The Engineer: All spacers not created equal

Don’t let anyone fool you when they claim there is no difference between one product and another.
David Heska, P.Eng.
May 29, 2018
By David Heska, P.Eng.
I was born and raised here in Canada and there are many things about our country that I’m extremely proud of. But I must admit whenever I hear someone refer to the United States’ Declaration of Independence, part of me wishes that this document was in our history and not theirs. Namely the second paragraph, which states, “We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable rights, that among these are life, liberty and the pursuit of happiness.”
Men and women are created equal, regardless of our race, ethnic origin, religion or status. While this is true, there are many examples of manufactured products where this is not true. Specifically, in the glass industry, it is becoming more and more apparent that spacers are not created equal.

An insulating glass spacer is placed within the unit to hold the glass panes at a fixed distance from each other, establishing the size of the interpane space. Typical spacers are four to eight millimeters wide and 12 to 14 millimeters thick. Traditional aluminum spacers are roll formed, inexpensive, highly conductive, and can create stress on the primary seals due to their expansion and contraction. As aluminum spacers decrease in popularity, three other types of spacers have become more common: stainless steel spacers, hybrid spacers, and flexible spacers. But which type of spacer is the best?

To answer this question there are a number of factors that must be considered and in this short article there is only room to briefly scratch the surface. Items to consider include strength, durability, conductivity, and price. There is a lot of information published by manufacturers and researchers comparing these factors and sometimes the information is inconsistent. One manufacturer claims that their spacer “is stronger and more esthetically pleasing” and that their spacer “reduces the stress on the primary and secondary seals.” The next manufacturer claims that their spacer “keeps the argon gas in place longer.” Then there are others who claim that the conductivity of their stainless steel spacer is the same as other flexible foam spacers.

Fortunately, the National Fenestration Rating Council publishes its Procedure for Determining Thermophysical Properties of Materials. Appendix C of this document (NFRC 101-2017 [E2A6]) is updated three to six times a year creating an industry-approved baseline for spacer density and conductivity values. Here are the published conductivity values for a few commonly used spacers (units in W/m∙K). The Cardinal stainless steel spacer has a conductivity of 14.187. The TGI NA stainless steel M-Spacer is 14.0. The Intercept Ultra stainless steel spacer is 13.63. The Quanex Spacer S2 Premium is 0.125. The Tremco EnerEdge silicone warm-edge spacer is 0.118. It should be noted that these values represent the spacer only and do not include the surrounding primary or secondary seals.

Over the past 20 years our company has developed an extensive electronic library of over 500 spacers that are used in computer thermal models. We recently completed a report for a client comparing 28 spacers. When modelling a simple 1.2-by-1.5-meter double-glazed IGU (without framing elements; three-millimeter glass with a low-E coating on surface 2 and 90 per cent argon fill) we determined that the U-value of the window ranged from 1.68 to 1.46 W/m2∙K and the condensation resistance ranged from 19 to 53. For a simple triple-glazed IGU with a variety of spacers, the U-values ranged from 0.89 to 0.64 W/m2∙K. This report is now a valuable tool for our team that we have been able to reference as we advise window manufacturers which spacers provide better thermally modelled test results.

Conductivity is only one of the factors that should be considered when determining which spacer to specify or install, but don’t let anyone fool you when they claim there is no difference between one product and another. There are differences and they matter and, unlike people, spacers are not created equal.


David Heska, P.Eng. is a director with WSP’s Building Sciences team in southwestern Ontario. He oversees the operation of the Hamilton, Kitchener and Windsor offices. David has been involved on window simulation projects as well as the design and replacement of windows in commercial and residential buildings. He can be reached at This e-mail address is being protected from spambots. You need JavaScript enabled to view it

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