Image Credit: Bruce Richey
More Musings of an Energy Nerd
Homeowners can now receive a federal tax credit for 30% of the cost of new energy-efficient windows. The credit was authorized by the American Recovery and Reinvestment Act (ARRA) signed by President Obama in February.
There’s just one problem with the new tax credit: the specifications for eligible windows were crafted by politicians, not window experts. The ARRA stipulates that eligible windows must have a maximum U-factor of 0.30 and a maximum solar heat-gain coefficient (SHGC) of 0.30 — requirements that have been dubbed “the 30-30 provision.”
Only low-solar-gain windows are eligible
There’s nothing wrong with the U-factor specification — except, perhaps, that it isn’t particularly stringent. The problematic provision is the SHGC spec.
By setting a maximum SHGC of 0.30, the ARRA actually excludes the best windows for cold climates. Cold-climate homes need windows with a SHGC in the range of 0.39 to 0.65; so if they comply with the tax-credit provisions, they’ll end up with windows that contribute to higher-than-necessary energy bills.
South-facing windows need a high SHGC
In Florida, solar heat gain from windows increases a home’s air conditioning load, so low-solar-gain windows are usually the best choice. Even in hot climates, however, high-solar-gain windows usually save energy during the winter.
Although some high-solar-gain windows — especially those facing east or west — can cause summer overheating, south-facing windows rarely cause overheating problems, especially if the windows are protected by a well designed roof overhang.
Since the average American family spends far more on space heating than on air conditioning, installing high-solar-gain windows on south walls makes sense for much of the country. Yet most U.S. window manufacturers have all but abandoned the market for high-solar-gain windows.
Low-solar-gain glazing is now the norm
During the 1980s, glazing manufacturers perfected spectrally selective coatings that made it possible to produce low-solar-gain insulated glazing.…
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33 Comments
High Solar Gain Glazing
Martin,
I've been struggling with this option. We are using Serious 900 series windows and HSHG is an option but our house will face about 40 degrees west of south so our south windows really face SW and our east windows face SE. We'll have proper overhangs for south sun but it won't be totally effective due to the west angle. Our south windows are also partially shaded by a high ridge and a mix of conifer and deciduous trees on the south and west. This is in a 9500 degree day climate. The question came back from the window manufacturer asking if we REALLY wanted to specify any HSHG glass like that is a bad idea.
A 9,500 degree day climate
Donald,
A 9,500-degree-day climate is cold. I'm not sure where you are, but I imagine that you live somewhere where few people have air conditioning.
I would certainly choose high-solar-gain glazing for the SE orientation -- after all, mornings are probably cool, even in the summer. The SW orientation is more of a judgment call. It depends in part on whether you have a low glazing ratio or a huge expanse of glass -- as well as how your overhangs are designed.
Be sure to do a RESFEN analysis and examine your options. Remember, on sunny days from September to mid-May, you'll definitely appreciate the solar gain.
Dual glazing
This is very insightful for dual glazed windows, however they fall far short of triple glazing. Did you know that dual glazed windows are no longer legally sold in most of Europe. Pan-european building code has mandated triple glazing as part of passive design as the minimum to pass the building code.
Of course when you have energy cost such as they have in Europe, you are more motivated to stop wasting energy.
I have had many people try to tell me we do not need triple glazing here in Southern California, and I always reply...show me a zero energy cost for heating and cooling and then I will believe you, until then I agree with the Europeans and recommend that no one should settle for dual glazed windows, it doesn't cost that much more for another layer of glass and a second air space in between. And the additional soundproofing over dual-glazing is a huge additional benefit, reducing noise pollution substantially.
pushing for net energy [email protected]
Different kinds of triple glazing
Wayne,
You're right — many window buyers should be taking a serious look at triple glazing. Fortunately, the Canadian window manufacturers that I listed offer both low-SHGC and high-SHGC triple glazing. For example, Thermotech Fiberglass offers triple-glazed casement windows with whole-window SHGCs that range from a low of 0.25 up to a high of 0.47. Anyone living in a cold climate should choose a high-solar-gain window with triple glazing.
