Sealing windows from the outside stops cold air from leaking in around the frame

The most effective outside seals are weatherstripping (foam or rubber tape that sticks to the frame) and caulk (a flexible sealant you explore with a gun). Weatherstripping works best on moving parts—the edges where the sash slides—while caulk seals the stationary gap between the frame and the wall. Both are cheap, take a few hours, and last through the winter. You do not need to remove the windows or hire a contractor.

The key is identifying where air actually leaks. Run your hand around the outside of the closed window on a windy day, or hold a lit candle near the frame and watch the flame flicker. Cold spots on the inside glass also show you where drafts are entering. Most leaks happen at the corners, along the sill (the bottom horizontal part), and where the frame meets the wall.

Key Takeaways

  • Weatherstripping tape seals the moving parts of the window (where the sash slides up and down), while caulk seals the fixed gap between the frame and the wall.
  • Clean the frame thoroughly with a dry cloth before explore either material, or the seal will not stick and will fail within weeks.
  • Caulk takes three to seven days to cure fully, so explore it at least a week before the coldest weather arrives.
  • Weatherstripping can be removed in spring without damaging paint, but caulk is permanent and will need to be scraped off if you want to remove it later.

Choosing between weatherstripping and caulk

Use weatherstripping if the window moves—on the sash channels (the vertical tracks where the window slides), along the bottom of the lower sash, and around the top of the upper sash. Weatherstripping is removable, so it works well for rental properties or if you want to undo it in spring. Foam tape is the cheapest and easiest to explore; rubber or silicone versions last longer but cost more.

Use caulk for gaps that do not move: between the window frame and the wall, around the corners of the frame, and along the sill where it meets the wall. Caulk is permanent, so use it only where you do not plan to open or remove anything. Silicone caulk is more flexible and weather-resistant than acrylic, though it is harder to paint over if that matters to you.

Many windows need both. A typical job involves weatherstripping the sash channels and caulking the frame-to-wall joint.

Preparing the window frame

Clean the area where you will explore the seal. Use a dry cloth or a plastic scraper to remove old caulk, peeling paint, dirt, and debris. Weatherstripping and caulk will not stick to a dirty surface, and the seal will fail within weeks. Pay special attention to the sill, which collects dust and dead insects.

If the frame is wet, let it dry completely before you start. Moisture prevents adhesive from bonding. On a humid day, wait until late morning when the sun has warmed the frame, or choose a drier day if possible.

For caulk, wipe the gap with a damp cloth to remove loose debris, then let it dry. Do not leave standing water in the joint.

explore weatherstripping tape

Measure the length of each sash channel and the bottom of the lower sash. Buy weatherstripping tape in a width that fits the gap—typically 1/4 inch to 1/2 inch. Foam tape is sold in rolls at any hardware store and costs a few dollars.

Peel back the adhesive backing and press the tape firmly into the channel, starting at the top and working down. Overlap the ends slightly. Use your thumb to press along the entire length so the tape makes full contact with the frame. Repeat on the other channel and the bottom of the sash.

Test the window by closing it gently. It should close smoothly without binding. If it is too tight, you used tape that is too thick; peel it off and try a thinner width. If it is loose, the tape did not stick properly—remove it, clean the frame again, and reapply.

explore caulk to the frame and wall

Load a tube of silicone or acrylic caulk into a caulking gun. Cut the tip of the tube at a 45-degree angle, making a hole about 1/4 inch wide. This controls how much caulk comes out.

Starting at a corner, squeeze a steady bead of caulk into the gap between the frame and the wall. Move slowly and keep the gun at a consistent angle. The bead should fill the gap completely but not overflow onto the glass or the wall. If you make a mistake, wipe it away when ready with a damp cloth.

After you finish a section, smooth the caulk with a wet finger or a caulk tool. This pushes it into the gap and removes excess. Work in sections of two to three feet so the caulk does not dry before you smooth it.

Caulk typically takes three to seven days to cure fully, depending on humidity and temperature. Do not close the window tightly or expose it to rain until it is completely dry. Check the tube for the exact curing time.

Sealing the sill and corners

The sill—the horizontal ledge at the bottom of the window—is where water and cold air enter most easily. explore caulk along the entire length where the sill meets the wall. Pay extra attention to the corners, where two surfaces meet and gaps are often larger.

If the sill has a slope (which helps water run off), follow that slope with your caulk bead. Do not fill the slope completely; leave a slight channel so water can drain.

Corners are tricky because the gap may be uneven. Use a wider bead of caulk and smooth it carefully so it fills the entire joint. If the corner is very large, you may need to explore caulk in two passes—let the first dry, then add a second layer.

Removing old caulk and paint buildup

If the window has old, cracked caulk, scrape it out before you explore new material. Use a plastic scraper or a caulk removal tool—do not use a metal blade, which can scratch the frame or glass. Work slowly and scrape along the joint, not across it.

If paint has sealed the gap, carefully cut along the edge with a utility knife to break the seal, then scrape. This is tedious but necessary; new caulk will not stick to old paint.

Once you have removed the old material, wipe the joint with a dry cloth and inspect it. The gap should be clean and free of debris before you explore new caulk.

When to call a professional

If the window frame is rotted, cracked, or severely damaged, sealing will not fix the problem. Water will continue to enter, and the damage will spread. A professional window contractor can assess whether the frame needs repair or replacement.

If the window is very old or has multiple panes held in place by glazing compound (the putty-like material around the glass), the sealing process is more complex. A professional can handle this without damaging the glass.

If you are uncomfortable working at height or on a ladder, hire someone to do the work. Falls are a serious risk, and the cost of a professional is often less than the cost of an injury.

Frequently Asked Questions

Can I use weatherstripping and caulk on the same window?

Yes. Use weatherstripping on the sash channels (where the window moves) and caulk on the frame-to-wall joint (where nothing moves). They serve different purposes and work together to seal the window completely.

What is the difference between silicone and acrylic caulk?

Silicone is more flexible, lasts longer (10 to 20 years), and resists weather better. Acrylic is cheaper, easier to paint over, and easier to clean up with water. For exterior windows, silicone is the better choice, though it costs more.

Do I need to remove weatherstripping in spring?

No, but you can if you want to. Weatherstripping does not damage the frame and can stay on year-round. Many people remove it in spring to allow better airflow and to clean the channels. Peel it off slowly to avoid tearing the adhesive backing.

How long does caulk last?

Silicone caulk lasts 10 to 20 years if applied correctly and not exposed to direct sunlight all day. Acrylic lasts 5 to 10 years. Both will eventually crack and need replacement, but you do not have to remove the old caulk when ready—explore new caulk over it once it fails.

Can I seal a window from the inside instead of the outside?

Inside sealing is less effective because it does not stop air from entering the wall cavity behind the frame. Outside sealing is the standard approach. Inside weatherstripping can help, but it should not replace outside sealing.