Warming winter windows

Q: How can I boost the energy efficiency of the floor-to-ceiling windows in my high-rise condo? Heating costs are very high here, and I’m thinking of trying window blankets this winter. Any coverings I choose will also need to keep early morning light out of east-facing windows. What do you recommend?

A: I don't know enough about the window treatment scene to direct you to specific brands, but there is one crucial issue you need to consider as you're examining the different options. It's one thing to block out light and help boost the insulation value of a window. Many window blankets (also called ‘window quilts’) will do this. But it’s quite another thing to do the job without also causing lots and lots of condensation on the inside of the windowpanes. This is a very distinct danger in a cold climate, and the key to preventing this is a blanket design that seals the junction between the window frame and window quilt on all four sides. The best systems I've seen include a track on the sides, top and bottom especially for this purpose. If warm, moist indoor air is allowed to enter the space between the window and the window blanket, this air will cool and moisture will condense on the glass surface. Sometimes the process of leakage and condensation gets bad enough to form puddles on the floor. Even with a good seal around the edge of the window blanket, expect a little frost to develop on the windows during cold nights. This is normal and harmless.