- Q. Do so-called “double-cell” insulating window shades perform as advertised? One salesperson claims R-3 if they are installed with an airtight edge seal. But what if they are installed without the perimeter track?
A.Paul Fisette, director of Building Materials and Wood Technology at the University of Massachusetts Amherst and a JLC contributing editor, responds: I can’t tell you whether your curtains are truly R-3, though that does not seem like an unreasonable R-value if they are installed with tracks. I have seen some shades advertised that claim values as high as R-8, which I’m skeptical about. You’ll definitely get the best performance and are more likely to get the full reported R-value from the shades if you use an airtight seal around the edge of the window. Without the seal and with the shades drawn, you will most likely find condensation around the edges of the glazing — an indication that warm air from the room is leaking in through the gaps between the shade and the window trim and reaching the cold condensing surface of the glass.
This is evidence that the shades are in fact insulating your windows to some degree and providing some energy benefit, because the window glass is colder than room temperature. Say, for example, that the room temperature is 70°F and the relative humidity is 50 percent; in that case, the dew-point temperature is around 49°F. If it’s a freezing night and you pull the shade, thereby preventing warm room air from contacting the glass, it may not take long for the glass to reach dew-point temperature. You’ll get condensation around the edges, where heat is lost to the window frame; with double-hungs, you’ll probably also have condensation just above the meeting rails, where air leaks are common. But if you leave the shade raised, the glass will stay in contact with the room air and remain warmer. You won’t get condensation, but you’ll be expending additional heating energy. So condensation is actually a sign that the shades are working to some degree — though the wetting is not good for the long-term durability of the windows. Therefore I would go the extra mile and install the edge seals.
One last note: In my climate (around 6,800 heating-degree days), you save approximately 2 percent on your heating bill for every degree you reduce your thermostat on a 24-hour basis. Because warmer surfaces radiate heat to colder surfaces, using thermal curtains can make a room feel more comfortable at a lower temperature.
