How to Seal Windows for Winter

Windows are the weakest link in a home's thermal envelope. Even a well-built home loses 25–30% of its heat through windows. In an older home with single-pane glass, that number can hit 40%.

The fix doesn't require replacing the windows. A few dollars of caulk, weatherstripping, and plastic film can cut heat loss through windows by 50–70% — for about $30–$50 per home, in an afternoon.

This guide compares the three main approaches — caulk, weatherstripping, and window film — and gives you a step-by-step for sealing an entire home.

First: find the actual leaks

Not all windows leak equally. Older double-hung windows leak most at the meeting rail (where the two sashes meet) and at the sides. Fixed windows rarely leak. Before you buy anything, find the leaks:

• Hand test: on a cold, windy day, run your hand along the edges of every window. Where you feel cool air, that's a leak.
• Incense test: light an incense stick and hold it near the window frame. If the smoke streams sideways, there's a leak.
• Flashlight test at night: have someone shine a flashlight around the outside edges of the window while you're inside in a dark room. Light leaking through = air leaking through.

Write down which windows leak and where. Most homes have 3–5 problem windows, not 15.

Option 1: Caulk (best for frame-to-wall gaps)

Caulk is the right tool when the leak is between the window frame and the wall. These leaks show up as a cracked or peeling line of caulk on the interior trim. Over time, caulk dries out and separates.

Materials

• Paintable acrylic latex caulk — $6–$10 per tube. One tube does 3–4 windows.
• Caulking gun — $10–$15 (one-time purchase)
• Putty knife — $5 for removing old caulk

Steps

1. Scrape out the old caulk with the putty knife. You don't need to remove every bit, just the loose, cracked material.
2. Wipe the area clean and let it dry.
3. Load the caulk tube into the gun.
4. Apply a thin, even bead along the gap. Don't overdo it — caulk is not a filler for large gaps.
5. Wet your finger and smooth the bead, pushing it into the gap.
6. Let it cure for 24 hours before painting or cleaning.

This is a one-time fix that lasts 5–10 years. Cost: $15–$30 for an entire home. Savings: $20–$50/year from reduced air infiltration.

Option 2: Weatherstripping (best for sash-to-frame leaks)

Weatherstripping is the right tool for leaks where the moving sash meets the frame. On double-hung and sliding windows, this is where most of the loss happens.

Types of weatherstripping

• V-strip (V-seal): $8–$15 per window. Slips into the gap between sash and frame. Most durable and most effective for windows.
• Adhesive foam: $5–$10 per roll. Peels and sticks to the frame. Cheaper but wears out faster.
• Felt: $5 per roll. Cheapest, least durable. Fine for a single season.
• Door sweeps (for windows): rarely used on windows but useful for any swinging casement window with a large gap at the bottom.

For most windows, V-strip is worth the extra cost — it lasts 5+ years vs. 1–2 for foam.

Steps for V-strip

1. Clean the frame where the strip will sit.
2. Cut V-strip to the length of the window opening.
3. Peel the backing and press it into the gap between the sash and the frame. On double-hung windows, you'll need it along both vertical sides of both sashes.
4. Test by closing the window and feeling for air leaks.

Cost: $30–$60 for the typical home. Savings: $40–$100/year.

Option 3: Window film (best for single-pane windows that won't open in winter)

Removable shrink-wrap film is the most effective single fix for single-pane windows that won't be opened until spring. It creates a dead air space between the glass and the film, which insulates almost as well as adding a second pane.

Materials

• Window insulation kit — $10–$25 per window. Includes the film and double-sided tape.
• Hair dryer — for shrinking the film tight
• Scissors

Steps

1. Clean the window frame with a damp cloth and let it dry completely.
2. Apply the double-sided tape around the entire window frame, about 1/4 inch from the wall.
3. Cut the film to size, leaving 2 inches of overlap on each side.
4. Peel the backing off the tape and press the film into place. Start at one corner and work your way around, smoothing as you go.
5. Use a hair dryer on high heat, held 6–12 inches from the film, moving continuously. The film will shrink and tighten. You'll hear a slight crackle — that's normal.
6. Trim any excess film with scissors.

Once applied, the film is nearly invisible (some kits are crystal clear), and it stops almost all air movement through the glass area. In spring, the film peels off cleanly with no damage to the paint.

Cost: $10–$25 per window. Savings: $30–$60 per window per winter, depending on the window.

Which option for which situation

• Frame-to-wall gaps: Caulk
• Moving sash: Weatherstripping
• Single-pane window that won't be opened: Window film
• Double-pane window with a broken seal: Window film (the seal is unfixable without replacing the pane)
• Older double-hung window that still opens: Weatherstripping + caulk

In most homes, the best result comes from combining weatherstripping and window film on problem windows, and caulk on any cracked frame seams.

Sealing an entire home: order of operations

1. Find the leaks (30 minutes)
2. Caulk frame-to-wall gaps on all windows (1 hour, plus overnight cure)
3. Apply weatherstripping to sash gaps (1 hour)
4. Apply film to single-pane windows that won't open (2 hours)
5. Test on a windy day with the incense stick again

Total time: one Saturday. Total cost: $40–$80 for a typical home. Total first-year savings: $80–$250.

What NOT to do

Don't seal every window permanently

You need at least one operable window in each room for ventilation and emergency egress. Film should only go on windows you won't open until spring.

Don't use spray foam on small gaps

Spray foam is for gaps over 1/4 inch. On small cracks it's messy and creates a bigger problem than it solves. Stick with caulk.

Don't cover vents or HVAC registers

If a window has a supply register or return grille nearby, don't block it with insulation or tape. The HVAC needs airflow.

Don't skip the exterior

Caulking the exterior of window frames (where they meet siding) is also important. Water intrusion from exterior gaps causes rot and mold. If you have exterior access, caulk both sides.

Frequently Asked Questions