Foam-filled vinyl may provide some structural strength and consumer appeal, but the foam core adds little to the overall window energy performance. The following is from my favorite fenestration publication, ENERGY DESIGN UPDATE. For you pros, if you don't have a subscription to EDU you're missing out on a lot of good info. An online subscription costs about $2,500/year, and a mailed written subscription is I think about $400. They test new products, challenge claims, and analyze issues relating to windows, glazing, fenestration, energy-efficient buildings, etc. Good stuff. Anyway, a while back they had an article called "The Dubious Benefits of Foam-Filled Vinyl Windows." Here Goes:
Some manufacturers of vinyl-framed windows inject urethane foam into the hollow vinyl frame to improve thermal performance and/or increase rigidity. Although a foam-filled frame is visually impressive when viewed in cross section, the actual benefits in terms of energy performance are apparently minimal.
As part of a recent research project, Enermodal Engineering of Waterloo, Ontario, modeled the thermal performance of vinyl frames with and without foam insulation. The R-value of the hollow frame was R-2.0; the foam-filled frame was only slightly higher - R-2.3. In a climatic region with 6,000 degree days per year (Denver, for example), that boils down to energy savings less than 50,000 Btu per window per year - about 30 cents' worth of natural gas heat.
Impact on overall window R-value
When installed in a double-glazed window with low-E glass, the slight improvement in frame R-value produced by foam filling has little effect on overall window R-value. Using the WINDOW 4.0 computer program developed by Lawrence Berkeley Laboratory, we calculated the overall R-value of vinyl windows with and without foam filling. The results, listed in Table 1, show only 0.1 improvement in [total unit] R-value.
The figures in Table 1 are theoretical calculations, but at least one laboratory measurement bears them out. Quality Testing Inc. of Everett, Washington, ran a series of tests on vinyl windows with and without foam filling. The results, listed in Table 2, were very close to the theoretical results in Table 1.
Building codes, condensation, and trade fairs
Given the minimal energy benefit of foam-filled windows, might there be other practical advantages? Maybe. One researcher pointed out that some building energy codes require minimum R-2.5 for windows. In the examples listed in Tables 1 and 2, the uninsulated window would not qualify; the foam-filled window would.
What about moisture condensation resistance? Will the higher R-value in the frame make the window more resistant to condensation? Only minimally. Dave Perich at Louisiana Pacific (LP), which sells foam-filled vinyl windows, told EDU that LP tests show that at sub-zero outdoor temperatures, the interior surface of a foam filled vinyl frame is only about 3°F warmer than that of an uninsulated frame.
In fact, says Perich, the main benefit of foam filling is marketing appeal. In the highly competitive replacement market, foam filling gives a slight marketing edge over noninsulated windows. The foam filling is very visible in crosssection at trade shows.
For more information on frame R-values, contact Enermodal Engineering, 368 Philip Street, Unit 2, Waterloo, ON N2L 5J1, Canada; A research study on frame R-values by Steve Carpenter of Enermodal is published in the proceedings from the Thermal Performance of the Exterior Envelopes of Buildings V conference, available from American Society of Heating, Refrigerating, and Air-Conditioning Engineers, 1791 Tullie Circle NE, Atlanta, GA 30329; or from Oak Ridge National Laboratory, Box 2008, Oak Ridge, TN 37830.