GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted
Energy Solutions

Window Performance 4 — Dealing with Edge Losses

Warm-edge glazing spacers and high-performance windows

Image 1 of 2
Warm-edge spacers. Edgetech makes the Super Spacer silicone foam glazing spacer, the most effective spacer for slowing heat loss.
Image Credit: Edgetech
Warm-edge spacers. Edgetech makes the Super Spacer silicone foam glazing spacer, the most effective spacer for slowing heat loss.
Image Credit: Edgetech
Comparison of interior glass surface temperatures at the glazing edge with different types of glazing spacers.
Image Credit:

Over the last three weeks I’ve focused on the major strategies for improving the energy performance of windows: adding extra layers of glass, increasing the thickness of the airspace between the layers of glass, adding low-emissivity coatings, and replacing air with a low-conductivity gas fill. These strategies all help to reduce heat flow through an insulating glass unit (IGU), and if we do a really good job with these strategies we can achieve center-of-glass R-values of R-5 or higher.

But these measures don’t do much to improve the energy performance at the edges of an IGU.

In the olden days, when windows were single-glazed and wood-framed, the window sashes insulated better than the glass. With the air films on both sides, an inch-thick wooden window sash provides about R-2, while a single layer of glass provides just half that. When we switched to double glazing, the glass and wooden sash insulated about equally.

With the advent of low-e coatings and low-conductivity gas fills, though, the glazing itself became better insulating than the frames and edges of the glass. All of a sudden, instead of the glass being the weak point, in terms of heat loss, the glass became better-insulating than the edges of the windows. A significant culprit of that window-edge heat loss is the heat-conducting glazing spacer that holds the two pieces of glass apart.

Better glazing spacers

Until recently most glazing spacers were made of hollow aluminum channel. Aluminum is an easy material for manufacturers to work with, and the cavity formed by the channel allows a desiccant to be added that adsorbs any water vapor that gets into the insulating glass unit (IGU) during manufacture.

The problem with aluminum is that it’s highly conductive, readily transferring heat from the warm inner pane of glass to the cold outer pane. Because of this heat loss, the inner pane of glass often cools off enough that water vapor from the indoor air condenses on it — and you get droplets of water forming on the inside of the window. If you have wood windows, that condensate often wets the wood, causing staining or even rot.

We indicate risk of condensation forming on a window using a standardized measure from the National Fenestration Rating Council, “Condensation Resistance.” This is expressed as a number between 1 and 100, with higher numbers indicating greater resistance to condensation.

So, what to do about it? Manufacturers have worked hard over the past several decades to deal with the problem. Here are the primary options:

Stainless steel

Stainless steel is just 1/15th as conductive as aluminum. Furthermore, stainless steel is a lot stronger, so glazing spacers made out of stainless steel can have thinner walls. Conductivity is proportional to the cross-sectional area of the material through which heat is flowing, so stainless steel glazing spacers are better for two reasons: lower conductivity and thinner walls. Indeed, stainless steel is rapidly displacing aluminum as the leading glazing spacer material.

Butyl rubber

Butyl rubber is a great sealant because it sticks really well to glass and other materials, and it’s also a good insulator. Rubber is 120 times less conductive than stainless steel and 1900 times less conductive than aluminum. To work as a glazing spacer, a thin reinforcing metal strip is often used to maintain the proper thickness. The strip of metal increases the conductivity (though the metal never contacts the glass); the spacer’s conductivity remains a lot lower than an all-metal spacer. A desiccant is incorporated into the butyl rubber.

Swiggle Seal, the first so-called “warm-edge spacer,” was introduced in 1979. The name refers to the thin ribbon of metal reinforcement that is in a wavy shape. While the edge of an IGU with low-e2 and a standard aluminum spacer has a condensation resistance of 19.3, according to testing done by Enermodal Engineering, with butyl rubber and a metal strip that condensation resistance improves to about 38. Swiggle Seal is manufactured by TruSeal, which is now owned by Quanex Building Products Corporation. While still found in some products, the success of stainless steel spacers has dampened the market for butyl rubber spacers in recent years.

Silicone foam

The least conductive glazing spacers are made of silicone foam. These inorganic foams don’t soften as much as butyl rubber and lose their shape, so they don’t require strips of metal reinforcement. Like the butyl rubber spacers, a desiccant is formulated into the silicone foam.

