There is no standard test for determining the R-Value of window coverings.  This is probably due to the multitude of variables in the real world, and especially the variable temperatures of the window and the room and the effect on convection currents on both sides of the covering.  A simple standardized test is proposed that will give a relative measure of Rw-Value for different coverings.  Modern CFD simulation will allow these results to be corrected for important variations from the test conditions. With windows being a major source of heat-loss in buildings, this is an area that will become more important as energy conservation becomes more important.

Convection currents are a source of confusion for window coverings. In normal R-value testing this effect is rigorously eliminated because in practice the effect disappears when 2 solid materials touch each other.  With window coverings, however, big air gaps between windows and the coverings are potentially desirable, but their effectiveness is dramatically reduced by convection currents.  And, unlike solid materials that have a more or less constant R-value with temperature, the window R-value will likely change considerably with temperature, due to the large temperature effects on convection currents. Hopefully, this effect will be determined by CFD.

Previous testing of window materials has shown that a 2’ x 2’ test area using a cold window face at 32 F creates a test that is very repeatable and useful for differentiating the R-Value of different materials.  A review of patents on window coverings showed thousands of patents, but without any standard testing the good ones could not be differentiated by the obvious bad ones.

Although an R-Value is measurable characteristic of a particular insulation, determining the value is complex. In the 70’s and early 80’s before standards, there were many false claims of R-values.  In the US standard tests were therefore mandated for all claims of “R-Value”.  Since there is no standard for window coverings we would have to be careful to always declare it was an R-value by our test method (Rw-value?) and not an official USA “R-Value”.  Over time our test might be accepted as a valid R-Value testing method.