Low-e Storms: The Next “Big Thing” in Window Retrofits
Official Webinar Transcript (September 9, 2014)
Pam Cole: Welcome, everyone. Again, this is Low-E Storm Window for Retrofits webinar. I’m Pam Cole and I am a research engineer with the Pacific Northwest National Laboratory (PNNL). I’m just going to give an intro to Building America. I’ve been working on Building Energy Codes program at PNNL for the last 14 years, and I’m currently involved in Building America’s efforts to resolve code and standard barriers with innovations. I also help with the technical support for Building Energy Codes across the US, answering questions on energy code compliance, and how to show compliance for residential and commercial buildings.
So an overview of Building America. There are actually 10 supported energy research teams, and four national labs. Each of these teams and lab partners, with dozens of industry professionals, develop and improve systems for residential homes. That is a big part of what Building America does. The best and the brightest in residential building energy industry can be found here. And we are lucky that we have the best and most knowledgble people and teams on this program with us today.
So what does Building America develop? We’re going to talk about low-e storms windows today, but Building America uses applied research to deliver building science solutions, using a four step-type framework. These innovation solutions are tested in homes and develop proven case studies of successes. They market those, and they can pinpoint certain solutions and best practices. Building America provides tools to residential building industry needs, and they ensure the innovations are applied correctly. They are always keeping an eye on energy performance, durability, quality, and actual affordability. And the final step of Building America processes is infrastructure development. . . this is the conduit to getting these innovations to the marketplace.
And so, Building America, what do we achieve? In nearly 20 years of Building America research, we have spearheaded combining ultra-high efficiency and high performance for both new and existing homes. We consistently are achieving these challenging tasks. And they are growing, and they are growing every year.
With that, we look at where we’ve been and where we are going. For example, in 1995 a home used three times more energy per square foot compared to a home today. And the indoor air quality, and the comfort and the durability problems – they were common back in 1995. But today, a home built to DOE’s Zero Energy Ready Homes specifications uses less than half of that energy and are more comfortable, healthy and durable. By 2030, what do we hold for the future? Building America will demonstrate that new and existing homes can produce more energy than they use. That is the goal.
So with window retrofit opportunities, it’s not a mystery to anyone on the line that windows are a major source of heating losses and gains in the building, and can significantly drive up residential energy bills. Despite this fact, probably 40% homes still have single pane windows. And in homes, they are inherently inefficient and are poor insulators. There are studies that have shown this. The presence of single pane windows in homes, you know, they have persisted over the last two decades. And [the number of single-pane windows persists] despite the fact that thirty million windows are replaced each year with higher performing insulated windows. In addition, almost half of all the double-pane windows are not high performing low-e windows, but are made of clear glass.
So there is a tremdous opportunity for technologies that improve existing window performance, such as low-e storm windows, to provide energy savings to a large segment of residences which, for the most part, which mostly accommodate moderate and low income households.
So cost-effective window retrofits. That’s always the big question, are they cost effective? DOE has supported technology development, market assessment, and early deployment activities of Low-E storm windows, which includes the recent climate-based modeling effort to determine where and what types of low-e storm windows are cost effective throughout the U.S. The results are impressive, demonstrating significant savings in a wide range of climate conditions, which can be achieved cost-effectively, and in most regions in the US. Tom Culp, he’s our first presenter, will talk more in detail about these climate modeling results.
So, a new look at Low-e storm windows, inside and out. Considering the amount of energy lost through windows of existing homes each year, this is definitely a retrofit, remodeling area. There is a need for a solution. And even though there are a lot of good window replacement technologies available to homeowners, window replacement is simply not an option for many homeowners. There are many reasons for this, including cost, affordability and for aesthetic reasons for those owners who want to keep the historical integrity of their home.
Storm windows have evolved through the years. A lot of people have certain ideas about storm windows, especially the look of them. Modern storm windows are not your grandmother’s windows, which had to be installed or removed every season, had to be cleaned almost monthly – modern storm windows are designed to blend in with the existing architecture, and to be permanently mounted, and are available as fixed and operable.
So let me get into the actual presentation introducing Tom Culp, who has been working with programs regarding low-e storm windows for 8 years. He has headed a DOE-funded research project from 2009 to 2013, to demonstrate residential and commercial low-e storm windows can cost-effectively lower energy cost.
Tom Culp: Thanks, Pam. The technology of low-e storm windows has been under development for about 15 years now, through the Emerging Technologies Program at DOE. DOE supported the national labs to do testing. It graduated to the Building America Program to work on field studies. Now, multiple groups are working on building transformation activities.
I’ll start off talking about the initial concept for Low-e storm windows, and some of the testing that has gone into its development.
In the late 1990’s, Lawrence Berkley National Laboratory (LBNL) suggested this concept, that a durable low-e glass coating could be incorporated into storm window designs for a cost effective, insulating and sealing measure for existing windows. You shouldn’t think about this as a separate window. Think of it as a way, or a measure, or a tool, to improve your existing window.