The dominant product on the market employing this technology is the Super Spacer, made by Edgetech in Cambridge, Ohio (which is now also owned by Quanex. Super Spacer is made of silicone foam with no metal reinforcement. Several additional layers are added to make the foam impervious to vapor — both to keep water vapor from getting in and to keep any low-conductivity gas fill, such as argon, from escaping. The condensation resistance of the above-described IGU with this glazing spacer is 44.9.

Bottom line

Along with minimizing the risk of condensation at the edges of windows, warm-edge spacers will improve the overall unit U-factor of a typical residential, double-glazed window by about U-0.02 Btu/hr·ft2·°F. For example, if the unit U-factor with standard aluminum spacers would be 0.30, the warm-edge spacers would reduce that to 0.28. That improvement (reduction in heat flow) might sound modest, but it adds up!

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. Watch for a forthcoming BuildingGreen special report on windows. To keep up with his latest articles and musings, you can sign up for his Twitter feed.


  1. user-1087436 | | #1

    The "Super Spacer" and argon.
    Thanks, Alex, for another informative survey. Question: We are buying windows from a manufacturer who uses the EdgeTech Super Spacer in most of its products. The sales rep, however, told us that the Super Spacer cannot be used with argon gas fill. He says the argon leaks out. According to him, EdgeTech is working on a newer version that will be suitable for use in an argon-filled IGU. Your article seems to indicate otherwise. Do you know anything more about this?

  2. Alex Wilson | | #2

    Silicone foam and argon
    Indeed, silicone foam is fairly permeable to gases like argon. However, I was under the impression that EdgeTech adds layers to the foam specifically to reduce that diffusion. I would be surprised if the current EdgeTech products on the market do not work with argon.

  3. gusfhb | | #3

    edgetech doesn't seem to agree. edit[with lack of argon retention]

    I think manufacturers prefer metal because they are used to it.

    personally I would recommend a double seal system[butyl plus something else] for anything.

  4. djt76 | | #4

    electrified spacers! the ultimate spacer??
    Hope you guys got something out of my post. I was looking up edgetech super spacers last year and i came across a new product. It was a low voltage heating element incorporated right into the spacer originally used in supermarket glass doors. It was actuallu used in residential house as the heat source! I would love to know if anyone has followed up on this developement or if it even makes any sense? I get mostly no as a response but not everyone agrees. what do you think?

  5. user-1087436 | | #5

    Reply to Alex Wilson
    Thanks for your reply, Alex. The rep was from Sierra Pacific Windows, our preferred supplier. We like the looks of their product, and the Edgetech spacer was an added bonus. I'll ask again about argon to be sure.

  6. ToGZ9JmdjB | | #6

    i been thinking about these windows for an upcoming project ( energy Retro )
    the 925 series boasts a Full-Frame Performance U value .11 and an R value of 9.1
    has anyone had any experience with these windows ?

  7. oberon476 | | #7

    this and that (and way too long)
    Silicone foam is permeable to argon, it's also readily permeable to air and moisture. The manufacturer of the silicone spacer uses a significantly less permeable mylar "coat" to cover the silicone foam because of the inherent permeability of the silicone foam.

    Marketing is a wonderful thing. If someone were to look at image 2 of 2 of the graphic that was used for this blog, one would see the amazingly superior energy performance of the foamed silicone spacer versus the other brands listed.

    And while the graphic is accurate when it proclaims "up to 16.6° warmer temperature at the edge of the glass" when compared with the aluminum spacer as shown; what the graphic fails to point out is that the edge temperature comparisons between silicone foam and the other spacers may not be exactly "apples to apples".

    Curiously, the originators of the graphic apparently forgot to mention that the temperatures as given are for clear glass over clear glass for all the spacers listed except the silcone foam where the temperature as given includes the use of a triple silver LowE coating.

    As printed the graphic is technically correct of course. When you compare the edge temperature of an aluminum spacer keeping two lites of uncoated glass apart with the silicone foam spacer holding a triple-silver coated lite away from a clear lite the edge temperatue difference really is close to 17° - which is a really significant difference.

    Of course when the other spacers listed also include a lite with a triple silver LowE coating, then the edge temperature differences tend to become substantially less (and as a disclaimer: I am NOT in any way, shape, or form calling out Alex or his really excellent articles on window and glass performance when I comment on the graphic. It is a graphic, nothing more, and it was included in the article in absolute good faith as a simple illustration I have no, absolutely NO disrespect meant to Alex in any way at all by my comments on the graphic!!)

