It’s no secret that windows are a liability when it comes to heat loss in homes. Conductance of quality insulated, low-e windows is typically 0.30-0.35 Btu/ft2hr°F, or about R-3. If installed in a wall that performs at R-20, heat lost through windows is 6-7 times higher (per unit area) than heat loss through the rest of the wall. Various window treatments have been used for years to reduce this loss
and make homes more comfortable. Working through the DOE Building America Program with partner Comfortex Window Fashions, SWA performed a simple study measuring how much window treatments can reduce heat loss.
SWA and Comfortex installed double-honeycomb shades in four windows in a relatively new Connecticut home. Two of the shades were equipped with “ComforTrak Plus™” – a system that includes side-tracks and a simple gasket to reduce air movement around the shade (see image). The two other shades did not have any side tracks.
SWA installed several temperature sensors behind each shade, in the rooms, and outdoors; temperatures were recorded for 3 weeks during February and March. Based on these temperature
measurements, SWA calculated approximate, effective R-values for the shades. When shades were installed without side tracks, the shades seemed to add R-1.2 to R-1.5 ft2hr°F/Btu to the window assemblies. When side-tracks were installed, effective R-values were typically between R-4 and
R-5. It’s clear that limiting air movement around the shade assembly has a dramatic effect on heat transfer. Infrared images corroborate this.
In the image below, taken from inside the house when both window shades were closed, the window shade on the left has side tracks; the shade on the right does not. The warmer temperatures behind the shade without tracks – along with the cold edges near the bottom – are a result of much more warm air moving in behind the shade and cold air moving back into the room near the bottom.
While the numbers generated from this simple study are approximate, it’s clear that insulating window treatments can make a big difference in cutting heat loss through windows. Systems like the ComforTrak Plus tested here – which limit air movement through the shade – can reduce heat loss even more.