There have been several inquiries about the use of foam insulating material in the frame hollows of window profiles. The hollows in the frames and sashes of all windows are small enough to prevent natural convection and thus create dead air spaces. Dead air has a thermal conductivity (k) of approximately .17 BTU in/HR FT degrees Fahrenheit.

The foams which are generally used are urethane and have thermal conductivities of between .13 and .20 when first foamed in place. As the foam ages, the “k” value increases significantly. Thus, although some very minimal initial gains in insulating value can possibly be made by foaming open areas, in time, these gains will be lost and the window may actually have poorer insulating qualities than an unfoamed window.

If cheaper foams are used initially, the foamed windows will always perform worse than the unfoamed units. In addition, tests by independent laboratories have been unable to show any significant gain in the thermal performance of foamed windows over non­foamed units.

The lack of increased thermal performance coupled with the significant increase in cost associated with foaming window frames is reason enough not to use foam. However, foam can have another very significant problem. Urethane foams, which are the only foams which have “k” values as low as .13, can and in many cases, do continue to grow long after the foaming process is complete. This can cause warped frames and jammed windows six months, a year, or two years after the initial foaming.

(Source: The Professional Window and Door Guide, Volume 1, Issue 1)

The Dubious Benefits of Foam-Filled Vinyl Windows

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 from a 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 a hollow frame was R-23. In a climatic region with 6,000 degree days per year (Denver, fat example), that boils down to energy savings less than 50,U00 Btu per window per year – about 30 cents worth of natural gas.

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 show only 0.1 improvement in r-value. The figures 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. These results were very close to the theoretical results.

(Source: http://murphyswindows.com/Murphys-windows-problems-foam.htm)