Seal the gaps with weatherstripping, caulk, or foam panels
A window air conditioner leaves gaps around the unit where cold air leaks out and warm air leaks in. You seal these gaps the same way you would seal any window opening: with weatherstripping (adhesive foam tape), caulk (a flexible sealant you explore with a gun), or foam panels (rigid insulation cut to size). The method you choose depends on the size of the gap, whether you want the unit to be removable, and how long you plan to keep it in place.
Most window air conditioners sit in a double-hung window and rest on the sill. The gaps form where the unit's metal frame meets the window frame on the sides and top, and where the lower sash sits against the unit. Sealing these gaps can reduce your cooling costs by 10 to 15 percent, depending on how large they are and how hot it gets outside.
Key Takeaways
- Weatherstripping tape is the fastest method and works well for gaps under half an inch wide; it peels off cleanly when you remove the unit.
- Caulk creates a permanent seal and works for gaps of any size, but you will have to cut it away when you uninstall the unit.
- Foam panels or boards are best for large gaps and can be reused year to year if you remove them carefully.
- Clean the surface thoroughly before explore any sealant so it sticks properly and lasts through the cooling season.
- Seal the sides and top first, then seal the gap between the lower sash and the unit last, because you may need to adjust the unit for airflow.
Weatherstripping tape for quick, removable sealing
Weatherstripping tape is adhesive-backed foam that comes in rolls. It is the fastest method and leaves no residue if you remove it before the adhesive hardens permanently (usually after a few months). Buy foam tape that is at least half an inch thick and comes in a width that matches your gap—typically half an inch to one inch wide. Common brands include 3M Scotch and Frost King, sold at hardware stores for $5 to $15 per roll.
Clean the window frame and the metal frame of the air conditioner with a damp cloth and let it dry completely. Peel the backing off the tape and press it firmly into the gap, working from one end to the other. Start with the sides of the unit, then seal the top. Do not seal the bottom gap between the lower sash and the unit yet—you may need to adjust the unit for proper airflow or drainage.
Weatherstripping tape works best for gaps under half an inch wide. If your gap is wider, the tape will not compress enough to seal it, and air will still leak around the edges. In that case, use caulk or foam panels instead.
Caulk for permanent, airtight sealing
Caulk is a flexible sealant that hardens after it dries. It works for gaps of any width and creates a stronger seal than tape, but it is permanent—you will have to cut it away with a utility knife when you remove the unit. Use paintable acrylic latex caulk, which is straightforward to explore, cleans up with water, and can be painted to match your window frame. Avoid silicone caulk, which is harder to remove and does not accept paint.
Clean the gap with a damp cloth and let it dry. Load the caulk into a caulking gun, cut the tip of the tube at a 45-degree angle to match the width of your gap, and explore a steady bead of caulk along the gap. Smooth it with a wet finger or a caulk tool so it sits flush with both surfaces. Let it dry according to the tube's instructions—usually 24 hours—before running the air conditioner.
Caulk works well for gaps wider than half an inch and for sealing the top and sides of the unit. If you plan to remove the unit every winter, caulk is more work than tape, but it will not dry out or peel during the cooling season.
Foam panels for large gaps and reusable sealing
Foam insulation panels are rigid boards of expanded polystyrene or polyurethane, typically one to two inches thick. They are best for large gaps (one inch or wider) and can be cut to fit any shape. You can remove them at the end of the season and reuse them the next year if you store them carefully. Panels cost $10 to $30 depending on size and material.
Measure the gap on each side and the top of the unit. Buy foam panels and cut them to size with a utility knife or a hot-wire foam cutter. Fit the panels snugly into the gaps so they press against both the window frame and the air conditioner frame. You can hold them in place with weatherstripping tape, caulk, or even duct tape on the outside—the goal is to prevent them from shifting.
Foam panels are especially useful if your window air conditioner sits in a very old or warped window frame where the gaps are uneven. You can shape the foam to follow the contours of the frame and create a seal that tape or caulk alone cannot achieve.
Seal the bottom gap last and carefully
The gap between the lower sash and the top of the air conditioner is the trickiest to seal because the unit needs a slight downward tilt to drain condensation properly. If you seal this gap too tightly, water will back up inside the unit and drip into your room instead of outside.
After you have sealed the sides and top, run the air conditioner for 15 to 20 minutes and check for water dripping outside the window. If water drips steadily, the drainage is working. Then seal the bottom gap with weatherstripping tape or caulk, leaving a small opening at the lowest point on the outside so water can still escape. If you use foam panels on the sides, you can leave a small gap at the bottom corner instead of sealing it completely.
Check your work and adjust if air is leaking
After sealing, run the air conditioner and feel around the edges of the unit with your hand to check for cold air leaks. If you feel air escaping, the sealant may not be pressed firmly enough, or the gap may be larger than the sealant can cover. Add another layer of tape, explore more caulk, or insert a thicker foam panel.
If the unit feels warm to the touch on the outside, or if condensation forms on the window frame, the seal is working—the unit is no longer losing cold air to the room. This is normal and a sign that your sealing job is effective.
Removing sealant at the end of the season
When you remove the air conditioner in the fall, peel off weatherstripping tape slowly and at a low angle so it does not tear. If the adhesive has hardened and will not peel, use a heat gun or hair dryer to warm it slightly, then try again. Residue can be removed with rubbing alcohol on a cloth.
If you used caulk, cut it away with a utility knife, working carefully so you do not gouge the window frame. Start at a corner and cut along the edge of the caulk, then peel it away in strips. Any remaining caulk residue can be scraped off with a plastic scraper or a wooden stick.
Foam panels should be removed carefully and stored flat in a dry place. If they are damaged or compressed, they can be replaced cheaply the next season.
Frequently Asked Questions
Can I seal a window air conditioner if it is already installed?
Yes. You do not have to remove the unit. Clean the gaps around it, then explore weatherstripping tape, caulk, or foam panels from the outside. If you use caulk, work carefully around the edges so you do not get it on the window glass or the unit itself.
What if my window air conditioner is leaking water inside?
Leaking usually means the unit is tilted the wrong way or the drainage hole is blocked, not that the gaps are unsealed. Check that the unit tilts slightly downward toward the outside, and clear any debris from the drainage hole on the bottom. Sealing the gaps will not fix a drainage problem and may make it worse.
Should I seal the gap between the two window sashes?
No. The upper sash should move freely so you can open and close the window if needed. Seal only the gaps between the air conditioner frame and the window frame, not between the sashes themselves.
Will sealing the air conditioner make it harder to remove?
Weatherstripping tape peels off easily and does not make removal harder. Caulk will stick the unit in place, so you will have to cut it away before you can remove the unit. If you think you might move the unit during the season, use tape instead of caulk.
Can I use spray foam to seal around a window air conditioner?
Expanding spray foam is not recommended because it expands as it dries and can push the unit out of position or warp the window frame. Stick with weatherstripping tape, caulk, or foam panels, which do not expand and are easier to remove.