first one is - with unvented attics are skylights still usable? If so, does the skylight shaft need to be insulated?
Bill: The answer to that is pretty short and it is yes and yes. Basically the shaft of the skylight would be considered an external wall so whatever the required R value for the external vertical wall of that house and that climate zone would be the same requirement for that shaft. That includes air barriers on both sides of the insulation.
Gail: Okay, thank you and we have a second question here. Could you expand on why airflow distance is so important for the soffit construction option?
Bill: Sure, this actually goes into a couple of concepts and one that Dave went into in a little more detail than I did and that's the issue of compact design where you are looking at compact duct design with drop soffits. It is integral to it. You want to have as few soffits as possible or practical that have ducts in them so it will be less expensive to build and easier to integrate into the floor plan. What that means is that if I've got a room that is 14 feet wide and I have say a high wall register in that space because there is a drop soffit behind it with ducts in it the air needs to flow at least 12 to 14 feet to the opposite wall in order to get complete mixing in the room. So it really becomes a comfort issue. Now the issue is too that as we get better building enclosures the requirement for the B2 output for the HVAC systems becomes a lot less, which means we have to move a lot less air to be comfortable, which means that each register has less CFM of air going to it. So the register that at one point with traditional systems might have 150 CFM with a high performance enclosure and a compact duct design I might only have 100 CFM or 80 CFM and now that same EDCFM needs to go a greater distance than it normally would because it's got less pressure behind it. The distance becomes important for comfort reasons and the issues and problems become exacerbated as it gets better envelopes and compact systems.
Gail: Okay, thank you. Another question is - how does crawl space ventilation requirements impact the thermal losses from ductwork in this location? You referenced 2 CFMs per 50 square feet as a requirement. Isn't this larger than the ventilation requirement for occupied spaces?
Bill: The requirement specifically is 1 CFM per 50 square feet. So if I have 1,000 square foot crawl space, let's say, that's 20 CFM of ventilation air. It is a code requirement. I'm not saying it's the right number or whatever but it's the code requirement and you could get to that 20 CFM in this case either by continually exhausting 20 CFM out of the crawl space through dedicated fan or you can have your HVAC system if it's a heated and cooling ducted system designed so that every time the system turns on 20 CFM of conditioned air is being directed to the crawl space. Not quite sure I get the reference to the rest of the question in terms of the requirement for the rest of the house but I think that may be referencing the total ASHRAE 62.2 ventilation levels. I'm not quite sure.
Gail: Then one final question for you Bill - what is the approximate incremental cost of employing buried ducts and encapsulated and buried ducts in an attic.
Bill: Great question and as I kind of eluded to in my presentation with just the dry buried duct system that we can do in dry climates without worrying about condensation. Really that is kind of a zero cost option. It just needs to be planned ahead and if we are smart about it we can actually save some money because as we drop our HVAC capacity needs because we have a more efficient system maybe we can save some money on going from say a 2 ton system to about a 2.5 system, which costs less. If I use compact duct arrangement then there is actually less linear dimension of ducts so that cost less. So you can actually get a cost savings there. On the encapsulated we found a couple of things. The cost of apply closed cell foam to the surface of the duct is about a dollar a board foot, more or less. So it is about a buck for a 1x1x1 inch. With a similar smaller compact duct system probably that would max out at probably 600 board feet, something like that, so nominally that would be about a $600 application. What we need to do is make sure that well...let me say it this way, the guy with the foam truck who is spraying the spray foam typically won't show up for less than $1,000. So if I'm doing a $600 spray foam application on my ducts probably that is also a good time to spray my band joists and my mud seals and that sort of thing with the same product. In that case I am still looking at what we would call nominally a $600 incremental cost.
Gail: Okay, thanks Bill. And we have a couple of questions here for Dave. The first one is, can you expand on the design challenges for selecting supply registers for compact duct systems?
Dave: Yeah, sure thing. Well, I think that Bill addressed that pretty well. I think that the only thing that I would add to that is I think it gets even more critical to select the throw and the heating mode. In the cooling mode if you just reach the exterior wall you may be okay even though we design for about 20% more throw than if it's just from a ceiling diffuser or a high wall diffuser but in the heating mode if you just get to the outside wall and don't travel down that exterior wall it leads to the design we used in our South Carolina test house. You'll likely get stratification. You'll have considerably warmer air towards the ceiling in the winter time and tend to have a colder floor, particularly for a slab on grade house. That's complicated if you have a gas furnace for example in the house. You're usually running that at a lower air flow, total air flow. So, that would compound the problem in the heating mode. In our design house we designed a throw, which ought to be a selected based on manufactured product data within acceptable moist limits, sometimes referred to as NCEF noise criteria or other manufacturers simply state a top velocity, which is sometimes 700 feet per minute. We're designing for throw plus about 4 feet or more down the exterior wall in that case with the idea that we will get good air mixing and avoid that stratification. Beyond that in certain rooms the designer may simply need to select a different location for that supply.
