How to Seal Windows for Winter and Stop Drafts

Cold air does not need an open window to get inside. Gaps around the sash, worn weatherstripping, and cracked caulk let warm air leak out and freezing air seep in, and windows can account for up to 30 percent of a home’s heat loss. Sealing them is one of the cheapest energy upgrades available, and a few tubes of caulk and a roll of weatherstripping can pay for themselves in a single heating season. Before you buy anything, it helps to understand how different window types and materials behave and where they leak.

Where Drafts Come From and How to Find Them

Heat escapes through windows four ways at once: conduction through the glass, air leaking around the sash, gaps at the frame-to-wall joint, and openings at the sill and stool. Glass conduction is the reason double-pane units perform better than single-pane, but air leakage is the part you can fix yourself without replacing anything.

The 30 percent figure comes from houses with single-pane windows and poor seals. Modern double-pane units cut conduction losses sharply, but the air-leakage share stays high in any home where the weatherstripping has worn out. Fixing the leaks is the fastest return, because the labor is measured in minutes per window and the materials cost less than one dinner out.

Common Air Leak Locations

Start your inspection at these spots:

• between the sash and the window frame;
• along the sill and the stool where the sash rests;
• at the joint between the frame and the wall or siding;
• around the lock, hinges, and hardware cutouts;
• between the upper and lower sashes of a double-hung window.

Interior storms and heavy curtains reduce conduction loss, but they do nothing for air leakage. A draft test tells you which problem you actually have, and most homes have both, so plan to seal first and insulate second.

Testing for Drafts

On a cold, windy day, run the back of your hand around the window perimeter and you will feel the worst leaks immediately. For a precise map, use a candle or an incense stick and move it slowly along the same path.

Using a Candle or Incense Stick

Hold the flame or smoke plume about an inch from the frame and travel the full perimeter, then cross the sash joints. Where the flame flickers or the smoke streams sideways, you have a leak. Mark each spot with painter’s tape and fix them in order of severity.

The repair you choose depends on the window style. Double-hung, casement, sliding, and awning window configurations each leak in different places, so match the fix to the hardware you actually have.

Caulk Around the Inside Trim

Choosing the Right Caulk

For painted interior trim, a paintable latex or acrylic latex caulk is the right choice because it dries flexible and takes paint without yellowing. Silicone stays flexible in wet areas but cannot be painted, so keep it for surfaces that do not need a finish. Match the caulk color to the trim paint, and buy a good-quality tube, since cheap caulk shrinks and cracks within a season.

Condensation on the glass is a separate signal. If the inside pane fogs in winter, humid indoor air is meeting a cold surface, and sealing the trim will not fix it. Lower the humidity with exhaust fans first, then seal; otherwise the trapped moisture can rot the sill behind the new caulk.

Applying a Clean Caulk Bead

A tidy bead seals better and looks finished. Work through it this way:

1. cut the nozzle at a narrow angle so the opening is about the size of a strand of spaghetti;
2. load the tube into the caulk gun and squeeze until the bead flows;
3. pull the tip steadily along the joint, letting the gun do the work;
4. smooth the bead with a wet finger and wipe away excess with a damp rag.

For single-pane windows, an interior storm panel or a shrink-film kit adds a second air layer and cuts the conduction loss by roughly half. These are the cheapest glass-level fixes and they combine well with trim caulking, because they solve different parts of the same problem.

For a visual walkthrough of the same sequence, the sealing windows for winter guide from This Old House shows professional technique for both caulk and weatherstripping.

Restore and Upgrade Weatherstripping

Types of Weatherstripping

Weatherstripping closes the gap between the moving sash and the frame. The right type depends on the window and the gap size:

When to Replace vs. Repair

If the existing strip is cracked, compressed, or missing sections, replace it. Some windows hold the weatherstripping in a narrow channel, and the old strip pulls out and the new one slides in without tools. When the gap is too large for the original seal, add a second layer of foam tape or upgrade to a thicker profile.

V-strip is pressed into the sash channel with the point facing the gap, and it works best on the sides and tops of double-hung windows. Foam tape compresses to fill irregular gaps and is easiest on casement and slider windows, where the sash closes against a flat stop. Cut each piece a quarter inch long and let it compress at the corners.

Builders follow the same priorities when air sealing windows and doors during construction, so the retrofit sequence matches professional practice.

Seal the Outside of the Window

Exterior Caulk and Sealant

The outside of the window is the first line of defense against wind-driven rain and drafts. Use an exterior-grade caulk rated for the siding material, and seal the joint between the siding and the window flange, the top of the trim, and the drip cap. Work on a dry day above 40 degrees Fahrenheit so the caulk cures properly.

Before caulking, check the drip cap above the window. If it is missing or rusted, water runs down the face of the window and into the top joint, and no caulk bead will hold against that. Replace the cap, then seal the joints below it.

Backer Rod and Spray Foam for Large Gaps

Gaps wider than a quarter inch should not be filled with caulk alone, because the caulk will sag and crack as the house settles. Stuff a foam backer rod into the gap first, then apply caulk over the top. Deep cavities around the frame call for spray foam, which expands to fill the space and seals far better than packed fiberglass.

Exterior caulk should be painted after it cures if the trim is painted, because bare sealant degrades in sunlight. Choose a caulk that accepts paint and give it a full day before you brush.

A spray foam air sealing pass around doors and windows handles the big holes that caulk cannot bridge.

The Cheapest Ways to Insulate Windows

Low-Cost Temporary Options

When the windows themselves are the weak point, temporary insulation beats replacement on cost:

• shrink-film window kits that tape to the trim and tighten with a hair dryer;
• rope caulk pressed into the sash channels, removed in spring;
• draft stoppers or foam strips along the sill;
• heavy thermal curtains that add a still-air layer at night.

Do the work in fall, before the heating season peaks, when the caulk cures at a decent temperature and you can still test each window with the house closed up. Most seals last three to five years, so put the draft test on a yearly calendar rather than waiting for a cold snap to find the gaps for you.

Cost Comparison of Sealing Methods

The table below shows what each method costs per window and how long it takes:

If the budget is tight, prioritize the rooms that face the wind and the rooms you heat the most. Sealing the living room and bedrooms first captures most of the savings, and the remaining windows can wait a year.

For older homes with original single-pane units, restoring old sash windows can tighten the seal without replacing the glass, which keeps the character of the house and cuts drafts at the same time.

Taken together, these steps form the winter home preparation checklist that most homeowners work through each fall, covering windows, doors, pipes, and heating systems. Start with the draft test, fix the worst leaks first, and the savings show up in the next heating bill.