Most heat loss at a window occurs through gaps between the sashes and the frame. Even small gaps can be big energy-wasters. On a windy day hold a piece of tissue paper or plastic wrap near the window and move it around. Wherever you see movement (either outward or inward), there is a significant leak.
The first step is to caulk, especially on the exterior, although some interior caulking is helpful as well. Also check that the glazing putty on the outside is free of gaps and seals tightly against the window. Where glazing is failing, scrape it out and apply new glazing. On the inside, see that the joint between the glass and the sash is sealed with paint.
Weather-stripping where two surfaces push together (the horizontals of a double-hung window and the verticals of a casement or sliding window) is straightforward. Where two surfaces slide against each other (the verticals of a double-hung window and the horizontals of a casement or slider) calls for more precision.
Most weatherstripping applications only take an hour or two to apply. Before you begin, make any needed repairs to the window and its frame.
Nail-on weather-stripping is the most durable choice if you have a wood window. Spring bronze works well for gaps that are consistent in width. It is also the best-looking product. Where the gap is large and uneven and looks are not as important, a tubular vinyl gasket is a good choice. Strips of felt are a poor choice because they don't seal well and are not durable.
Self-stick weather-stripping is easy to cut, making it the most convenient choice. If you are choosing self-stick V-strip tape, be sure it is made of EPDM (ethylene-propylene-diene-monomer), which stays flexible for many years, even when exposed to extremely low temperatures.
Foam weather-stripping is easy to apply and fills large and uneven gaps effectively. However, it usually doesn't last long. Open-cell foam is the best at bouncing back after being compressed, but it can only be used on the inside. Closed-cell foam is weather resistant but short-lived.
For quick but temporary sealing, use rope caulk, which can be removed when the weather gets warm.
Cut a caulk tube's tip using a sharp utility knife; a straight, clean cut contributes to a smooth caulk line. Some people prefer to cut at a steep angle, while others prefer a nearly straight cut. Cut near the tip for a small bead of caulk. Some caulk tubes require the seal be broken with a wire or long nail through the tip.
It takes a bit of experience to produce a smooth bead of caulk, so start caulking in an inconspicuous location. Get in a comfortable position and rest the tip against the joint. Squeeze the trigger until caulk emerges, then continue to squeeze as you move the tip along the joint.
If the bead looks good, leave it alone. Otherwise use your finger to tool it; this tends to smear the caulk, but it does ensure sticking on both sides of the bead.
For an added measure of weather-stripping during cold months, unroll and press in rope caulk where the sash meets the stops, between the top of the lower sash and the bottom of the upper sash, and in the pulley. The window cannot be opened while the caulk is in place, so remove it in the spring.
Test to be sure you will be able to close the window before you apply thick weather-stripping to the underside or top of a sash. To apply self-stick foam, first make sure the surface is clean and dry. Cut the foam with scissors or tin snips. Peel off the backing and press the foam into place.
Cut spring bronze with tin snips to fit precisely. Taking care not to bend the metal as you work, hold the piece in place and drive in the little bronze nails. Drive two or three nails, test to be sure the window will close, then drive the rest of the nails.
Tubular vinyl can seal large cracks, though it does detract from the appearance of a window. Cut the strips with scissors. Close the window and press the vinyl firmly into place while you drive the small nails.
To seal a window for the winter season only, purchase a shrink-wrap window cover kit. Apply the double-sided tape all around the casing, then carefully apply the plastic to the tape. Use a blow dryer to shrink the plastic and make it taut.
To install spring bronze along the sides of a lower sash, raise the sash all the way up. Cut the bronze to the height of the sash. Slip it up the sash as far as needed, nail in place, then close the sash to test.
Self-adhesive foam works well for a casement or sliding window. The foam comes in various thicknesses; test to be sure the window will close after you apply it.
Repair glazing putty if it is cracked, missing in spots, or curling from the glass. Use a putty knife or chisel to scrape away loose putty. Clean the area, apply a little linseed oil, and apply putty.
For a casement or sliding window, apply V-strip to the side of the sash or jamb for a good seal. Make sure the surface is clean and dry. Cut the V-strip to fit and hold it in position. Peel back the paper as you press the self-stick strip in place.
Apply caulk around all exterior window molding. Also caulk the inside of the molding and under the sill.
A double-hung window has a number of potential leak points. Most prominent are where the bottom of the upper sash meets the top of the lower sash, where the sashes slide against the jamb, and where the bottom sash meets the stool. These gaps must be sealed with weather-stripping. Gaps can also be found on the outside of the casing, the underside of the stool, and the apron. To seal them, paint and caulk.
If you have installed a replacement window or replacement sashes, the cavity for the sash weights can be filled with insulation. Fill the opening by gently stuffing with fiberglass insulation or with spray foam (the nonexpanding type is easiest to control).
Cold winter air can come in through the pulley slots. Plastic covers are available or you can press in removable rope caulk to fill the gaps. Or simply cover with duct tape. Remove the caulk or tape before opening the window in the spring.