Home Exterior

How to Seal Drafty Windows and Cut Your Energy Bill

A drafty window isn't just uncomfortable — it's your furnace heating the outdoors. An afternoon with caulk and weatherstripping is one of the cheapest energy fixes in the house.

First, find the leaks

Cold air doesn't always come through the glass. Most of it sneaks in around the frame, under the sash, or through a failed seal you can't see. Pick a cold, windy day and work window by window — you'll find leaks in minutes.

• Hand test: run your palm slowly around the frame, the sash joint, and the sill. Cold spots mark leaks.
• Smoke test: hold a lit incense stick near the edges. Wavering or pulled smoke means moving air.
• Paper test: close the window on a dollar bill. If it slides out without drag, the sash isn't sealing.
• Light test: at night, have someone shine a flashlight around the frame from outside while you watch from inside.
• Look for peeling, cracked, or missing caulk outside, and gaps where the trim meets the wall inside.

Match the fix to the gap

The single most common mistake is caulking something that moves. Caulk seals stationary joints — where the frame meets the siding or the trim meets the wall. Weatherstripping seals the parts that open and close. Use one where the other belongs and you'll either glue a window shut or watch the seal tear apart by spring.

• Gaps where frame meets wall or siding — exterior caulk.
• Gaps between sash and frame on an operable window — weatherstripping.
• Gaps wider than about 1/4 inch — backer rod first, then caulk over it.
• Large gaps around the rough opening (visible with trim removed) — low-expansion window-and-door foam, never standard expanding foam, which can bow the frame.
• Cold radiating off the glass itself with no air movement — that's a glazing problem, not a draft. Insulating film or a storm window is the fix.

Caulking the frame

1. 1Scrape out old caulk completely with a putty knife or caulk removal tool. New caulk over old fails fast.
2. 2Clean the joint and let it dry — dust, chalky paint, and moisture all break adhesion.
3. 3Check the temperature. Most exterior caulks want 40°F (5°C) and rising; read the tube.
4. 4Cut the nozzle at a 45° angle, starting small — you can always widen the opening, but not shrink it.
5. 5Run a steady bead in one pass, pushing rather than pulling the gun so the caulk gets forced into the joint.
6. 6Tool the bead immediately with a wet finger or caulk tool to press it into both surfaces.
7. 7Let it cure fully before painting — usually 24 hours, longer in cold or humid weather.

For exterior work, use a siliconized acrylic latex or a polyurethane rated for exteriors — both take paint. Pure silicone lasts longest and stays flexible, but nothing sticks to it, so save it for spots that will never be painted. Skip anything sold simply as "painter's caulk" outdoors; it isn't built for weather.

Weatherstripping the moving parts

Measure the gap before you buy. V-strip (tension seal) is the most forgiving choice for double-hung sashes: it tucks into the channel, stays invisible, and springs against the sash. Foam tape is the cheapest and easiest, but it compresses permanently and typically needs replacing every year or two — fine for a sash you rarely open. Silicone or rubber bulb seal costs more and lasts far longer on casement windows and sash bottoms.

• Clean the surface with rubbing alcohol and let it dry — adhesive-backed strip fails on dust or grease every time.
• Measure and cut each piece before peeling the backing. Butt the ends together; don't overlap them.
• Install with the window closed where you can, so the strip compresses to the right thickness.
• Test the window afterward. If it won't latch, the strip is too thick — step down a size rather than forcing it.

When film or a storm window makes more sense

Single-pane windows lose heat through the glass no matter how well you seal the edges. Shrink film — a plastic sheet taped to the trim and tightened with a hair dryer — creates a still-air pocket that noticeably cuts both heat loss and condensation, for a few dollars a window. It's seasonal and it does obscure the view slightly, but for a rental or a window you won't open until spring, it's hard to beat. Cellular shades and heavy lined curtains help too, mostly at night. Interior or exterior storm windows cost more and last for decades, and are worth pricing out for older homes where replacing the windows isn't realistic.

What it's actually worth

The U.S. Department of Energy estimates that sealing air leaks throughout a home, combined with adding insulation, can cut heating and cooling costs by roughly 10 to 20 percent. Windows alone won't get you the whole way — but caulk and weatherstripping run well under a hundred dollars for a whole house, so the payback is typically a single season. Do the windows on the windward side of the house first; they leak the most.

One caution: a very tight house needs deliberate ventilation. If you seal aggressively and start seeing condensation on windows or a stuffy smell, run bathroom and kitchen exhaust fans longer rather than reopening the leaks.

Make it a yearly habit

Caulk cracks, foam tape flattens, and V-strip works loose — a seal you did three winters ago probably isn't sealing now. The job only stays done if someone checks it each fall. Add your home to Upkeepery and it schedules a draft check before heating season and reminds you when the weatherstripping is due for a look, so you're not rediscovering the same cold window every January.