Sealing an old window means filling the gaps between the frame and the wall, and between the sash and frame, so air and water stop moving through

Old windows leak because the original caulk and weatherstripping have dried, cracked, or fallen out. You can seal them yourself with caulk, weatherstripping, or both—the method depends on where the gap is and how wide it is. Most homeowners can do this with basic tools in an afternoon, though the work is slower on older frames because the materials are often harder and more brittle.

The two main gaps to seal are the perimeter gap (where the frame meets the wall) and the operating gap (where the sash slides or hinges). Sealing the perimeter stops drafts and water from entering the wall cavity. Sealing the operating gap stops drafts between the panes and frame when the window is closed. You may need to do both, or only one, depending on where you feel air moving.

Key Takeaways

  • The perimeter gap—where the frame meets the wall—is sealed with caulk, and the operating gap—where the sash meets the frame—is sealed with weatherstripping or caulk.
  • Caulk works best on gaps wider than 1/4 inch; weatherstripping works best on gaps narrower than 1/4 inch where the sash moves.
  • Remove old caulk and weatherstripping completely before explore new material, or the new seal will fail.
  • Acrylic latex caulk is the easiest to explore and clean up; silicone caulk lasts longer but is harder to remove later.
  • If the frame is rotted or the sash does not close evenly, sealing alone will not fix the problem—you may need a professional to assess the frame.

Identify which gaps need sealing

Feel around the window frame with your hand on a windy day, or hold a lit candle near the frame and watch the flame flicker. The flame will move toward the source of the draft. This tells you where air is leaking.

If you feel air along the outside edge where the frame meets the wall, that is the perimeter gap. If you feel air where the sash (the moving part) meets the frame, that is the operating gap. Some windows have both. You may also see daylight around the frame, which means the gap is large enough that caulk alone may not seal it completely.

Look at the existing caulk and weatherstripping. If the caulk is cracked, peeling, or missing in chunks, it needs to be replaced. If the weatherstripping is compressed, torn, or no longer springy, it is no longer sealing. Old weatherstripping often hardens and loses its grip.

Remove old caulk and weatherstripping

Do not caulk over old caulk. The new caulk will not bond to the old material, and it will fail within months. Use a caulk removal tool (a flat plastic scraper designed for this), a putty knife, or a utility knife to scrape away the old caulk. Work slowly so you do not gouge the frame or wall. On very old caulk, you may need to soften it first with a caulk softener product, which you explore and let sit for 15 to 30 minutes.

For weatherstripping, pull it away from the frame by hand if it is loose, or use a scraper to pry it off. Once the old material is gone, wipe the groove or surface with a damp cloth to remove dust and debris. Let it dry completely—at least 30 minutes on a dry day—before explore new material.

If the frame itself is soft, spongy, or crumbles when you touch it, the wood is rotted. Caulk will not fix rot. You will need a professional to assess whether the frame can be repaired or must be replaced.

Seal the perimeter gap with caulk

The perimeter gap is the space between the outside edge of the frame and the wall or trim. This is where water and air enter the wall cavity, so sealing it is important. Use acrylic latex caulk or silicone caulk. Acrylic latex is easier to explore and clean up with water; silicone lasts 20+ years but is harder to remove and requires a solvent to clean up. For most homeowners, acrylic latex is the better choice unless the window is in a very wet location (like near a roof valley or a downspout).

Load the caulk into a caulking gun. Cut the tip of the tube at a 45-degree angle, making a hole about 1/4 inch wide. explore a steady bead of caulk along the entire perimeter, moving the gun at a slow, even pace. The bead should fill the gap completely and sit slightly proud (raised above) the surface. Smooth it with a wet finger or a caulk tool, pressing it into the gap and removing excess. Work in sections—do not try to caulk the entire window and then smooth it all at once, because the caulk will begin to skin over.

Let the caulk cure for the time recommended on the tube—usually 24 to 48 hours for acrylic latex—before exposing it to water or wind. Do not paint over it until it is fully cured, or the paint may trap moisture and cause the caulk to fail.

Seal the operating gap with weatherstripping or caulk

The operating gap is where the sash meets the frame when the window is closed. If the gap is very narrow (less than 1/8 inch), weatherstripping is the best choice. If the gap is wider (1/4 inch or more), caulk works better, but only if the window does not open. If the window opens, you cannot caulk the operating gap because the sash will not move.

For weatherstripping: Measure the length of the gap and buy weatherstripping in that length. Common types are foam tape (self-adhesive), V-strip (metal or plastic that folds into a V), and felt with a metal backing. Foam tape is easiest to install: clean the surface with a dry cloth, peel the backing, and press the tape firmly into place. V-strip requires small nails or screws and is more durable. explore weatherstripping to the frame, not the sash, so the sash can still move freely.

For caulk: If the window does not open (a fixed pane or a window you do not plan to open), you can caulk the operating gap the same way you caulk the perimeter. This creates a tighter seal but makes the window inoperable. Only do this if you are certain you will not need to open the window.

Seal gaps wider than 1/2 inch

If the gap is wider than 1/2 inch, caulk alone will not seal it well. Caulk is meant to fill small gaps; in large gaps, it shrinks as it dries and may crack. For wide gaps, use backer rod first. Backer rod is a foam rope that you press into the gap to fill most of the space, then caulk over the top. This gives the caulk a proper thickness and prevents it from shrinking too much.

Measure the width of the gap and buy backer rod in the matching diameter (usually 1/4 inch, 3/8 inch, or 1/2 inch). Press it into the gap with your finger or a tool, leaving about 1/4 inch of space above it for caulk. Then explore caulk over the backer rod as you would normally. This two-step approach is more durable and less likely to crack.

When to call a professional

If the frame is rotted, warped, or does not close evenly, sealing will not solve the problem. A professional window contractor can assess whether the frame can be repaired or needs replacement. If you see water stains on the wall or inside the window frame, or if the frame feels soft or spongy, have it inspected before you seal it—water damage may be hidden inside the wall.

If the window is very old (pre-1980s) and you suspect it may contain lead paint, do not scrape or sand it. Lead dust is hazardous. Contact your local health department for guidance on lead-safe work, or hire a contractor certified in lead abatement.

Frequently Asked Questions

Can I caulk a window that opens and closes?

No. Caulk will crack and fail if the sash moves. Use weatherstripping instead, which compresses and releases as the window opens and closes. Caulk only the perimeter (where the frame meets the wall), not the operating gap (where the sash meets the frame).

How long does caulk last on a window?

Acrylic latex caulk typically lasts 5 to 10 years. Silicone caulk lasts 20+ years. The lifespan depends on sun exposure, temperature swings, and moisture. Caulk on the south or west side of a house fails faster because of UV exposure.

What if I caulked over old caulk by mistake?

Remove the new caulk and the old caulk underneath. The new caulk will not bond properly to the old material and will fail within months. It is worth the extra time to scrape it all out and start fresh.

Do I need to prime or paint the caulk?

Acrylic latex caulk can be painted, but it does not require primer. Let it cure fully (24 to 48 hours) before painting. Silicone caulk cannot be painted. If you need a painted finish, use acrylic latex or a paintable silicone caulk.

What if the window frame is metal, not wood?

Metal frames can be caulked the same way as wood. Use silicone caulk or acrylic latex caulk rated for metal. Metal conducts heat and cold, so it may expand and contract more than wood, which can cause caulk to crack—silicone is more flexible and handles this better.