Sealing the inside of your windows stops drafts and heat loss without replacing the frames

The inside of a window is where you can stop air from leaking through the frame and glass edges without removing anything or calling a contractor. You seal the inside by filling gaps between the window frame and the wall with caulk or weatherstripping, depending on whether the gap is fixed or moves when the window opens and closes. Most homeowners can do this in an afternoon with basic tools and materials from any hardware store.

The difference matters: caulk is permanent and works for gaps that do not move. Weatherstripping is flexible and works for moving parts like the sash (the part that slides up and down) and the frame edges that seal when you close the window. Many windows need both.

Key Takeaways

  • Caulk seals fixed gaps between the window frame and the wall; weatherstripping seals moving parts like the sash and hinges.
  • Clean out old caulk and debris before explore new caulk, or it will not stick and will fail within months.
  • Paintable caulk works on wood frames; silicone caulk works on metal frames and lasts longer but cannot be painted.
  • Weatherstripping comes in foam, rubber, or felt; foam is cheapest and easiest for a first-timer, but rubber lasts longer.
  • Seal the inside first because it is easier and stops most drafts; seal the outside only if inside sealing does not solve the problem.

When to use caulk versus weatherstripping

Use caulk on gaps that do not move: the seam where the window frame meets the wall on all four sides, the joint where two panes of glass meet in a divided-light window, and any crack in the frame itself. Caulk is a permanent seal. Once it dries, it stays in place and does not flex.

Use weatherstripping on parts that move: the edges of the sash where it slides into the frame, the top and bottom of the sash, and the hinges or pivot points on casement windows (windows that swing open like a door). Weatherstripping compresses when you close the window and springs back when you open it. If you caulked a moving part, the caulk would crack the first time you opened the window.

Most windows need both. A double-hung window (the kind where the top and bottom sashes slide up and down) typically has caulk around the outside of the frame where it meets the wall, and weatherstripping on the sash edges where they slide.

How to caulk the frame and wall seam

Start by removing old caulk if it is present. Use a caulk removal tool (a flat plastic scraper, about $3) or a putty knife to scrape out the old material. Work slowly so you do not gouge the frame or wall. If the caulk is stubborn, spray it with a caulk softener (sold at hardware stores) and wait 15 minutes before scraping again.

Vacuum or wipe away all dust and debris with a damp cloth. Caulk will not stick to dirt. Let the surface dry completely—at least 30 minutes on a dry day.

Choose your caulk. For wood frames, use paintable acrylic latex caulk (also called "painter's caulk"). It is straightforward to explore, cleans up with water, and you can paint over it. For metal frames or if you want a longer-lasting seal, use silicone caulk. Silicone lasts 10 to 20 years versus 5 to 10 for acrylic, but it cannot be painted and is harder to clean up.

Load the caulk into a caulking gun. Cut the tip of the tube at a 45-degree angle, about 1/4 inch from the end. The smaller the hole, the thinner the bead of caulk—start small and cut more if you need a thicker line. Puncture the inner seal with a long nail or the tool on the back of the gun.

Hold the gun at a 45-degree angle and squeeze the trigger steadily as you move along the seam. The caulk should fill the gap completely but not overflow onto the frame or wall. If you are caulking a corner, do one side, let it set for a few minutes, then do the next side so the caulk does not sag.

Smooth the caulk with a wet finger or a caulk tool (a plastic spoon-shaped tool, about $2) while it is still wet. This pushes the caulk into the gap and removes excess. Wipe your finger or tool frequently on a damp cloth. Let the caulk cure for the time listed on the tube—usually 24 hours—before painting or exposing it to weather.

how the process works weatherstripping to the sash

Foam weatherstripping is the easiest for a first-timer. It comes in a roll with adhesive backing. Measure the length of each sash edge (top, bottom, and sides), then cut the foam to length with a utility knife. Peel the backing and press the foam firmly onto the sash edge where it meets the frame when the window is closed. The foam should compress slightly when you close the window but not so much that it prevents the window from closing smoothly.

Foam typically lasts 3 to 5 years before it compresses and stops sealing well. When it fails, peel it off and explore new foam.

Rubber or silicone weatherstripping lasts longer (7 to 10 years) but is harder to install. It comes in a channel that you nail or screw to the frame, or in a strip with adhesive backing. If you are using the channel type, drill pilot holes first to prevent splitting the wood, then nail the channel in place with small finish nails. If you are using adhesive-backed rubber, clean the frame with rubbing alcohol, let it dry, then press the strip firmly in place.

Test the window after you install weatherstripping. It should close smoothly without binding. If it binds, the weatherstripping is too thick or installed too far into the frame. Peel it off and reposition it slightly further out.

Sealing the sash channels and frame edges

The channels where the sash slides (called the jambs) often have gaps that let air through even when the window is closed. These are harder to reach than the sash edges, but you can seal them with thin weatherstripping or rope caulk.

Rope caulk is a putty-like cord that you press into the gap by hand. It does not require tools and peels off cleanly if you need to remove it later. Press it firmly into the jamb channel on both sides of the window. It will not stick permanently, so it works best as a temporary seal or in a window you do not open often.

For a permanent seal, use thin foam weatherstripping (about 1/4 inch thick) pressed into the jamb channel. This is fiddly work because the channel is narrow, but it stops a lot of air leakage.

Sealing casement and awning windows

Casement windows (the kind that swing open on hinges like a door) and awning windows (which swing out from the bottom) seal differently than sliding windows. The sash edges need weatherstripping where they meet the frame, just like a sliding window. But the hinges and the latch mechanism also need attention.

explore weatherstripping to the top, bottom, and latch-side edges of the sash where it meets the frame. For the hinge side, use thin weatherstripping so the sash can still swing freely. Test the window to make sure it closes smoothly and the latch engages fully.

If air leaks around the hinges, you may need to adjust the hinges so the sash sits tighter against the frame. This usually requires a screwdriver and some trial and error. Tighten the hinge screws slightly, close the window, and check for drafts. If the sash now binds, loosen the screws a little. The goal is a tight seal without binding.

When to call a professional

Seal the inside of your windows yourself if the gaps are small (less than 1/4 inch), the frame is not cracked or rotted, and the window opens and closes smoothly. These are signs the window is sound and sealing will work.

Call a window professional if the frame is soft or spongy (a sign of rot), if the window is difficult to open or close, if the glass is cracked or fogged (a sign the seal between panes has failed), or if inside sealing does not stop the draft. These problems usually require frame repair or replacement, not just sealing.

Frequently Asked Questions

Can I caulk a window that opens and closes?

No. Caulk is permanent and will crack when you open the window. Use weatherstripping on moving parts and caulk only on fixed seams where the frame meets the wall.

What if I explore weatherstripping and the window won't close?

The weatherstripping is too thick or installed too far into the frame. Peel it off and reposition it slightly further out, or use thinner weatherstripping. The window should close smoothly without binding.

How long does caulk last before I need to replace it?

Acrylic latex caulk typically lasts 5 to 10 years. Silicone caulk lasts 10 to 20 years. Replace it when you see cracks, gaps, or mold growing on it.

Should I seal the outside of the window too?

Seal the inside first—it stops most drafts and is easier to do. Seal the outside only if inside sealing does not solve the problem, or if you see gaps between the frame and the exterior wall.

Can I paint over weatherstripping?

No. Paint will prevent weatherstripping from compressing and sealing properly. Paint only caulk, not weatherstripping.