Of course, if your existing window is degraded and needs replacement, you should do it with the best window possible. But practically speaking, this is not an option for many home and building owners, because of cost or historic preservation concerns, or the existing window is fine, and doesn’t need replacement, though you still want to upgrade its performance. In that case, low-e storm windows and panels are an ideal way to improve your windows at a much lower cost. It does this in three different ways. First, it provides air sealing of the primary window. I’ll talk about some of the case studies that show a 10% reduction of air leakage of the entire home. Next, a low-e storm window creates a “dead air space” which reduces conduction and convective heat losses across the prime window. And third, the low-e glass reflects the radiant heat back into the home.
This concept is best seen through infrared field images. This was taken outside of a home in winter. As you can see, light colors shows heat loss, while the darker areas show where it’s more insulated. In the center there is the original window, while left and right are retrofitted low-e storm windows. As you can see, the low-e reduces heat loss significantly.
To test the concepts –going through a little history – first, in 2000 to 2002, LBNL used their Mobile Window Test facility (MoWitt) to do side-by-side detailed testing of different window options. There is a lot more in this study from Joe Klems that you can look up, but basically, they demonstrated that a low-e storm window plus a single pane primary window performed basically the same as a new double-paned low-e replacement window. They discovered that a low-e storm window plus a primary window performed the same as new double-pane low-e replacement window.
Additionally, a little later on, LBNL did some work with Building Green, Peter Yost’s group up in Vermont, doing some infrared imaging. This slide shows on a Vermont winter night, an image taken from the interior – so in this case, the dark colors show heat loss and the light colors show more insulating performance.
So the lab testing looked quite positive showing that we can gain significant improvement in the performance of both exterior and interior panels. DOE wanted to take this to the next level with field studies. In 2003-2006, a field study took place in Chicago, co-funded by the DOE and HUD, and the primary researcher was NAHB research center (I think they are called Home Innovations Research Lab now), where they did monitoring of six weatherization homes that had single glazing, and were retrofitted with low-e, exterior storm windows. The study showed that the heating load was reduced by 21% and the simple payback period was 4.5 years for the low-e storm windows. Of course, in replacement windows, the payback period is considerably longer than that. Additionally, when doing blower door tests, the overall home air leakage was reduced by about 7% for the overall home, not just the window. That’s fairly significant.
Chicago is obviously a very cold climate, and we wanted to do a similar study in a more mixed climate. I participated in a DOE-funded field study between 2011-13, again with NAHB Research Center, as well as Larson Manufacturing, and QUANTAPANEL. We took ten older homes in the Atlanta area that had single glazing, and tested different storm window technologies. The low-e windows showed, over a 2-year period, approximately 15% heating savings as well as 2-30% cooling savings. There is large variability, especially on the cooling side. That’s because these are real-life field studies - variations between homes, amount of trees around them - and these are occupied homes. So there is a lot of variability, but very positive results in general, on both the heating and cooling side. Later on, Sarah Widder from PNNL will talk about the test studies done in the PNNL test homes, which are a more controlled environment.
On the air leakage standpoint, in this case, these homes and these single pane windows were particularly leaky. Adding the low-e storm windows reduced the overall home leakage by 17%, just by introducing this modern-style, low-e storm windows.
In addition to the energy efficiency, we also polled the occupants on other benefits. They rated these higher than the energy efficiency, because that’s what you notice in the home. The number one benefit was the improved home appearance. These are designed to be aesthetically pleasing , fit in with the architecture, and improve curb appeal. Significant benefits included reduced drafts, improved comfort on sitting by the window, as well as reduced noise were significant improvements cited by the occupants.
In addition to single-family homes, we wanted to test out low-e storm windows in multifamily buildings. We tested 2 fifty-year old, large apartment buildings in the Philadelphia area, 101 apartments overall. These had single pane, metal frame windows. They already had old, clear storm windows, but these windows were really poor quality, so we replaced them with modern, low-e storm windows. Just by replacing those windows, over a one-year period we saw an 18-22% reduced heating energy use, and a 9% reduced cooling energy use. The air leakage test was done on select apartments and showed that the air leakage was reduced by 10%, which is consistent with the other studies. So very positive results in both the single-family homes and apartment buildings.
I did want to also mention that low-e storm windows and panels are not just for low income weatherization homes. Obviously, that’s one of the first targets, but they are really appropriate for any building type. Low-e storm windows are a good option for homes of all types, historic buildings, commercial buildings, large buildings.
Although they are applicable to almost any building type, one of the first targets we looked at was government weatherization programs, but also the general, weatherization market for upgrading existing homes. In 2009, the ability to include low-e storm windows was added to the National Energy Audit Tool (NEAT) a weatherization software package used by many state weatherization programs.
The following year, low-e storm windows were added to Pennsylvania’s Weatherization Measure priority List for single-family homes. This was due to a NEAT analysis of 37 home types in four cities. In this case, the key output we are looking for is the SIR, or Savings-to-Investment Ratio. Under weatherization programs, the SIR must be greater than one to qualify. When we did the NEAT analysis, the SIR was between 1.4 and 2.2 over single pane windows, and 1.3 to 2.1 over metal-framed, dual pane windows. The main study was done for natural gas-heated homes, but when you used propane or electrical resistance heating, the SIR was much higher. In all cases, it was proven to be cost effective for the Pennsylvania weatherization program, and they added it to their Priority List.