    There are some really good European window companies that do a really good job of helping to keep heat inside a home. They have U-values that make most North American U-values look anemic by comparison. But, as Martin H. has pointed out on several occasions, once again we aren't always dealing with a straight apples to apples comparison when comparing Euro performance values to NFRC performance values...we don't test the same.

    I would suggest reading Martin's blog's on the subject, but my personal opinion (and take it as you will) is that the Euro testing protocol differences can add as much as 20% to the preformance values of Euro windows when compared to NFRC numbers. I am certainly not trying to suggest anyone is "cheating" in any way, because no one is, the testing is simply different. Once again I would suggest reading Martin's really excellent blogs if they want more information on the subject.

    North American glass packages absolutely match those produced in Europe. Glass quality, coatings, spacers, are quite comparable. Where Euro window makers excel is in their sash/frames. There are no deep dark secrets or magic formulas - but there are philosphy and geographical differences between Euro and North American window manufacturers which result in a different focus on product performance.

    Most folks who visit GBA. I think, are very attuned to the performance values of the various materials and products that they use in their homes...kind of the point of the site. People who are interested in windows are certainly no exception.

    Mention of any of the "big five" Canadian fiberglass window companies, as well as the serious contender in the US, is sure to set off a lively and interesting discussion, often including direct comparisons to Euro windows.

    Mention one of the "big" American window companies on the other hand and many folks tend to dismiss them as using "lower" or older technology...something less than state-of-the-art. And some people ask "why" the big companies aren't leading the way in the energy performance market.

    In the window world (and so NOT talking about the franchise company with the same name), the "big five" Canadian companies are really just niche players. Only one of them (Inline) even ranks in the top 100 list of largest window companies in North America. This is certainly not a bad thing, and I would suggest that this allows them a flexibilty that large companies simply don't have.

    In any industry it is going to be difficult for the largest companies to compete in niche markets where smaller companies can thrive and grow. I think most folks would agree that this is a really good thing...healthy for the economy as well as for people who are looking for those niche products...imagine walmart controlling the world...

    I suspect that at least part of the reason that it has taken a while for some of the big American window companies to get into the "energy performance window wars" is simply because until recently they simply didn't consider that niche to be worth the effort to pursue. I suspect the economic downturn now has them looking in that direction...and now they are looking to add to their market share by getting into those formerly niche markets which may prove interesting considering their deep pockets and budgets.

    Long enough ramble...

  8. djt76 | | #8

    Serious windows.?!?.the best in the world?!?
    According to our fearless leaders they are the best in the world.

    As someone who was recently involved in a Serious windows installation I could say a lot about my experience there. I will only say that you should definitely be doing all your homework on these windows how/where they are made and be very carefull about how you go about decoding all these numbers you keep seeing. Its not just the numbers that matter its the whole package/reliability. The darn thing has to keep the rain out for many years too ya know! We're always so worried about temp we forget that weathering is a huge part of what windows must withstand. I would want my window frames to have been field tested for AT LEAST ten years before i gave it any confidence.

    I think Greg's comments are right on the money btw, I am going to dig into the Canadian windows next.

    Thanks for the perspective Greg. And I will try to keep it short from now on too. I've been thrust into this exciting field and I'm getting a lot of differing opinions seemingly every day. I'm finding this site an execellent resource and a voice of reason. Thanks again. Anyone want to start a new window company?lol

  9. djt76 | | #9

    the whole window package, batteries not included
    Thanks for commenting on my comment Alex and for your interesting articles as well!,

    I have been so busy with selecting the right windows for a passive house project so I'll just share a few insights everyone here.

    I attended the building energy2012 conference in Boston a few weeks ago primarily to meet all the European window reps. One of my first visits was to a Marvin rep set up nicely in the center of the building proudly displaying their triple "pain" line. Well i went right up to this nice young lad and asked him when Marvin will upgrade there spacer tech to a silicone faom based spacer?

    His response was, well non- informative. He looked confused about what i meant but musterd up the line..... "Marvin would never use a cheap foam material like that, we only use metal its the strongest ...."
    Ok we're in Boston at the center of the alternative building/ energy movement here in the northeast! What are you even doing here! Jeeez lets go meet some Europeans already. Very impressed with all their dispalys and each product looked stronger than the next! A local manufacture i spke with told me the big guys here in the states are not going to upgrade their spacers/frames until they absolutely have to, since it would require retooling their production lines. Shame.

