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Musings of an Energy Nerd

Choosing Window Glazing

How to specify the right U-factor and SHGC for your climate zone

New windows are usually delivered with an NFRC sticker affixed to the glazing. This Marvin window (left) is typical; the black-and-white NFRC sticker can be seen at the lower right side of the window pane. A closeup photo of the sticker is shown on the right. This window has a U-factor of 0.20 and a SHGC of 0.27.

If you are choosing windows for a new home, you have a lot of decisions to make. You’ll need to decide on the frame material (wood, vinyl, or fiberglass?) as well as the operational style (fixed, double-hung, slider, awning, or casement?). Not all manufacturers offer all options, so anyone seeking something unusual has to research the products of many different window manufacturers.

If you are a bewildered homeowner or builder, the easiest way to tackle these choices is to take a few small bites at a time. In this article, I’ll focus on glazing options.

Reviewing the basics

The last time I wrote a primer on glazing was in 2010, when GBA published All About Glazing Options. Most of the information in that article remains relevant today, so curious readers may want to start with that article.

In this article, I won’t attempt to repeat all of the basic information I presented in 2010; however, here are a few important bullet points:

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  1. bwsct | | #1

    HI Martin,
    Thanks for this great article. I'm remember reading somewhere you had written an article about how to choose a window manufacturer. I'm a homeowner who has gone down the rabbit hole of choosing which window manufacturer to use. I'm leaning toward Tilt/Turn windows in climate zone 5A but there are so many different companies to look at.

    Could you provide a link to that article if its something you've written about in the past and it will give me invaluable info along with this article.

    1. MartinHolladay | | #2

      You might want to read these two articles:

      "How to Order Windows"

      "What Windows Should I Buy?"

  2. russellchapman | | #3

    Great stuff Martin, as always. Thank you.

    A few thoughts. Any US or Canadian manufacturer who has paid to have their products rated by the NFRC is almost certain to apply the NFRC sticker for the whole window. I've heard that's actually required by the NFRC as a participant, but haven't confirmed that fact myself. Either way, the testing is crazy expensive and very few manufacturers will make that investment and then not be promoting it.

    In my experience, whenever I've seen an NFRC label showing the center of glass it was simply not removed when the manufacturer bought insulated glass from someone else. By rights, they should be removing that label and replacing it with one representing the whole window data once they've glazed it into a sash/frame, but sometimes this may be accidently missed. In MANY cases I've seen smaller manufacturers who do not test their products to NFRC standards intentionally leave these labels on there to provide something, anything. The description on the label should say if it's an IG or whole window if you look close, but most folks don't.

    Lastly, not an advertisement for Cardinal Glass by any means but all the manufacturers you mentioned here use them so let's unpack. Cardinal commonly offers LoE 180, 270/272 (nearly identical), and 366 as it relates to this discussion. Yes, they do a ton of others but through the lens of SHGC these are your top choices. 180 is a single coat of silver oxide that offers a visible light rating of 80 ON ONE SINGLE PIECE OF GLASS. 270/272, two layers of silver per coating stack and VT follows the pattern. 366 is a triple layer of silver per coating stack and that single piece offers a VT of 66. The lower the VT, the lower the SHGC. In other words, the high SHGC product would want to look at using 180, the low 366, and if you don't know shoot for the middle with the traditional option of 270/272. All the specific branding you mention above is confusing to no end, but not every manufacturer subscribes to the policy of overcomplicating things. I spend far too much time translating this for architects, it should be standardized across the industry imo.

    PS, the best way to see the NFRC data these days is to request a quote from your local retailer, and tell them you want the NFRC performance data. Most everyone's quote tools have this info and it's the click of a button away on your actual quote usually. Cheers and thank you again Martin. I always appreciate your articles on glass & glazing.

    1. MartinHolladay | | #4

      Assuming that a contractor orders windows from an American window manufacturer, it is certainly true that the paper stickers on windows delivered to a contractor's job site include NFRC ratings for the whole window -- so you are absolutely right about that. Whole-window ratings represent the standard method required by NFRC for reporting U-factor and SHGC numbers. That's all fine and good (and, for the record, I never claimed that these paper stickers have center-of-glass numbers; they don't.)

