Some years ago, representatives of an engineering firm in Winslow came to our studio on Madrone Lane and argued that a window shade could not insulate well because the cold has already passed through the glass and is inside the house. To test that assumption — and to prove them wrong — we installed a Hunter Douglas Duette honeycomb shade in one of their windows.
After running tests with all of their instruments, the engineers found that the shade, when fully lowered, worked very well in keeping the cold trapped in the 3-inch air pocket between the back of the shade and the cold window glass. They ended up installing the honeycomb shades in all of their windows.
This insulating air pocket also works for metal venetian blinds, such as the Levolor brand. Fabric shades do insulate much better than metal blinds but we have found that even metal blinds will keep a room warmer for about one to two hours. If you have large glass doors near your dining table and they are covered by a venetian blind, lower the blind completely just before dinner.
Sitting at the table, you will feel warmer for the time it takes you to finish that meal. It takes that long for the metal slats to cool down and to start transferring the cold so that you can feel it on your body. Just as with fabric honeycomb shades, the air pocket between the cold window glass and the window blind is the real insulator.
It is best to not leave shades all the way down for several days. When the sun shines in winter or when it’s overcast in the summer, raise the shades or blinds all the way up and let the built-up moisture dissipate.
Also, do not close off rooms that are rarely used. Leave the doors open at night or when you leave the house. Rooms need to breathe, otherwise mold and mildew will take over.