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...
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