Gail: Okay, and then one other question for Dave. What criteria are you using to determine if non-SPF encapsulated buried ducts in humid climates work? Is minor chronic condensation of any extent acceptable?
Dave: That's a really good question. Our goal is to determine the amount of insulation that will prevent condensation but looking at the dynamic moisture conditions inside the attic I think it's a good question that may be answered and that is if there is some moisture collecting on a surface at what point does that become a problem? So again our goal for the project is to prevent condensation but looking at those dew point curves in the attic and within the insulation, trying to determine what is going on there, it likely could be that if there is some condensation that it does not cause a problem. Again our design is to prevent that. Again, we've done some preliminary modeling and so the next step for us is to actually test some different configurations in that humid climate.
Gail: Okay, and then we have a few questions for Jordan and Francis. The first one is - is there a recommended materials and process specification for manual duct sealing?
Jordan: Yeah, this is Jordan. So in the report that I referenced we do have a set of specifications just for the manual sealing as well as specific material recommendations.
Gail: Okay, and the next question is - contact duct versus small diameter high velocity duct. Are they one and the same or what are the parameters that differentiate each of them?
Francis: I think that's for Dave.
Dave: Oh, okay. I can try that. They do have some similarities. The high velocity systems generally do have a single compact return close to the air handler. They do have certainly less area because they are smaller and they are smaller because they're much higher velocity than a standard conventional air system. So they are not the same thing. I think even though there are some similarities the duct layout would vary because on the high velocity system that's driven by, you generally need so many...a supply, a diffuser for every x number of square feet. You're generally getting about the same volume of air, flow of air, from each of those two inch inside diameter ducts. So, they come in high velocity so you either need to put them in a corner of a room where they are directed downward from the ceiling. Sometimes they can be put up high in a room, a high wall blowing towards an outside wall, but you have to be I think particularly careful with noise and throw for those. They can cause unwanted drafts if they are not located just how the manufacturer recommends those get installed.
Gail: Okay, thank you and back to Jordan and Francis. Did you perform pre and post total external static testing on systems having ducts sealed?
Jordan: Francis, can you address that?
Francis: We did not do any static tests of the system before and after. No.
Gail: Okay, another question - what happens to the Aeroseal that doesn't contribute to the seal? Does it enter the rooms and drop out on horizontal surfaces as a residue? How do you get rid of the excess Aeroseal?
Francis: That really is a great question and one of our concerns when we first started looking at the Aeroseal system. So, if you imagine you have ducts between floors, and their leaking, some of the sealant is going to escape into the living area. In bad cases when you have a lot of duct leakage it will look a little bit like a fog. There's a couple of things to consider with that. One, we have dried particles so when the particles settle out they're dry. They aren't sticky so you may find it looks like sheetrock dust when we've done a sheetrock project. On some of your furniture there is some residue that you can wipe off. The second thing is that if we anticipate or notice that that is happening generally we bring a hepa filter fan. So we circulate the air through hepa filters. We have a couple of those that we put in the space to gather up the particles that have escaped or system and are floating around in the house but you will see that and that will be a concern for some homeowners.
Gail: Then a related question would be - are there any issues with indoor air quality with Aeroseal and what type of chemical products are being used in Aeroseal?
Francis: As far as I know it's a vinyl and it's considered to be pretty inert. I know LDL did some testing on it and unfortunately I'm not real familiar with that but they're not concerned with either the air quality of it being inside ducts. They've also done longevity studies of the Aeroseal material itself being inside ducts. Like I said, I don't have the details with me but they have pretty much given both of those the thumbs up in terms of being concerned about air quality and longevity of the process.
Gail: Okay and another question - do you deduce that the similar savings found between the two methods is that both sealed high static pressure leaks thoroughly or do you think there is another factor?
Jordan: I suppose that's a possibility. It may just be that the sample size was not sufficient to capture the true effect and there might have just been natural variation. Francis, do you have any thoughts on that?
Francis: You know I think that the hand sealing that we did in the attic was above and beyond what typically you would get, especially in affordable housing. The fact that these homes had these type of ducts for a long time for instance. Nobody wants to get in the attic and do that work. It's difficult work to do right. We paid pretty much attention to making sure that they did address the hand sealing in the attic correctly and addressed all the registered, which I don't think is typical. In my opinion, probably the hand sealing got a little better performance because of the attention that we paid to that than we would normally. I think effectively we sealed most of the same holes. I think that's partly the answer to the question but I don't think that's typical. I don't know if that really answers your question or not but that's kind of my assessment from what we saw at the homes.
Gail: Okay, well thank you everyone and panelists before we take our quick survey do you have any additional or closing remarks you'd like to make? No?
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