NUMBERS GAME. Another means by which manufacturers are claiming improved performance ratings for their windows is through the increase of the number of air chambers that are in the frames of their vinyl windows. Five manufacturers say they increased the number of air chambers that are in their vinyl windows during the past 2 years.
The typical increase is to 12 air chambers from six; Pella puts 18 air chambers in its 350 Series. However, Pella estimates that the 18 air chambers that are in its vinyl window improves its U-factor rating over models that have eight air chambers by 0.02, which is the same amount that manufacturers of hybrid windows say their vinyl adds to wood. Because additional air chambers aren’t the only design variable in any new window model, it’s impossible to say whether having more air chambers translates into higher prices.
The frames of vinyl windows aren’t solid pieces but are constructed of a series of chambers. Adding air chambers breaks up the convection, or the circulation of air that can transfer heat from one surface to another, that occurs in open air space, so more chambers can reduce heat flow, says Ray Lamb of National Certified Testing Laboratories, which independently tests windows. However, the extra plastic that’s used to create additional chambers also conducts heat, he says.
Certified product testers and simulators say it’s impossible to determine at which point chambers become too numerous to improve a window’s U-factor rating effectively. The maximum number of chambers varies greatly from window to window, they say, based on how those chambers integrate with other design factors, such as how the chambers are positioned relative to exterior and interior surfaces and whether the chamber walls run parallel or perpendicular to other components that might transfer heat. Two experts suggest that too few windows exist that lean on this concept to determine conclusively whether the design actually works. If it does, other manufacturers likely will follow suit.
Lamb points out that the walls that are in air chambers add support to an otherwise open space within a window’s frame—similarly to how supports are added to a bridge—which translates into a stronger window frame. All of the vinyl windows that have triple-pane glass include chambered designs, and we found several windows that are capable of accommodating triple-pane glass that have fewer than 18 air chambers and cost, on average, $68 less than the Pella 350 Series window ($372).
An upgrade to triple-pane glass will net you a 0.11 improvement in U-factor, which computer models suggest will reduce your annual heating costs by 8 percent, or $75, depending on where you live and your home’s configuration, heating system and insulation. An upgrade to triple-pane glass in the Pella 350 Series window costs 30 percent more, or $111 per window. So, if you replaced, say, 10 windows, it would take 10 years for you to recoup your costs.
Consequently, we believe that you shouldn’t become mesmerized by the number of air chambers that a vinyl window has or by hybrid designs that add vinyl to wood windows. Instead, you should focus on a window’s performance ratings, which can be found on the National Fenestration Rating Council label that’s affixed to the window or to its packaging or that’s published in the product’s literature.
ON FILM. Glass that has thin layers of film suspended within the air space that’s between the panes of double-pane windows isn’t new, but the number of windows that use this technology increased in the past 3 years. Three manufacturers—Hurd, Kensington and Marvin—now use suspended film in their windows, with a fourth, Ply Gem, possibly on the way in 2014. Serious Windows (now Kensington) was the only manufacturer that we found that sold this type of window nationally in 2010.
Suspended-film technology provides the same benefits as triple-pane glass does but without the weight and structural requirements. The film improves a window’s U-factor and in some cases its solar heat-gain coefficient, which measures how much of the sun’s heat that a window rejects.
Because suspended-film glass weighs one-third less than triple-pane glass does per square foot—or about 17 pounds less per 3-by-5-foot window—that equals less labor and fewer achy muscles if you install the windows yourself.
Pricewise, we found that windows that use suspended-film glass are slightly more expensive compared with those that have triple-pane glass. For example, Hurd’s 3-by-5-foot clad-wood casement window that has suspended film costs $925. The triple-pane version costs $900. Both provide insulation of R-4 (equal to a U-factor rating of 0.25).