So we wanted to work with PNNL to do this Pennsylvania experiment throughout the United States. We expanded the NEAT analysis to include 39 different home types and 22 cities across all eight climate zones. There’s a lot to this study and you can download it from the PNNL website, and a reference will be at the end of the presentation. This slide [graphic of the United States, showing climate zones] shows one of the key conclusions that came out of the cost effectiveness analysis. In addition to NEAT, we also did a RESFEN analysis, which is the go-to program [for windows], to confirm the results.
This map shows that low-e storm windows and interior panels installed over all single pane windows, as well as all double pane metal frame windows with clear glass, were cost effective in climate zones three through eight, with SIR ranging from 1.2 – 3.2. In climate zone 3, solar control low-e storm windows are recommended, and above zone 3 [see slide], the normal low-e storm windows are recommended.
We also did the test over double-pane wood or vinyl-framed windows, which – adding a low-e storm window or panel – would then turn it into a triple low-e unit, basically. This was found to be cost effective in climate zones 6 through 8, as well as the eastern part of zone 5, which has higher heating fuel costs, such as Pennsylvania, New York. In this case, the SIR was 1.1 to 1.9. Again, this is for natural gas heating, and if you do it with propane or electrical resistance heating, then it would be recommended over a much larger range, because these are higher cost fuels.
So taking these really positive results about the field studies, and some of the weatherization and climate-based modeling we did, one group we are working with now is the Consortium for Energy Efficiency (CEE), which includes a lot of utilities and energy efficiency program administrators. They have formed a windows working group of CEE members and industry stakeholders to work on advancing the uptake of efficient fenestration products throughout the United States and Canada --both new and replacement windows, and window attachments such as low-e storm windows.
One of the outcomes of this group was the CEE Window Product Overviews, published in February as a resource for CEE members and program managers. It really goes into detail for options and technologies for addressing windows in a utility or energy saving program. Currently, there are some CEE subgroups that include windows attachments, including low-e storm windows. The next meeting is in Salt Lake City as part of a CEE industry partner meeting on Sept 17-18.
We added a slide on code compliance to answer some questions on low-e storm windows and panels. Storm windows are already exempt from the energy code, because you are improving the energy performance of what is already there, which is a good thing, and the energy code recognizes that.
As for other building requirements, such as structural, wind load, and fire resistance requirements -- these are generally going to be met by the primary window and the building façade. It’s not a concern as long as the low-e storm window or panel is not adversely affecting that performance.
So the main thing you have to worry about with low-e storm windows or panels is to check whether the windows will be used in a hazardous location defined by the building code. For instance, an interior panel installed near a bathtub or near a swimming pool. In this case, the window requires tempered glass. Very easily done, you just need to specify tempered glass for that application.
The next question we get a lot, is ‘Do I have to apply for a permit?’ That really comes down to local jurisdiction. Our experience is that this can be a DIY [do-it-yourself project] for a homeowner, or a contractor can do it. Most jurisdictions do not require a permit to install low-e storm windows, but you should always check.
The third question we get is ‘Should I check with my homeowners association before installing?’ The answer is ‘yes.’ Just like any exterior modifications to the home. In general, they should approve it, if they are looking at modern low-e storm windows. As noted earlier, they are made to fit in with the architecture of the home, they are aesthetically pleasing, and they are preferred in historic preservation programs. But if it is a concern or issue, you can also consider interior panels.
Looking into the future before I hand this off to Sarah. Low-e storm windows were recently integrated into the FEDS model, to support most Federal building energy audits.
DOE is supporting the development of a new organization and Certification and Rating program for Window Attachments, which includes a wide range of products, including storm windows.
We are working with CEE to develop tools and resources for energy efficiency programs, and we are working directly with utility and weatherization programs to provide technical assistance. Contact Katie Cort, the lead for PNNL, or contact myself, or Sarah Widder. I know there are other people on the call. Kerry Haglund has been working with another group in Minnesota that also happens to go by the acronym CEE but is a different energy efficiency organization. There’s a lot of work out there, so contact one of us for a lot more details.
So next I’ll hand it over to Sarah Widder. Sarah is an engineer at PNNL, and she’s an expert who’s worked in a wide range of areas related to energy-efficiency in homes and buildings, not just windows, but all components of housing. She works for the Building America program in particular. She’s an expert on side-by-side test homes at PNNL. Today she’ll be talking about some of the results with low-e storm windows.
Sarah Widder: Thanks, Tom. And good morning or afternoon to everyone. I’m Sarah Widder and I’m an engineer at PNNL, and I’m going to talk a little about low-e storm windows in the PNNL Lab Homes. The Lab Homes are a great expansion of some of the case studies and lab studies that Tom spoke about. It is sort of a compromise between lab testing and full-on field testing. We get a lot of precision in our answers in a Lab Home, because we simulate occupancy and we mitigate against a lot of the confounding factors that you’ll see in field experiments. We can look at the whole house energy performance, which is not always possible in the lab. And with that, I’ll get into it.