    The whole approach to the Eoropean windows is different and the relative "strength" of these windows is unquestionably superior than almost anything I've installed from the North America. I just visited a jobsite in Brooklyn with a rep from the company we chose. We closely inspected the installed double pane, tilt and turn windows for the first time. I left very impressed except for the fact the installer obviously was confused by what he had and was not particularly interested in EF permonance whatever. It was fancy, the end.. These windows are incredibly stout but there are still major issues with them as well. These are: cost, difficult installations, screens, shipping and handling..etc

    I see no reason why these European windows can not be matched by North American manufacturers. When it comes to vynil windows we could build these windows easliy right now, with only some minor tweeks.
    I'm just guessing here but i think there's a vynil clad/wood window co. in the US who could probably produce exceptionally good and affordable window frames and glass if it wanted to. hmmmmm? Where'd that stimulus money go again? Oh that's right what a serious debacle that was/is! Forget the Gov't, industry can lead the way, can't they?

    Here on Long Island there's a all vynil window company advertising great U values, foam spacers and foam filled frames and now up to 50% off your engery bill! Come'on that's total BS TRI-State wf. Anyone who knows anything about windows could confirm that. Please don't kill the real deal!

    The real question is how long do we have to wait for the type of windows maufactured in the US that will have the following four features: very low U value, combined with very high SHG, in a water tight frame, with the best gaskets and spacers available!! This would make building near zero/passive/sustainble A LOT easier! And I beleive it could trigger the type of market growth we need to rebuild America in a logical fashion. I've found some smaller manufactures that are pretty close but just shy of achieving these standards right now. Let's get'er done man! Do we have to sign a pettition or something??

    Bottom Line. The Europeans have been cleaning our clocks in the window department for years and its starting to get embarassing. Wndows are the most difficult aspect of constructing a true passive/near zero home. Do we really need the threat of world wide disaster before we make improvements/changes to our products? I don't get it... (with head shaking)

  10. Stephen Thwaites | | #10

    Spacer Marketting/Fudgetting Continuum
    Maybe i'm naïve but, if one bicycle maker said their bicycle is significantly lighter than a competitor's i would expect that they were both weighed on the same planet – that they were both subject to the same gravitational force.

    Having used Edgetech's sightline temperature comparison for years, I was surprised by Greg Smith's comments about the different baselines for Edgetech's comparison of edge of glass temperatures. But after doing some basic quick comparisons i believe he is correct.

    The temperature range between warmest and coldest edge spacers for the same glass condition is, in fact, much less than 16F. For double glazed units it's likely about 5F-ish and for triple glazed units it's likely about 12F-ish. (this is actually backed up by the data in a graph on Edgetech's site from an NRC study -

    In my view, the SuperSpacer is good enough to stand on its merits. My experience with residential windows is that switching from a metal spacer to a foam rubber spacer DRAMATICALLY reduces condensation related call-backs.

    Introducing a variable baseline is an uneccessary and needless trip onto the marketting / fudgetting continuum.

    Stephen Thwaites
    Thermotech Fiberglass Fenestration

  11. YpVNwkFpwm | | #11

    help with distilling the very tech info you have provided
    I need confirmation in what I thought I read on this site-
    I need to replace roughly 12 windows in my home. I live in Colorado so I am considered 'cold' geographic area for purposes of window climate. I need some commentary on whether I am on the right page. I should be looking for windows with a high SHGC, triple glazing, and high VT.
    It has been SO difficult to find a rep that is fully experienced and truly offers what the best options are for your particular needs.
    Your blog has been so helpful for me….thank you.
    If I have not listed what I should be looking for in my replacement windows- please correct me! Thanks again!

  12. GBA Editor
    Martin Holladay | | #12

    Response to Karen Linden
    You're on the right track -- except that you may be better off choosing orientation-specific glazing. (In other words, your west windows may need different glazing from your south windows. In many cases, west-facing windows need glazing with a low SHGC.)

    For more information, see:

    All About Glazing Options

    Choosing Triple-Glazed Windows

    High-Solar-Gain Glazing

  13. YpVNwkFpwm | | #13

    Your education to a novice like me was invaluable. Thank you again and please wish me luck!

  14. tnedelsky | | #14

    Foam Insulated Glass Sapacers
    What is the R value per inch of thickness for Edgetech silcone foam?

Log in or create an account to post a comment.



Recent Questions and Replies

  • |
  • |
  • |
  • |