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Choosing Triple-Glazed Windows

Balancing U-factor and solar heat gain

Posted on Mar 26 2010 by Martin Holladay

Since 1977, when Sweden introduced its stringent energy code, almost all new homes in Sweden have been equipped with triple-glazed windows. Here in the U.S., where energy codes are more lax, triple-glazed windows are still rare.

For a minority of U.S. builders, however — especially cold-climate builders of superinsulated homes — triple-glazed windows are considered essential. Since few U.S. manufacturers offer high-solar-gain triple-glazed windows, most Americans get these windows from Canadian manufacturers.

Look for a low U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. and a high SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1.
In any climate, a window with a low U-factor performs better than one with a high U-factor. The lower the U-factor, the better. (For more information on low-U-factor windows, see “Passivhaus Windows.”)

Most cold-climate builders want windows with a high solar heat gain coefficient(SHGC) The fraction of solar gain admitted through a window, expressed as a number between 0 and 1. (SHGC) and a high visible light transmittance (VT). After all, solar gain helps heat a house during the winter. During the summer, when solar heat gain is less desirable, a properly sized roof overhang will shade south-facing windows during the hottest hours of the day. (For more information on the desirability of windows with a high SHGC, see “High-Solar-Gain Glazing” and “Windows That Perform Better Than Walls.”)

Most low-U-factor windows have a SHGC that is unacceptably low — at least for cold climates. Designers of cold-climate houses have to balance conflicting needs — the need for windows with a very low U-factor and as high a SHGC as possible. Most builders end up choosing triple-glazed windows with a U-factor ranging from 0.19 to 0.26 and a SHGC ranging from 0.39 to 0.47.

The higher the visible light transmittance (VT), the better. Windows with a low VT look gray and depressing. To get an idea of what range you’re looking for, consider the advice of Robert Clarke, a technical specialist at Serious Energy and the former president of Alpen Windows. According to Clarke, any window with a VT below 0.40 “would not be ethical to sell as clear glass.”

When comparing U-factor, SHGC, and VT specifications between manufacturers, be careful. While leading manufacturers provide whole-window values (as required by NFRC labeling rules), less reputable manufacturers often trumpet glazing-only numbers. These glazing-only numbers often seem to show much better thermal performance than whole-window numbers, but they are misleading. To avoid comparing apples to oranges, insist on whole-window specifications that include the window frames.

Suppliers of triple-glazed windows
The five leading Canadian manufacturers of triple-glazed windows with pultruded fiberglass frames are Accurate Dorwin, Duxton, Inline Fiberglass, Fibertec, and Thermotech Fiberglass.

New England builders looking for less-expensive triple-glazed windows have sometimes settled for triple-glazed vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). windows from Paradigm Windows in Maine. Like almost all of the triple glazing offered by U.S. manufacturers, however, all of Paradigm's triple glazing has a very low SHGC.

A new entry into the field of high-performance windows is Serious Energy of Sunnyvale, Calif. Although Serious doesn’t offer triple-glazed windows, it sells low-U-factor windows with Heat Mirror glazing. (Heat Mirror glazing has two panes of glass with one or more suspended plastic films between the inner and outer pane.)

Assembling manufacturers’ information on U-factors, SHGC, and VT is time-consuming. To help out window specifiers everywhere, I’ve created a table showing these specifications for casement windows from seven manufacturers. (This table, or one like it, may or may not appear in my upcoming Fine Homebuilding article on triple-glazed windows.)

Note: The second table includes Energy Rating (ER) numbers; ER is a window-rating system developed in Canada. For more information on ER, see “Windows That Perform Better Than Walls” and Stephen Thwaites March 27 posting, below.

Last week’s blog: “Forget Vapor Diffusion — Stop the Air Leaks!”

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Image Credits:

  1. Thermotech
  2. Inline Fiberglass
  3. Fibertec

Nov 16, 2010 10:03 PM ET

Response to Bernard
by Martin Holladay

Assuming their performance specifications are identical, then you should choose the window you prefer based on hardware, appearance, or the reputation of your dealer for service.

Nov 26, 2010 9:51 AM ET

window cost savings

A local solar building designer (Don Roscoe) makes his own fixed windows out of cellular PVC (to make the frame) and IGU's from a local glazing manufacturer (usually PPG).
The cost for a 20sf IGU with triple glazing, 2 lowE, argon fill, & superspacer is around $300ea in quantities of 5 or more.

Nov 26, 2010 10:39 AM ET

1 15/16" thickness better than 1 3/8"

I modeled a few window configurations in LBNL Window 5.2.
I used 3mm clear glass (ID# 9801) for the outer lite, and LOF-EA2 (ID#9921), and 90% argon fill. A good manufacturer who uses a Sparklite tester will have an average of 90 to 95% argon fill.
With an overall thickness of 1 3/8", that leaves room for 1/2" between the glazing layers. The result in Window 5.2 is a COG U-value of .159 Btu/h-ft^2-F.
Increasing the inner gap to .563" improves the U-value to .157.
With the inner and outer gap at .563", the U-value is still .157, as it is with .563" and .625".
With both gaps at .625", the U-value is .158.
I think the best configuration is .688 and .787", which gives a COG U-value of .161.
With CEN environmental conditions, which are more in line with the typcial heating season of Toronto than NFRC-2001 conditions, the U-value is .144. With CEN and 2 gaps of 1/2", the COG U-value is .163.
When looking at whole window U-values instead of COG, there is an even bigger improvement.
Even with a superspacer, the U-value through the spacer is much worse than COG. So a total spacer thickness of 1.5" will have 33% less heat loss than 1" (2 x 1/2" spacers).