Before I dive into the results, I wanted to acknowledge the supporters of the Lab Homes work. The Lab Homes have been on the PNNL Richland campus for three years. It wouldn’t be possible without these supporters.
I also wanted to acknowledge the climate that this testing was done in. Tom talked about where low-e windows are cost effective, and here in Richland, Washington, we are in climate zone 5. Based on modeling, low-e storm windows should be cost effective, which was validated by some of the modeling that Tom discussed. So we are in the southeastern part of Washington. It’s the desert, so we get really hot summers and cold winters. We measured Lab Homes low-e storm window performance in both the summer period and winter period.
So the Lab Home characteristics. We have two manufactured homes that were provided by Marlette, which is a manufactured housing manufacturer in Hermiston, Oregon. That was a cost-effective way to get two pretty identical homes. We specified them a little differently than a standard Marlette home so they would be representative of more of a 1970s era housing stock that we typically see in the Pacific Northwest region. You can see it has down specification levels of R-22 floors, R-11 walls, and R-22 ceiling. I’ve highlighted here one thing that stands out for this experiment, and the first experiment we did in the Lab Home, looking at the performance of highly insulating, triple pane windows. We have a significant amount of window area in the home, which allows us to exaggerate the benefit or the impact of these window technologies, and get a better signal.
This is just a floor plan of the Lab Home. You can see how it is laid out. It looks like a typical home: three bedrooms, a master bath, and a regular bath. And I just want to point out, up is south. There is a sliding glass door facing south and significant west-facing glass in the dining and living room, which is also beneficial for examining how low-e windows or the highly insulating windows can improve comfort in the home. Especially with the setting sun and afternoon heat we experience in the summertime.
These homes are highly monitored and controlled, and that’s what gives us such a precise measurement at the end of these experiments. We have commercial lighting panels as the circuit panels. We have 42 individually monitored breakers so that we can measure, uniquely, almost every load in the home. Half of them are controllable, which is how we simulate occupancy. We turn on and off lighting, appliances, and generate sensible loads that are representative of people and equipment. We have identical occupancy patterns that are representative of a typical occupant in both the homes. I know there is lot of variability in occupancy patterns, and we could do a whole webinar about that, but in these homes, the really nice thing about the way they are laid out, is as long as we do it identically in both the homes it becomes a null factor, so it is not so important to get the occupancy correct, as long as it is identical in both the homes.
We also have temperature and relative humidity monitors throughout the homes for ambient air temperature and comfort within the home. We have mean radiant sensor, which is in the second picture down [see slide] that sort of measure how a person would feel like next to a window. We also have interior and exterior surface glass temperature measurements, so we can look at conduction through the glass. We take samples of our data every minute, but we collect it on a couple of different intervals.
So I’ve talked about how the Lab Homes are unique. The main thing is that it has built in control. We have one house that has baseline windows, and we’ll talk about that later, but it has clear glass, aluminum framed, double-pane windows, and another identical house in every way except for the one technology we are trying to evaluate. That allows us to precisely look at the impacts associated with that technology, which gives us more exact information than in field studies.
But that is dependent on how identical the homes are. We are assuming that these buildings are operating the same. When the buildings were first sited, we made an initial building construction comparison to verify similar performance, and the homes were as similar as you would expect, and a lot more similar than you would get with a stick-built housing, because they are manufactured homes.
Before we do each experiment, we do an active baseline or null test. Before the homes are retrofitted, we operate both homes for a week with full occupancy and HVAC control. Just to ensure similar energy performance before the retrofit. Anything we see that is varying from that baseline we can attribute to that technology.
So let’s get into the experiment now. Here is a description of the window characteristics. The baseline windows are double pane, clear glass windows with an aluminum frame. We have a lot of these un-retrofitted, 1960s – 1970s ranch homes in the Pacific Northwest.
The U-Factor, which is a measure of thermal resistance; the source heat gain coefficient (SHGC), which is a measure of how much solar heat will come through the window and get into the house - which is beneficial in the heating season, not so great in the cooling season, and the Visual Transmittance (VT) is how easy it is to see through the glass.
So you can see we had significant reduction in the U-Factor and SHGC with the low-e storm windows. I will mention that these numbers for the low-e storm windows – 0.33 and 0.53, and the patio doors as well, were generated via modeling by a company using LBNL’s Window Therm model. They are created in accordance with NFRC testing techniques, but there’s no official NFRC certified ratings available for low-e storm windows right now, or other window attachments. Which is one of the barriers to really getting these technologies incentivized in the market, because we don’t have a clear way to describe their performance in comparison to other window fenestration products.
