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 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 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.
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.
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.