      The problem with center-of-glass numbers is that they show up on window manufacturer web sites (most egregiously on the Andersen web site), not on the paper stickers.

      You're also right, unfortunately, that "the best way to see the NFRC data these days is to request a quote from your local retailer, and tell them you want the NFRC performance data." While this statement is true, it isn't particularly helpful information for comparison shoppers. These days, builders can choose from 30 window manufacturers or more, and requesting quotes on all 30 is an extremely inefficient way of discovering performance information -- inefficient for the contractor, and no fun at all for the local building supply reps, who don't relish frivolous requests for quotes.

      The creation of the NFRC standard was intended to make it easier for architects, builders, and homeowners to select windows based on window performance. For now, these numbers are extremely difficult to discover -- which means that the goal of the creators of the NFRC system has not yet been achieved.

  3. Expert Member
    Michael Maines | | #5

    Excellent overview, Martin. My one quibble is regarding cost of triple pane. While they always cost more when comparing double glazing to triple glazing among the same product lines, I can get a triple glazed Mathews Brothers window for half the price of a double glazed Marvin Elevate (as one example). Whether that's a fair comparison I can't say; they are very different windows. But you can get pretty affordable triple-glazed windows.

    1. freyr_design | | #6

      I agree, I think the costs are not as substantial as they perhaps once were. I recently got a bid looking at both double and triple in the exact same frames and the cost was 13% difference. While this is not nothing, it seems well worth it in most instances (in my opinion) from solely a comfort standpoint

      One other thing I think might be worth noting is the derating of U value over time as some of the more exotic filler gases leach out. Many of these super low U-value windows use these to achieve these numbers, which you pay a premium for, and which benefit seems fleeting.... or perhaps this is not an issue?

  4. [email protected] | | #7

    "180 is a single coat of silver oxide that offers a visible light rating of 80 ON ONE SINGLE PIECE OF GLASS. 270/272, two layers of silver per coating stack and VT follows the pattern. 366 is a triple layer of silver per coating stack and that single piece offers a VT of 66."

    Actually the VT ratings (as well as U and SHGC), are based on an IG unit comprised of two lites of 3mm float glass and not just a single coated lite. VT for 270 and 272 is 70% and 72% respectively, but LoE-366 is actually 65% due to a minor tweak a long time ago, and 180 is actually 79% VT because it replaced the original LoE-179 and was labeled as 180 because 179 already existed.

    1. russellchapman | | #12

      thanks for setting me straight!

      1. [email protected] | | #22

        You’re welcome. As Martin pointed out, the details of the various products can be very confusing for anyone not closely associated with the specifics.

  5. Deleted | | #8


  6. [email protected] | | #9

    In defense of companies not listing NFRC data on their website, while this is relatively doable for smaller companies that offer a limited selection of products, larger companies like Andersen or Marvin have literally millions of different performance value combinations within the NFRC depending on window type or style plus different product lines within each style. Unfortunately, there really is no remotely manageable way that they could offer anything of value or to that level of specific information directly to potential customer's on a company website, leaving the NFRC website as the only really viable option for researching product performance.

    Taking just Marvin's picture or fixed window lines alone, there are 54 different window lines within this category, each with thousands of individual window performance options available. Looking specifically at Marvin's Ultimate Wood Casement Picture Window (1 of 54 styles listed, selected at random - CPD MAR-N-355), there are over 27,000 individual performance value entries (product specific CPD numbers / product descriptions) just for this single window line. Multiple glass packages and window size differences result in U factors within this window line ranging from .15 to .46 and SHGC from .11 to .61. VT, condensation resistance, and other performance values vary just as much as U factor and SHGC within the product categories.

    For Marvin, or comparable size company, to try to offer and overview of performance values of their specific products on their website with any level of inclusiveness is simply not realistically feasible, and while listing center-of-glass on a company website might offer some information, ultimately that certainly isn't the answer either since as long as the identical glass package is installed, CoG performance would be the same regardless of window size, style, or sash and frame material used in window construction. Pretty much worthless for predicting actual window performance. While center-of-glass can be a useful tool for comparing glass packages alone, it's not something that would be consumer friendly or of any real value for them.