Then I compared the performance we are seeing from the baseline windows with these exterior low-e storm windows attached, to the highly insulated windows primary windows that we retrofitted in our first Lab Homes experiment. As you can see, we got a little bit better U-factor (thermal resistance performance), and significantly reduced Solar Heat Gain Coefficient (SHGC), because those had two low-e coats, which would be great in the southwest in a heating-dominating climate, but limited our heating season savings [in the northwest].
I’m not going to spend much time on this slide. Tom covered, in great detail, how low-e storm windows save energy. These are just three visual depictions of it. Just like a double pane window, they are creating an additional dead air gap that will decrease conduction. It will reflect heat back into the space, and can decrease air leakage.
I am going to spend a little bit of time on how low-e storm windows are installed. Low-e storm windows are very easy to install. These are all the tools that are needed, so any homeowner can do it with a screwdriver and measuring tape. Window or general contractors can easily install low-e storm windows; it isn’t a complicated technology to install. You just need some caulk and a caulking gun, something to wipe up the caulk, a screwdriver or screw gun will make this even easier, and measuring tape. Measuring tape is probably the most important piece.
The first step to specifying and installing your storm window is measuring your existing window opening. This is probably the most important step to make sure your low-e window will fit properly over your existing, primary window. This is applicable for both exterior and interior low-e storm windows. Exterior storm windows can be installed in two different configurations, either an overlap installation, which is what we did in the Lab Homes and we will see some pictures of that later, or a blind stop configuration where it fits into the window opening.
Manufacturers typically have specific instructions for the dimensions that they are looking for, so follow the manufacturer instructions. Windows can be custom ordered from window distributors, or some big box retailers. There are also some common sizes available that you can purchase off the shelf.
So measure, measure, measure. Typically for a slider or any horizontal opening, you’ll want to measure the top, the middle, and the bottom, and make sure you use the smallest measurement so that your window will fit into the window opening.
For an external installation, I recommend the blind stop installation. You first want to dry fit the window, so hold the exterior window up to the window opening, and make sure the screw holes all land on solid wood. You want to make sure that the storm window and the primary window open in the same direction.
Then just take the window down, caulk around the opening, put the window back in place and screw in. You only want to caulk around the top and sides of the opening. Don’t caulk the bottom, exterior storm windows have weep holes designed into the bottom. They have a little plastic gasket on the bottom to help make sure that the bottom edge is as airtight as needed. But those windows will help with drainage, if any condensation should occur between the primary window and the exterior window.
For an interior installation, the process is very similar. The windows are only available in the blind stop installation type. You will typically have a trim piece that will go between the primary window and the interior storm window to make sure you have a good thermal break between the two windows, and a sufficient gap you can see in the upper right hand corner where we are pointing to the screw holes that will be in the low-e storm window frame for an interior frame in a blind stop installation. You can see the contractor installing it in the lower left window. You want to make sure that your low-e coating is facing the right side. Sometimes the manufacturers put a sticker on them, but they also feel different, so you can rub the sides of the window, and the side with the low-e coating feels squeaky. If you run your fingers on it, it will squeak. It just has higher friction to it.
Interior low-e storm windows don’t have weep holes, so you want to caulk around the whole opening because they are meant to be the primary air bearer. If condensation, particularly from inside generated moisture is a concern, an interior low-e storm window might be something to consider.
[Slide shows a list of URLs]
So there are a lot more details and instructions on how to install low-e storm windows. We made a YouTube video that describes the instructions. There’s more information in the Building America Solution Center. Also there is information on the efficient window coverings website, which has a lot of information on efficient windows in general.
So the results from our Lab Homes experiment. We saw an average of 10% whole house energy savings, from the low-e storm windows, compared to 12% for triple-pane primary windows, and that’s really significant savings from a pretty cost-effective technology. I’ve shown just the average daily energy savings here. Those are extrapolated from our seasonal measurements using just a heating degree day extrapolation. We’ve also generated some energy models to corroborate those numbers using EnergyPlus, but these are just heating day regressions. You can see for the low-e storm windows, we saw better savings in the heating season, the winter, than the summer. That is the opposite in the highly insulated windows because of the low solar heat gain coefficient.
We also measured the air leakage performance of the low-e storm windows, and we did some sensitivity blower door testing, to try to figure out which window was the primary air bearer for the Lab Homes experiment. Manufactured homes have a reputation for being pretty air tight, because there is better quality assurance that can be done on the manufacturing floor than out in the field when you are building a stick-built house. For us, the primary windows were the manufactured homes windows, and they were primary air bearers. Those energy savings numbers we saw before, 10%, could probably be improved since those are really just from the improved thermal performance of the low-e. We are not getting a lot of benefit from decreased air leakage in our experiment, which would probably not be the case in the field.
I’ll spend a little bit of time describing the table.
Lab Home A, that first line in the table that is italicized – that is just the unmodified home, so you can see how similar Lab Home A and Lab Home B are. You can compare the first and second lines where both the primary and secondary storm windows are closed. We did not seal the bottom edge of the storm windows.