More information here:
http://www.thermotechfiberglass.com/case2.htm
High SHGC Underrated
A high SHGC is the underrated window characterisitic for northern housing. Here's an example to drive the point home.
Consider a north facing window in Toronto. (Toronto is warm by Canadian standards, but fairly typical for the Northern US). Whether it has a low or high SHGC glazing the north facing window is a net energy loser. However, it is more of a net energy loser if it has the better insulating low solar gain low-e, than if it has the poorer insulating high solar gain low-e. So even on the north where there is no direct sunshine, there is enough diffuse solar energy to play a very significant role in the energy balance of a window.
To be fair the difference between the two options for a north facing window is on the order of 1-2 kWh/ft^2 - so even if you heated with electricity, the difference in cost is on the order of $.10-.20/ft^2 of north facing window area over a northern house's 200 or so day heating season. Not alot of money.
Nevertheless the counterintuitive near parity of the two glazing options for a north facing window points to the underratedness of the SHGC.
"underrated is an understatement"
In the US, we are just far too spoiled and kingly to play around with passive solar. We all have (phoney) window shutters but haven't closed them at night for a century or so, and would be too lazy to close them now to get the best net btu out of south facing windows in the winter. Thermal mass is nothing but our fat American asses sitting around a fireplace.
Thermal mass
Rick,
Come now, you're being too harsh. The average U.S. home has other types of thermal mass — like a plasma TV, and granite countertops from Brazil, and all those empty soda cans in the recycling bin. It all adds up.
30/30 window
Blinds between the glass with low-e is a great fit it does both u factor with low-e hard coat clear glass on outside lite with the tilt and slide blinds great product check it out http://www.Royaltechwindows.com the windows are made in michigan and shipped across the country
BSC Info Sheet
I was just reviewing Building Science Corporation's "Information Sheet #001" titled "Residential Best Practices Criteria". Under the heading of windows, they recommend the following: For Climate Zones 1-3...max U value .40, max SHGC .35. For Climates Zones 4-8...max U value .35, max SHGC .40..
I would have hoped that these guys would have had no limit on SHGC's for northern zones...Any comment from anyone at BSC??
BSC comment welcome
Garth,
I have spoken to Joe Lstiburek on this issue. I would be happy for him to comment here directly. Although I hesitate to speak for him, I'll tell you what I remember. He worries that:
(1) The number of air-conditioned homes keeps increasing, and
(2) Production builders don't understand orientation, and need to be able to flip all of their home designs to any orientation in order to use these designs in large communities with lots that face any direction.
These concerns apply to many of Lstiburek's clients, but not to most GBA readers (I hope). Using high-solar-gain glazing requires thinking. For builders who can't think, it may be best to choose low-solar-gain glazing. You don't want to put massive amounts of high-solar-gain glazing on the west side of your house.
AC concerns
I can certainly understand Joe's concerns regarding AC. I live in southern Saskatchewan (9000 HDD) and even here, AC use is increasing. Most new homes are being built without regard to window orientation, but partly because most builders believe that low SHGC windows and AC will take care of any problems. I think that large areas of west facing glass of any SHGC and in any climate zone, result in an energy pig. The only solution regarding production builders would probably be code mandated...
For people who don't use A/C,
For people who don't use A/C, and leave their window open the whole summer, low Solar Gain windows are really pointless.
SHGC
My question is if higher SHGCs really matter on northern orientations - to the point of designing it into a home in a heating dominated climate? As Stephen Thwaites post would imply, they still gain from diffuse solar energy but it doesn't sound like much.
It matters, but not much
The amount of heat gained through northern windows during the winter is not great. Nevertheless, you might as well be harvesting heat instead of losing it. A low solar heat gain window will admit less heat and will often have a lower visible transmittance (be darker) -- something you don't want.
There is no danger of a north window leading to summer overheating, so the bottom line is: in a heating climate, be sure to select high solar heat gain windows for the north side of your house.
Good points on low vs. high SGHC
You’ve hit the nail on the head. It’s important to know what to expect and what is best for your area when thinking of buying windows.