    1. MartinHolladay | | #10

      You have articulated a problem that I am aware of. That said, as an English major and editor, I feel there is a solution. Here's a stab at a solution: use English!

      Let's try this: "Here at Martin Windows, we offer a variety of glass packages, but the following six types of glazing are by far the most popular. Precise NFRC values for U-factor and SHGC can be provided by your local window dealer on request, but the following information may prove useful. This information is based on a window that measures 30 inches wide by 40 inches high; of course, varying the window dimensions will result in different values.

      1. If you choose Glass option 1 (double low-e glazing with moderate SHGC), the NFRC rating for U-factor will be A [a stated value], and the rating for SHGC will be B.

      2. If you choose Glass option 2 (double low-e glazing with a low SHGC), the NFRC rating for U-factor will be C, and the rating for SHGC will be D.

      3. If you choose Glass option 3 (triple low-e glazing with a moderate SHGC), the NFRC rating for U-factor will be E, and the rating for SHGC will be F.

      4. If you choose Glass option 4 (triple low-e glazing with a high SHGC), the NFRC rating for U-factor will be G, and the rating for SHGC will be H.. ..."

      ...and so on.

      In other words, try to give customers some information. I know it's possible.

      The other approach is the Loewen approach -- make a searchable table, like the NFRC table, but restricted to one manufacturer's offerings, and using plain English to narrow down options.

      You wrote that listing NFRC data "is relatively doable for smaller companies" but harder for "larger companies like Andersen or Marvin." But remember -- larger companies have more economic resources to tackle the problem and provide a technical solution.

  7. [email protected] | | #11


    I really like your approach. I think that your idea could be very useful and easily doable as a place to start for people who are looking for window selection guidance. Of course my cynical side would suggest that a percentage of marketing folks might be less inclined to offer that level of easily accessible and comparable performance detail, but I definitely think that others might see it as a plus, especially if they were selling the product with the best numbers.

    I have attached another possible option that came to me after I read your reply, intended for people who have a compulsive need to compare numbers side-by-side (looking in the mirror here), illustrating performance comparisons of three different glass packages used in four different window styles.

    The performance values listed are totally fabricated and not intended to suggest any window company or product.

    1. Expert Member
      Michael Maines | | #13

      Perhaps this information is something GBA could host rather than relying on individual companies to all follow the same format?

      1. MartinHolladay | | #20

        I can certainly imagine GBA assembling the information you'd like to see. But it would be a Herculean task, and a frustrating one.

        In general, window manufacturers don't readily provide technical information to journalists or editors. "Why not?" is a mystery, but it has to do with marketing departments informing engineers that they are not allowed to speak to journalists.

        Manufacturers establish policies that instruct all employees to direct all questions to the marketing department -- and as a result, a journalist is stuck talking to someone with a marketing degree.

        1. Expert Member
          Michael Maines | | #21

          Martin, not all window manufacturers make it difficult. It's easy to find tons of data on the Sierra Pacific website, for example: It's the first hit that comes up searching Google for Sierra Pacific Window Performance Data. Or, starting from their home screen, it's three fairly intuitive clicks away. Perhaps a difference is whether the company's customers tend to care about the numbers.

          As a designer, I often ask window company architectural reps questions and they have always sent me performance values if I have asked. More often, I send them target ranges of values and they send me what they offer that's close to what I want. I order the meal, not the ingredients.

          1. MartinHolladay | | #23

            Thanks for your comment. I'm really glad to hear of window manufacturers that are doing better than average when it comes to sharing performance data. Hats off to Sierra Pacific.

    2. MartinHolladay | | #18

      Your suggested table is even better than my suggested words. Let's hope some window manufacturers have marketing experts who read GBA.

  8. AC200 | | #14

    I think it's in some companies interest not to have the data accessible. A lot of people don't really know or care to learn about LoE IGUs. They think as long as it's LoE argon they are all the same. Let alone understand the I89 extra hard coat on the inside surface.

    I asked for performance details before I bought my windows and they sent me a a 10 sheet spreadsheet. It made choosing easy based on the tradeoffs Martin outlines, but you need to want to know and ask for it. I don't know of many window sales people that will walk though every orientation and room use with their customers. Manufacturers should at least make a spreadsheet like that available for those who ask.