Some people ask, don’t the weep holes decrease the benefit you might get from decreased air leakage from a low-e storm window because it is not as airtight as possible? That’s not why the weep holes are there; they are there for condensation, but we did do some experiments to try seal that bottom edge and see if improved air leakage performance, but from our testing, it wasn’t significant. That’s due to the design of the weep holes. They are designed to prevent air leakage into that dead air space, because they are only at the bottom and they don’t have an escape at the top.
The other thing I’ll mention about the energy savings performance of the low-e storm windows we saw in the Lab Homes. . . for the heating season, the majority savings occurred on sunny days at night, when it was the coldest outside. You can see that in the graph on the left. The blue line is Lab Home A which has the baseline windows, the red line is Lab Home B, which has the low-e storm windows. We are also seeing the outdoor temperature on the right axis on the dotted line.
You can see there is more of a difference in the morning when the morning, before the sun has risen, and separates again in the evening. On this sunny day, we didn’t see many savings in the middle of the day. If it was cloudy, we have graphs similar to this, we see that the difference is maintained throughout the day.
The graph on the right is the summer time, with the maximum amount of savings occur in the afternoon when it’s the sunniest and hottest. We just wanted to note that it is similar to the peak power periods that utilities experience, similar to a many effciency measures. This can also be a way of decreasing that peak power use in the summer time.
Tom talked a lot about the cost effectiveness of low-e storm windows. This is just the results we generated from our specific Lab Homes experiments. I have a couple of costs we used for low-e storm windows: a low cost, a medium cost, and a high cost, based on installation data and how much the windows cost, and general cost information about low-e windows. The total installed cost was between $1,500 and $2,000. That annual savings was based on the heating day regression I mentioned earlier. So we have a simple payback period of 5-7 years.
On the last line, I added information on the R-5 windows. They had a little better annual savings, but the payback was over 20 years.
Replacing your windows can be an excellent idea, but the decision is not typically tied to payback reasons. If we are looking at cost-effective retrofits, low-e windows are a good technology when window replacement is not being considered.
References (including URL links)
Many reports are available on the Building America Solution Center. This is a database of energy efficient home construction techniques and technologies. It provides specific installation guidance, including code compliance information, videos and cad drawings, climate specific information – a great resource for utilities, builders, and contractors to decrease energy use in homes.
Question and Answer Session:
Q. Can you provide information on the sash – is it metal, is it thermally broken, and does it make a difference?
A. You can use any material in these products, but the vast majority of them are aluminum. Usually a durable, painted aluminum. It can come in any color, but the sash itself is aluminum. But I think the question is asking about whether it needs to be thermally broken or not. The way a low-e storm window is installed, whether they are on the outside or the inside, there is a thermal break there between the storm window and the primary window that’s either on the wood brick mold on the outside or on the inside you have the surrounding sill. You’re not attaching it right to the primary window. Additionally, in some cases, there is a wood blind stop in there as well that creates a thermal break. In the end, the material of the sash itself does not matter, other than from a structural or aesthetic standpoint. There is no thermal bridge there.
A sash can be made from many different materials, but is typically made of durable, painted aluminum. The material does not matter, except from a structural or aesthetic standpoint.
There is a thermal break between the storm window and the primary window, whether a low-e storm window is installed on the inside or outside. It can be on the brick mold on the outside, or surrounded by the sill on the inside, but it is never attached directly to the primary window. In some cases, a wood blind stop creates a thermal break.
Q. Is there a recommended air gap for spacing?
A. In terms of performance, you should have a half-inch or more. You know, you can generate these charts that look at the u-factor as a function of your air space, but it’s pretty flat, it’s stable for an air gap, once you’re about a half inch or greater. If you were at a quarter inch, that might be too tight. It is still better than nothing, but it is not going to be as high performance. In reality, you are limited by the geometry of the window opening and the sill, and everything around it. And so most are going to be somewhere between a half inch to several inches. It is not real sensitive.
To ensure the highest performance, the air gap should be a half-inch or more. You are limited by the geometry of the window opening, the sill, and everything around the window. Most air gaps will range between a half-inch to several inches. A quarter inch might be too tight, and will lessen the performance of the window.
Q. Will they work with a hopper or awning-type window?
A. Yes. It depends on the type of product. So if you have a casement, an awning, or a hopper, or any sort of projecting product that projects outward, then you would want to put an interior low-e window panel over that. They can be fixed… but of course the whole point is you want be able to open it to ventilate it, so there are also operable interior panels. You would open the sash and then operate the projecting unit. If it happens to be a project-in casement - they are not as common, but they do exist - then you would want to put on an exterior panel. You can do it, but it may then restrict whether you use an exterior or interior panel.
If you are working with a product that projects outward, such as a casement, awning, or hopper, you should use an interior low-e window panel. There are interior panels that will allow you to open the sash and operate the projecting unit. Exterior low-e windows can be used with products that project inwards.