Wow
If heat-gaining windows are recommended for a 9,000 degree day area, I better find a company who will make some for me. Our dd is 14,000, and this could be important. I'm glad I found this site; I'm learning a lot and challenging pre-conceived ideas (like radiant floor heat in a super-insulated house.) Much to learn. j
determining the degree day number
How do you calculate the degree day number that I see used in many of these entries? Also, I'm a little confused. Should high SHGC windows be used only for south facing, only for north facing, or for both? What about east and west windows?
Degree Days
Steph,
Heating degree days are based on climate. They are usually listed by city; for example, ASHRAE Fundamentals has always maintained HDD information.
There are several Web sitest that include HDD information, including
http://www.degreedays.net/
http://www.huduser.org/portal/resources/UtilityModel/hdd.html
"SHGC" is an acronym that stands for "solar heat gain coefficient." It can be caluclated for any type of glazing. Some windows have a high SHGC, some have a low SHGC -- but every window has a SHGC.
The best SHGC for a home's windows depends on climate, orientation, and whether or not the windows have summer shading. There is no single answer to your SHGC question.
a replacement window consumer looking for answers
Thanks for your reply. I apologize because I am very much a novice at all this, but we have a house full of windows that are going to need to be replaced over the next few years and I would like to make an educated decision before investing a bundle. I have been reading as much as I can and ran across your website this weekend. Up until then, I had believed that the recommendations based on the government tax credit were what I should follow. Now I'm second guessing it all. The trouble is that I'm not an expert. How does an average consumer figure out what are the best windows for their particular house? Are there consultants that I should be able to locate? Where do I begin? I did review the sites you recommended for degree days and, if I understand correctly, the location of my house has 7200 heating degree days and 1100 cooling degree days. I live in Boise, ID. Almost all of my windows face the south and we do have a large overhang.
Is it true that the low-E glazing reflects summer sun and lets in the heat of the lower winter sun? That is what one of the Marvin brochures indicated. But if the window has a low SHGC, then doesn't that mean that the glazing does not even allow much of the winter solar gain?
Thanks again for your help.
Low-e glazing
Steph,
No, it is not true that "low-e glazing reflects summer sun and lets in the heat of the lower winter sun."
The effect of a low-e coating is to lower a window's U-factor. That's good. However, knowing a window's U-factor tells you nothing about its SHGC. You can have a low-e window with a low SHGC, and you can also have a low-e window with a high SHGC.
If you want a window that doesn't admit solar heat, choose a window with a low SHGC. If you want a window that admits solar heat, choose a window with a high SHGC. There is no such thing as a window with a variable SHGC (except for a few experimental windows that use an electrical current to vary the window's tint — windows that aren't sold for residential use). A window can't tell the difference between the winter sun and the summer sun.
You can limit summer overheating on the south side of your house by adjusting the roof overhang so that the windows are shaded during the hottest time of the summer and unshaded in the middle of winter.
In Boise, Idaho, you probably want windows with a high SHGC on the south side of your house. The only exception would be a house with a very large amount of south glazing that suffers from summer overheating.
manufacturers
This is a great site. Thanks to all who post here. I'm in Baltimore, with 4500 heating degree days and 1100 cooling degree days. I'm planning an addition that will have many south-facing windows. I'm thinking I should seek out high SHGC with low-E to maximize passive wintertime solar heat. Would you agree this this? I see that Cardinal lowE2-179 and Guardian ClimaGuard make the glass, but I can't find a manufacturer who puts it into an all-fiberglass window. Anyone know of a well-built window with this glass? Maybe I need to look to Canadian window manufacturers?
Fiberglass windows with high SHGC glazing
Richard,
If you want a fiberglass window with high SHGC glazing — look north. There are several good Canadian window manufacturers to choose from, including Thermotech, Inline, Fibertec, Accurate Dorwin, or Duxton.
Thank you, Martin. I will
Thank you, Martin. I will look into them. I knew of Thermotech already, but would like to see if the others might make single- or double-hung windows.
R-value
I've started talking to Serious Windows and Thermotech. I've started to see R-value numbers floating around. What does that refer to?