    1. OldBean | | #15

      My sales person (also the company's owner) submitted a proposal for "high efficiency windows". He was flummoxed when I asked for NFRC data for the several lines of windows (one manufacturer) he was selling. He said no one ever asked for that information. He said they are lowE with argon which is high efficiency. Of course, his installers do not use foam either. They use the pink stuffing. He has been in the window business, quite successfully, for 20+ years. I'm sure he is not a rarity.

    2. MartinHolladay | | #19

      It sounds like you got excellent service from your window rep; that's great. It would be encouraging for the industry if the type of service you received were more common.

      That said, the service you received is only helpful if a customer has narrowed down his window selection to a single manufacturer and a single line of windows. If someone is shopping for the best choice among a variety of manufacturers, the type of spreadsheet you received doesn't exist.

  9. Trevor_Lambert | | #16

    My personal experience is that high solar gain is still a good idea in my climate (SW Ontario). The advantages in winter are self evident. In the summer, despite having higher than the average amount of glazing, my cooling needs are dictated by humidity removal. Any amount of reduction of sensible heat by shading or lower solar gain glazing would be counterproductive.

    1. Expert Member
      MALCOLM TAYLOR | | #17


      You got your user-name back!

  10. user-6544780 | | #24

    Hi Martin,
    I have a post and beam house with SIP panels, 6" in walls, 10" in ceiling. I have double paned windows which on cold nights leave condensate on the bottom of the window, seeping into the wood and molding. This despite ventilation that always keeps humidity well under 50% indoors. I think people building a tight house need to know that double pane windows aren't good enough, I wish I had done triple pane.

    Design/Build firm Knickerbocker Group in Boothbay, Maine are building small 500 and 800 sqft ADU's with beautiful triple pane windows from Europe. They say, the windows are less expensive than Marvin double glaze. We need to get the word out and start moving to Triple glaze windows.

    Valli Genevieve

    1. MartinHolladay | | #25

      Valli Genevieve,
      You didn't mention your climate zone. The colder the climate, the more that triple-glazed windows make sense. I certainly agree that anyone who can buy triple-glazed windows for less than the price of commonly available double-glazed windows should buy the triple-glazed windows -- that's a no-brainer.

      I'm not sure why your windows have so much condensation, but there are several possibilities. One possibility is that your hygrometer isn't giving an accurate reading of your indoor relative humidity. Another possibility is that you are using curtains or blinds at night -- either of which can increase the chance of condensation.

      1. user-6544780 | | #26

        Hi Martin, I am in Rockland, Maine. it pretty much follows the outside temperature. If it is below 25 F outside, we see condensation at the bottom of the windows. We don't use shades or curtains, we did, but you are right, it makes the issue worse. We also thought about interior storms, but your cautions about how unlikely they are to be tight enough to prevent moisture between them and the window, have given us pause. We are stuck wiping dry the windows each cold morning and using a weak solution of bleach and water to clean the lower frames to kill the mold. Other than that, our tight, passive solar house, works beautifully. Dehumidifiers work but not enough if its really cold, and would make the interior air too dry anyway. Not to mention the use of so much energy to run them. Best solution by far, if I had known 8 years ago, when I designed and built my house, was to spring for European Triple pane windows. Building a tight house now, there is really no reason not to, and this condensation problem doesn't exist in the Maine houses built to PassivHouse standards.

        Valli Genevieve

  11. RosetteRemodeling | | #27

    What about air infiltration ratings? Doesn't this significantly impact the window's performance? A casement window performs better than a double hung window with identical U-factor and SHGC. When I get a window quote I can often find the U-factor and SHGC but not air infiltration. Is it a pipe dream to think I could compare windows on that metric as well? Thanks for another great article!

    1. Expert Member
      Michael Maines | | #28

      Air leakage is currently not reported very finely on NFRC labels; they provide three options but any decent window should fall into the lowest category, ≤0.1.

      Everything else equal, a double hung will typically leak more air than a window that closes against a gasket (awning, casement or tilt/turn) but I've had houses with double-hungs test extremely low for air infiltration (0.12 ACH50) so I don't consider the difference to be as large as some people do when it comes to new, high-quality windows.

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