Q. On a double-pane film, is it on the outside pane for solar control? How is solar control low-e different on storms?
A. To do a low-e storm window, you have to have a durable low-e coating because, even though it’s somewhat protected, it’s still going to see the elements, and humidity and moisture and all that, and so you are a little more restricted on the types of coating than you are on a sealed, insulated glass unit in a new window. [For] the vast majority of these…there are coatings from all different companies, but there are products available that are solar-controlled, pyrolytic durable low-e that can be used. You would want the solar control layer to be towards the outside layer, so you would only use those products on exterior low-e storm windows. This is very good for Climate Zone 3, like Georgia, the Carolinas, across the southern, south central region of the US. In terms of performance numbers, you can estimate the U-factor and solar heat gain over different types of primary windows. Over a single pane, wood window, which is very common, both a regular low-e storm window and a solar control low-e storm windowwill have a U-factor of about .34. The solar heat gain for a regular low-e storm window might be between .45 up to 0.50, and with a solar control glass .35 to .37, somewhere in that range.
A low-e storm window needs a durable low-e coating to protect it from humidity and moisture. When using a solar-control, pyrolytic low-e coating, it should be used on exterior low-e storm windows. This works well for climate zones 1 through 3, which includes the southern and south central region of the United States. A single pane, wood window with a low-e storm window will have a U-factor of about .34. The solar heat gain for a regular storm window with solar control glass might be between .45 up to 0.50, and with a solar control low-e, it will be around .35 to .37.
Q. Are there condensation issues with installing double-pane storm windows with double-pane windows?
A. Storm windows are usually a single layer of low-e glass, or you can use other materials. Whether you put it over a double-pane window, or a single pane window – I guess the way you think of condensation is, whatever [window] is most to the interior of the home you want to be tight, because you don’t want that warm moist, humid air from the home to leak out and hit the cold, outer surface. So if you are putting in an interior panel, those are designed to be airtight, so it stops that warm, moist air from leaking through to the primary window. They are very good with condensation issues to begin with. If you are doing an exterior storm window, in practice, you want to seal the existing window as best you can. Make sure the weather stripping is in good shape, and that’s also why we have weep holes in exterior storm windows. Not just to let liquid moisture to get out, but to also help with any condensation issues.
A2. If condensation is a concern in a particular home because the ability to seal the primary window is an issue, or you don’t think you can seal the primary window, then you just might want to consider interior storm windows because you can make sure the primary air barrier is on the inside window, whether that’s your low-e storm window or your primary window.
Storm windows are typically a single layer of low-e glass. Whatever is closest to the interior of the house should be tight, so that moist humid air from the home does not leak out and react to the cold, outer surface of the window. An interior panel is designed to be airtight, so leakage should be rare. When using an exterior storm window, try to seal the existing primary window tightly and make sure the weather stripping is in good shape. Weep holes in exterior storm windows will also help with any condensation issues.
Q. Why is NEAT used instead of the Building America BeOPT? How have low-e storm windows been tested in the NEAT version 8.9? There has not been an option for low-e storm windows, only the low-e windows.
A. NEAT added the ability to include storm windows back in 2009, version 8.5 or later. There are storm windows listed, but the default is a clear glass storm window. When you go into the set-up library, you can change the property of that storm window to make it a low-e storm window. You pull out the emitance for that glass that is used in the storm window. For all these products the number is 0.16.
Regarding why we use NEAT instead of BeOPT, mainly because we were targeting that initial analysis at state weatherization programs, such as what we did in Pennsylvania. There they use NEAT as a standard tool to calculate the savings investment ratio (SIR). Presumably, you would come up with similar results in BeOPT as long as you get the right properties in there.
NEAT is often used by state weatherization programs to calculate savings investment ratios (SIR). BeOPT should result in similar results as long as the same properties are entered.
NEAT added storm windows in 2009. The default is a clear storm window, but you can alter it to obtain a low-e storm window calculation. Go to the set-up library and change the property of the storm window emittance to 0.16. (See document entitled “How to Model Low-e Storm Windows in NEAT and other Software,” on /eere/buildings/events/building-america-webinar-low-e-storms-next-big-thing-window-retrofits).
Q. Is Building America developing a standard performance benchmark for the u-factor or solar heat gain coefficient or infiltration?
A. Building America is not themselves, but DOE has set aside funding for this endeavor. Under this program, “CRAFT” is a new organization that will be developing a standardized certification and rating program for all fenestration attachments, including blinds, shades, solar screens, window film, and low-e storm windows. That effort will start this fall . In the meantime, you can estimate u-factor and solar heat gain using Window 6 and Therm 6 software tools.
(Again, this is another place we could probably add that link with the “How to Model Low-e Storm Windows in NEAT and other software” which includes the list of typical U, SHGC, VT numbers)
Q. Have the storm windows been tested in climate zones 1 and 2 with a perforated screen, such as an inflector or eco-flector?
A. Low-e storm window analysis for climate zones 1 and 2 were limited to calculation analysis. There were no field studies. Energy savings were achieved in climate zones 1 and 2, but they were highly dependent on the home type and the payback period was over a wide range. It is recommended that climate zone 1 and 2 should be studied on a case-by-case basis.
