If your double-hung windows are too loose, they may rattle in the wind, admit street noise and leak large volumes of air, causing drafts and wasting your precious energy dollars. Too tight, and you’ll find them difficult or impossible to open and close. Finding the right balance is easy. Try a sharp blow to the center rail, near the locking point, with the palm of your hand or a rubber mallet. That can break the bonds that sometimes form between painted surfaces, in this case the two rails. If that fails, try tapping a wood block against the sides of the sash to break a similar bond between the stops and sash. Be gentle; too hard a tap can crack the glass. If the problem is a painted-over joint, it’s best to cut it, either with a window zipper (a serrated-blade tool designed for this purpose) or with any flat-blade tool, such as a putty knife or spackle knife. Hold the blade flat against the face of the sash and push the blade’s edge into the joint as you draw the tool along the surfac