Nov 26, 2010 8:04 PM ET

New TripleGlazed Windows
by David Jenkins

We recently replaced all of our old large casement windows with triple glazed windows from Loewen and Accurate Dorwin. Our triple glazed Accurate Dorwin windows cost about the same as double glazed Marvin fiberglass windows and the Accurate Dorwin quality is excellent! The Loewen windows were a bit pricier, but that is to be expected due to the douglas fir construction. We are thrilled with these windows.

Dec 13, 2010 6:30 PM ET

choosing windows for each side of a VT passive house
by Jay

I'm in the final stages of my design. I've gotten some good advice from this forum so far (thanks again, esp to Martin) but am in a little bit of a quandary about how important it is to select for different U and SHGC dependent on which side of the house the windows will go on.

The house will be approx 30ft x 48ft with the long axis aligned to true East-West. The south side will get full sun all day. The west side is also unshaded. Both south and west walls will be walk out basement with ICF, north and east basement will be buried. The east may be shaded early in the AM but if the sun ever decides it wants to shine while I'm up on my lot then I can probably identify and remove some trees to reduce or eliminate this.

I'm probably going to go with either Inline 325 (either Dual Low E Hard Coat or Triple Low E Hard Coat x2) or Thermotec (possibly 211 gain #3, 322 gain #3 #5 or 321 gain #5). Most windows will be out swing casements (as I understand these seal better against air infiltration) and a few fixed (like on the top of the west wall gable end) plus one sliding door, 2 double french doors and 2 inswing standard doors type TBD.

I've seen a few things here saying that even for north facing windows the higher ER rated ones can have a net gain due to passive light. Only one paper was cited and even that paper seemed to be based on a single side by side study. So is there really any strong, conclusive evidence that choosing a higher ER rated window for the north side (as opposed to going with optimizing U values without concern for SHGC)?

Likewise, on the East and West walls can anyone give me an approximate quantification of the effects of choosing windows that balance U and SHGC to maximize heat gain versus just optimizing the U values alone?

Is it only a modest gain like 10% or less, or despite the fact that at least on Dec 21st the sun won't even directly strike those walls and for much of the rest of the winter will still be at a low angle to them, can you really achieve a substantial heat gain on the East and West sides of a home?

BTW the reason I ask is because sometimes discussions on the various energy efficient building forums I've seen focus on what is technically possible and don't always consider the cost vs benefit trade offs. Or at least it isn't always apparent from the discussion whether the tradeoffs were considered.

Because these kinds of windows are pricey (initial cost estimates appear to be about $60/sf based on casing dimensions) IMO I'm not convinced it makes sense to spend an extra 20% or more (which can be $300 or more per window) to buy windows that maximize heat gain on certain walls when it might take 20yrs for that extra heat gain to pay for the additional cost.


Dec 13, 2010 7:29 PM ET

Response to Jay
by Martin Holladay

1. Don't sweat the decision on the north windows; it doesn't really matter what the SHGC is to the north.

2. It's fine to use low-SHGC on the west side if you want, even in Vermont.

3. South, of course, must be high-SHGC.

4. In Vermont, I believe that high SHGC windows make sense on the east side. Mornings are cool, even during the summer, and a little morning heat is almost always welcome.

5. When in doubt, model your options with RESFEN or PHPP.

Dec 30, 2010 11:58 AM ET

ER vs u and SHGC in comparing windows for passive solar
by Jay Hersh

Thanks for the tips.

For the Canadian windows I notice they use a rating system called ER. From what I've read it's supposed to account for the U and SHGC values and give you an indication of total performance based on calculations done for a standard window size. If I understand this correctly it shows you the relative net gain of a window.

So, for example, a window with a lower SHGC can actually outperform one with a higher SHGC (for passive solar applications) if it has a higher U value that's enough to reduce heat loss by a large enough amount to more than offset the lower passive solar heat gain, and thus result in a higher net total gain.

The windows I'm considering have ER values for casement types starting at 30 and going to 41 and for fixed starting at 30 and going to 48.

Are there any caveats to know when using ER as a means of comparison?


Dec 30, 2010 12:17 PM ET

Response to Jay Hersh
by Martin Holladay

I think you have a good understanding of ER. If you live in a cold climate and you care about solar gain during the winter, the higher the ER, the better.

I don't know if it's a caveat, but an argument can be made that ER is not very relevant for north-facing windows. I could easily understand a decision to specify a north window based on U-factor without regard to ER.

Jan 17, 2011 9:09 AM ET

best windows for sound
by Domenic

live on highway in new house sound driving me crazy...some recommendations pls

Jan 17, 2011 9:23 AM ET

Response to Domenic
by Martin Holladay

I suggest you post your question on our Q&A page. That way you are more likely to receive answers from GBA readers. Here's the page:

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