Solar screens were not tested with low-e storm windows, so it is unknown whether screens would provide an additional benefit, although it is reasonable to expect the reduced solar gain will have a benefit in these zones.
Q. Since these products have a NEAT cost effectiveness greater than one, do you expect them to be cost effective for energy efficiency programs according to cost effective tests?
A. Yes. That was the point of the paper we did with PNNL. We looked at the climate based modeling we did on twenty-two cities to calculate that SIR. We also used RESFEN as a second method to look at cost effectiveness. With RESFEN we found a return on investment of between 7 and 10%. Together with the SIR determined by NEAT, we came up with those maps showing what the ranges are. We tested across 39 different home types, it was always proven to be cost effective in those zones. You need to look at your particular area, but that data may already show that for where you are, that it would be cost effective for a local incentive program or energy efficiency program.
Studies show that low-e storm windows are effective across a wide range of climate zones and building types. See Database of Low-e Storm Window Energy Performance across U.S. Climate Zones. [https://basc.pnnl.gov/resources/database-low-e-storm-window-energy-performance-across-us-climate-zones.] for more details.
Q. Are building permits required to install low-e storm windows?
A. Check with your local jurisdiction. In most cases, a permit is likely not necessary, since the structural performance of a house is not affected. You should also check with your homeowners association, since storm windows may alter the appearance of a house, although interior low-e panels are always a good option.
**Due to time constrains, some questions were not addressed during the webinar timeframe. The following questions were not addressed during the webinar, but were answered with individual e-mails to participants.**
Q. Follow up question to air gap, with too much of a gap, isn’t there a tendency for the air to cycle up the inside and down the outside reducing performance?
A. Yes, that is true and there is an optimum air space gap for the lowest heat transfer – too small and the U-factor goes up from not having enough dead air space, and too big and the U-factor can go up a little from having convective loops within the air space. For air, the optimum is about 1/2 inch, but if you use WINDOW6 software to model it, the change is really not that much. See figure below, although we obviously only care about air (top line). Even with that small increase above 1/2 inch, a U-factor of 0.3-0.35 is still far better than the original single pane U-factor of over 1. The concern is more about being too small of a gap.
Q. How many contractors are actually aware of the low-e storm windows? I live in Cleveland, Ohio and I’m not sure if I have heard contractors mention this a lot.
A. You are correct – there probably are not many contractors who have heard of this product and who talk about it. We have found this to be one of the barriers to getting wider market acceptance and adoption of this product. Storm windows have traditionally been installed as a do-it-yourself project. In one sense, this implies a very easy and affordable installation – which is good news – but on the other hand, it means the contractor market for this product is not well developed. This is one of the reasons we produced this webinar on the topic, as we are hoping to get the word out that this is a very affordable and effective weatherization fix that home performance contractors could easily add to their list of retrofit measures.
Big box retail stores (Menards, Home Depot, and Lowes) are starting to display some low-e storm window products in their stores, and they will often have contractor assistance available for installations, which might help fill this void a bit more. We have also been trying to develop some contractor retrofit guides and checklists through Building America’s Solution Center. Hopefully we will get a little more traction in this area. If you need any other information or would like to pass along any of our information to local contractors, please feel free to do so.
Q. Some of the windows in old homes (close to 100 years) are very custom in how they fit into a sill, is it possible to use existing storm window frames and replace them w/ the low-e storm windows?
A. The modern storm window manufacturers are capable of custom products. They have to be, since especially in older homes, every window can be different. But they can make custom sizes and shapes (arch tops, etc.), and installation can be accomplished in a number of ways – overlap onto brick mold, blind stop inside openings, interior panels on different sill positions, etc. Sometimes the only restriction is whether there is enough width on the sill for the storm panel to fit, although even when there is not, you can always add a surrounding trim piece. It is possible that new sashes with low-e glass could be made to fit into the existing storm window frames, but to be honest, it is probably easier and possibly less expensive to get a custom fit new low-e storm windows instead of trying to have low-e glass put into the older sash.
Q. Who could we talk to or e-mail someone about our invention which would work very well with this to save even more energy and save energy in other scenarios.
A. DOE does offer ‘grants’ for energy ideas (e.g., technologies that save energy). These are competitive grants and I’m not quite sure this is what you are after, but the site (http://www1.eere.energy.gov/financing/index.html) may have some helpful information. There are, of course, other sites where people can take their ideas or prototypes, and many are associated with universities like this one: http://www.techtransfer.harvard.edu/inventions/ip/patents/. If you haven’t done so already, you might also consider checking out some of the resources we mentioned during the webinar to see how your technology measures up with the savings from some other technologies.
Q. How can we estimate savings in our hot humid climate without doing a HERS rating?
A. You can use NEAT or RESFEN, just like we did in our climate-based paper. And you can use any other software as long as you put in estimates of what the total U-factor and SHGC are before and after, for the original window and the original window with the low-e storm window. Our climate-based modeling study includes those representative U and SHGC numbers for several different types of primary windows. This is also mentioned in that handout I have attached on how to model low-e storm windows in NEAT and other software.