R-values
Steph,
A window's R-value is the inverse of its U-factor. In other words, R=1/U and U=1/R.
SHGC + VT
When I visited NESEA last week I made sure to leave our building plans with Accurate Dorwin, Inline, Armaclad (which I later regretted since I've researched heat mirrors) and Marvin Integrity.
Everything I've read here and other research indicates that if I want an SHGC of .4 and VT of at least .79 I should try to get LoE 179 (Cardinal). We are building a passive solar house, so I want these specs for the southern windows and will settle for NFSC 3, 3 elsewhere.
I just heard back from Marvin Integrity and they said LoE 179 was not an option. LoE in the 200 and 300 was available, with abysmal visible transmission.
I refuse to live with dark windows!
If I remember correctly, Dorwin also has a low VT.
What else can I do? We start building in 3 weeks. Please, your advice.
High VT windows
Vita,
You set a high bar for VT — which is OK, it's your house. If you're aiming for a VT of 0.79, I think your only choice is clear double glazing (with no low-e coatings or argon). That is an available option from many window manufacturers. According to the Thermotech Web site (www.thermotechfiberglass.com), their casement windows with clear double glazing have a U-factor of 0.42.
That's a pretty lousy U-factor, of course, but it will give you the very high VT (glazing-only VT of 0.81) you are seeking.
If you want a much better U-factor -- for example, a whole-window U-factor of 0.23 -- you could choose LOF EA2 glazing. But then your glazing-only VT drops to 0.68.
Your list of manufacturers could be expanded. Try Thermotech (www.thermotechfiberglass.com), Duxton Windows, and Fibertec as well as the ones you listed.
Window Comparison
We have struggled to find relevant information to compare energy efficient windows. We finally went through the exercise to put all of the information in one place. You can find our window comparison spreadsheet with performance info and pricing on our website. http://energetechs.com/resources/window-comparison-spreadsheet/
Feel free to comment or send us information on any other windows you have data on.
The link is deceased. I replaced over the last two years my south and west facing windows with triple glazed windows with not one but two layers of low-e glass. Frankly I can't see the difference visually. It makes me wonder whether I got any low-e glass at all. I bought a cheap infrared thermometer which shows a one degree C higher temp for the windowglass than the plastic frame. This was at night at a temperature of -15C outside. Today, sunny and 8.5C outside the window and window frame inside in direct sun are about 7.5C hotter than the frame not in direct sun. The most noticeable difference are the absence of leaks around the windows. When I installed them 25 years ago I stuffed the gaps around the window frame with fibreglass insulation which had become riddled with one eighth inch wide tunnels, made by insects judging from the casings left behind. Also forget about talking to anybody through the window unless you open it. As far as I am concerned the jury is still out on the value of the low-e glass.
Response to Russ
Russ,
My response is the same as the last time you posted this link on a different GBA page:
As far as I can tell, you forgot to answer an important question: are these whole-window specifications (including the frame) or glazing-only specifications? As you probably know, European manufacturers usually provide glazing-only specifications (especially for SHGC), while US manufacturers usually provide whole-window specifications (as required by the NFRC).
Without a clear statement on this question, it's hard to know whether the spreadsheet is useful.
Low-E 179
Your article recommends Low-E 178 as one option for High SHGC. The Marvin website offers Low-E 179 again for high SHGC. Is their a substantial difference?
Response to Gib
Gib,
Cardinal has introduced Lo-E 179 as a replacement for Lo-E 178. It's just as good as, or better than, the product it replaces.
Thanks for your post. I will correct the text of the article.
Window tuning for northern climes
Excellent article outlining all the reasons for properly tuning windows according to climate rather than convention, and good resources to boot. I have been advocating for 5 years that we consider window orientation and evergreen and deciduous tree cover to optimize solar heat gain in our heating-dominated climate - with only limited success in part due to the difficulty of finding even semi-convenient sources for glazing. Many folks just don't want to mess with it regardless of the economic or sustainability consequences. More articles like this one may begin to educate enough people to actually see the trend shift towards thoughtful window tuning.
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