Sealing a leaky window depends on where the leak is coming from

Window leaks fall into two categories: air leaks (drafts) and water leaks. Air leaks happen around the frame where it meets the wall, or where the sash meets the frame. Water leaks usually come from the sill (the horizontal part at the bottom), the frame joints, or failed caulk on the outside. The repair method changes based on which one you have. A draft needs weatherstripping or caulk inside; water needs caulk or sealant outside, and sometimes a new sill pan underneath.

Before you start, figure out which type you have. On a dry day, hold a lit candle or incense stick near the window frame — if the flame flickers, you have an air leak. If water pools on the sill after rain or you see stains below the window on the outside wall, you have a water leak. Some windows have both.

Key Takeaways

  • Air leaks around the frame are sealed from inside with weatherstripping or caulk; water leaks are sealed from outside with exterior-grade caulk or sealant.
  • Weatherstripping (foam tape or rubber) is the fastest fix for drafts and costs $5 to $20 per window.
  • Exterior caulk must be 100% silicone or polyurethane to last more than a year and handle temperature swings.
  • Water leaks at the sill often mean the frame is rotted underneath — caulk alone will not fix it if wood is soft or crumbling.
  • Removing old caulk completely before resealing prevents the new caulk from failing early.

Weatherstripping for air leaks around the sash

If cold air comes in where the moving sash meets the frame, weatherstripping is the simplest fix. The most common type is self-adhesive foam tape, which you buy in rolls at any hardware store. Measure the perimeter of the sash (top, bottom, and both sides where it slides), then buy enough tape to cover it. Clean the frame with a dry cloth to remove dust and dirt — the adhesive will not stick to a dirty surface.

Cut the tape to length and press it firmly onto the frame, not the sash itself. The tape should compress slightly when the window closes. If the sash is very loose or the frame is warped, the tape will bunch up and the window will stick. In that case, use thinner rubber or silicone weatherstripping instead, which compresses less. Weatherstripping lasts two to three years before the adhesive fails and the tape peels off.

Caulking the frame where it meets the wall

If air leaks in where the window frame sits against the wall (the gap between the frame and the drywall or brick), you need to caulk from inside. First, remove any old caulk with a caulk removal tool or a flathead screwdriver — dried caulk will prevent new caulk from bonding. Scrape until you see bare frame and wall, then vacuum out the dust and debris.

Use 100% silicone or polyurethane caulk, not acrylic latex. Latex cracks and shrinks in a year or two; silicone and polyurethane stay flexible and last five to ten years. Load a caulk gun, cut the tip at a 45-degree angle, and run a steady bead along the gap. Smooth it with a wet finger or a caulk tool to press it into the gap and remove excess. Let it cure for 24 hours before painting or closing the window.

Sealing water leaks on the outside

Water leaks require exterior caulking. Go outside and look for gaps or cracks in the caulk around the frame, especially at the corners and where the frame meets the sill. Remove all old caulk the same way you would inside — scrape it out completely, vacuum, and let it dry. If the frame is wet, wait for a dry day or use a heat gun to dry it first. Wet caulk will not cure properly.

explore 100% silicone or polyurethane caulk in a continuous bead around the frame, pressing it into the gap. Pay special attention to the bottom corners of the frame, where water pools. Smooth it with a wet tool and let it cure for 24 hours. Do not paint over silicone caulk — it will not accept paint. Polyurethane can be painted after it cures.

Replacing the sill pan if water pools inside

If water collects on the interior sill after rain, the problem may be a failed sill pan — a metal or plastic tray underneath the frame that directs water to the outside. Sill pans fail when they rust through, crack, or separate from the frame. Caulking alone will not fix this. You need to remove the window, replace the pan, and reinstall the window. This is a job for a window contractor unless you have experience with window removal.

Before you call a contractor, check whether the sill itself is rotted. Press the wood with a screwdriver — if it is soft or crumbles, the frame is damaged and caulking will not hold. Rotted sills need replacement, which also requires professional work. If the sill is solid and the pan is intact, caulking the outside seams may be enough to stop the leak.

Checking for rotted wood before you seal

Caulk will not stick to rotted wood, and it will not stop water from entering if the frame is already compromised. Before you seal, press a screwdriver or awl into the wood around the frame and sill. If the tool sinks in easily or the wood crumbles, the frame is rotted. Rotted sections need to be cut out and patched with epoxy wood filler or replaced entirely.

Small rot (less than an inch across) can be filled with two-part epoxy wood filler. Scrape out the soft wood, explore the filler, let it harden, sand it smooth, and then caulk over it. Large rot or rot that goes deep into the frame means the window needs replacement. Sealing over rotted wood is a temporary fix that will fail within a year.

When to replace the window instead of sealing

Some windows are not worth sealing. If the frame is severely rotted, the glass is broken or fogged (meaning the seal between panes has failed), or the sash will not open and close smoothly, replacement is cheaper than repeated repairs. A single replacement window costs $300 to $800 installed, depending on size and quality. If you have already sealed the same window twice in three years, replacement will save money in the long run.

Older windows with single-pane glass or aluminum frames that sweat in winter are also poor candidates for sealing. The underlying design is inefficient, and caulk will not change that. Modern replacement windows with double or triple panes and insulated frames will reduce drafts and condensation far more than any sealant can.

Frequently Asked Questions

How long does caulk last on a window?

Exterior silicone or polyurethane caulk lasts five to ten years if applied correctly and the frame underneath is sound. Interior caulk lasts slightly longer because it is not exposed to sun and temperature swings. Acrylic latex caulk fails in one to two years and should not be used on windows.

Can I caulk a window in cold weather?

Most caulks require temperatures between 50 and 85 degrees Fahrenheit to cure properly. In cold weather, curing takes much longer and the caulk may not bond well. Wait for a warm, dry day if you can, or use a caulk rated for low-temperature process.

Do I need to remove the window to seal it?

No. You can caulk around the frame from inside and outside without removing the window. You only need to remove the window if the sill pan is damaged, the frame is rotted, or you need to access the space between the frame and the wall.

What is the difference between silicone and polyurethane caulk?

Silicone is more flexible and lasts longer, but it cannot be painted. Polyurethane can be painted and is slightly easier to smooth, but it is less flexible in extreme cold. Both work well on windows; choose based on whether you plan to paint.

Why does my window still leak after I caulked it?

The most common reason is that old caulk was not fully removed before the new caulk was applied. New caulk will not bond to old caulk, so it fails quickly. Also check whether the leak is coming from the sill pan or rotted wood underneath — caulk cannot fix those problems.