Chances are that when you installed your window treatments, you were focused on privacy and decorative style, not keeping the house warm. But the fact is that thicker curtains can perform the valuable wintertime service of limiting the heat loss that would otherwise naturally occur through windows. That's why many savvy homeowners choose thermal-lined curtains. A do-it-yourself alternative is to sew fleece or flannel onto the back side of the existing window treatments. During the day, though, don't forget to draw open the curtains to allow for solar heat gain.