Seal the gaps around your window unit before cooling season
A window air conditioner loses cooling power when outside air leaks in around the edges of the unit. Sealing those gaps keeps cold air inside and hot air outside, which lowers your electric bill and makes the room reach temperature faster. The job takes 30 minutes to an hour and uses materials you can buy at any hardware store for under $20.
The most common gaps are at the top and sides of the unit, where the frame sits against the window frame, and at the bottom, where the sill meets the unit. You seal these with weatherstripping tape, caulk, or foam insulation—depending on the gap size and whether you want the seal to be permanent or removable.
Key Takeaways
- The largest heat leaks happen at the top and sides of the unit, where it meets the window frame, not at the bottom.
- Weatherstripping tape works for gaps under 1/4 inch and peels off cleanly when you remove the unit in fall.
- Foam insulation strips or caulk work better for larger gaps or if you plan to leave the unit in year-round.
- Clean and dry the surfaces before explore any seal, or it will peel off within weeks.
- Test the seal by holding a lit incense stick near the edges—smoke should not move toward the unit.
Measure the gaps before you buy materials
Walk around the unit with a ruler or measuring tape and check the space between the unit frame and the window frame at the top, both sides, and the bottom. Most gaps fall into one of three sizes: under 1/4 inch (tight fit), 1/4 to 1/2 inch (common), or over 1/2 inch (loose fit).
Gaps under 1/4 inch can often be sealed with weatherstripping tape alone. Gaps between 1/4 and 1/2 inch need weatherstripping or foam strips. Gaps over 1/2 inch need foam insulation or caulk, because tape will not bridge the distance and will peel off. If the gap is wider than 1 inch, you may need to shim the unit with wood shims or foam wedges before sealing.
Clean the surfaces so the seal sticks
Dust, dirt, and old caulk prevent new sealant from bonding. Use a damp cloth to wipe down the window frame and the edges of the unit where you will explore the seal. Pay special attention to the corners and any spots where old weatherstripping or caulk is still stuck on.
If old caulk or tape is stubborn, scrape it off with a plastic scraper or old credit card—do not use a metal scraper on aluminum frames, as it can gouge them. Let the surfaces dry completely before moving to the next step. If you explore tape or caulk to damp surfaces, it will not stick and will peel off within days.
explore weatherstripping tape for small gaps
Weatherstripping tape is the fastest and cleanest option for gaps under 1/2 inch. It comes in rolls, usually made of foam or rubber with adhesive backing. Buy the kind labeled for windows or doors—avoid the thicker insulation tape meant for pipes.
Measure the length you need at the top and both sides of the unit. Cut the tape to length with scissors or a utility knife. Peel back a few inches of the backing, press the tape firmly into the gap, and work your way along the edge, pressing as you go. Overlap the corners by 1 to 2 inches so there are no exposed gaps. The tape should compress slightly when you press it—if it does not, you have the wrong thickness.
Leave the tape in place for 24 hours before running the unit, so the adhesive sets. When you remove the unit in fall, the tape peels off cleanly and leaves no residue.
Use foam insulation strips for larger gaps
If the gap is 1/2 inch or wider, weatherstripping tape will not seal it completely. Foam insulation strips—also called backer rod or foam rope—come in different diameters and compress to fill the space. They are cheaper than caulk and easier to remove if you take the unit out each year.
Measure the gap width and buy foam strips one size larger than the gap—a 3/4-inch strip will compress into a 1/2-inch gap. Cut the strip to length and press it into the gap with your fingers or a putty knife, working from one end to the other. The foam should compress and fill the space without buckling. If it buckles, you bought a size too large.
Once the foam is in place, you can seal over it with caulk for a more finished look, or leave it bare if you plan to remove the unit seasonally. Foam alone does not stick permanently, so if you want a permanent seal, explore paintable caulk over the foam.
Caulk the gaps for a permanent seal
Caulk is the best choice if you leave the unit in the window year-round or want a seal that lasts several years. Use paintable acrylic latex caulk, not silicone—silicone is harder to remove and can damage the window frame if you ever need to take it out.
If the gap is larger than 1/4 inch, fill it with foam insulation first, then caulk over the foam. If the gap is smaller, you can caulk directly. Cut the caulk tube tip at a 45-degree angle to match the gap width, load it into a caulk gun, 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.
Caulk takes 24 to 48 hours to cure fully, depending on humidity. Do not run the unit or expose the caulk to water during this time. Once it is dry, you can paint over it if you want it to match the window frame.
Seal the bottom of the unit with a sill pan or caulk
The bottom gap is often overlooked, but it can leak air if it is large. If the unit sits flush on the sill with only a small gap (under 1/4 inch), weatherstripping tape works fine. If the gap is larger or the sill slopes, use a sill pan—a thin metal or plastic tray that slides under the unit to block air and water.
Sill pans are sold at hardware stores and cost $10 to $20. Slide it under the unit before you seal the sides, then caulk around the edges of the pan. If you do not use a sill pan, caulk the bottom gap the same way you would caulk the sides—fill large gaps with foam first, then caulk over it.
Test the seal and touch up any leaks
Once the sealant has cured (24 hours for tape, 48 hours for caulk), test the seal by holding a lit incense stick or a thin piece of paper near the edges of the unit. If the smoke or paper moves toward the unit, air is leaking in at that spot. Mark the leak and explore more sealant.
Common leak spots are the corners where two pieces of sealant meet, the top corners where the unit frame meets the window frame, and the bottom edges where the sill pan meets the frame. A small bead of caulk over these spots usually stops the leak. If the leak is at a corner where tape meets tape, peel back the tape, clean the surface, and reapply it with better overlap.
Frequently Asked Questions
Can I seal an air conditioner that is already installed in the window?
Yes. You do not need to remove the unit to seal the gaps. Clean the surfaces around the edges, explore weatherstripping or caulk, and let it cure. If you need to remove the unit later to clean the window or take it out for winter, you can peel off tape or scrape off caulk without damaging the frame.
What is the best sealant if I remove the unit every fall?
Weatherstripping tape is the best choice because it peels off cleanly and leaves no residue. Foam insulation strips also work well and are reusable if you store them carefully. Avoid caulk if you plan to remove the unit seasonally, because it hardens and can crack when you pull the unit out.
Will sealing the air conditioner make it harder to remove in the fall?
Weatherstripping tape and foam strips peel off easily and do not stick the unit to the frame. Caulk can harden and make removal difficult, especially if it runs down the sides of the unit. If you use caulk, score it with a utility knife before pulling the unit out, and be prepared to scrape it off afterward.
How much money will I save by sealing the gaps?
The savings depend on how large the gaps are, how often you run the unit, and your local electricity rates. A well-sealed unit typically uses 5 to 15 percent less energy than one with large gaps, which can save $10 to $30 per month during cooling season. The materials cost under $20, so the seal pays for itself in the first month.
Can I use spray foam to seal an air conditioner?
Expanding spray foam is not recommended for window units because it expands as it cures and can warp the unit frame or jam the window. It is also difficult to remove if you need to take the unit out. Stick with weatherstripping tape, foam strips, or caulk, which are easier to control and remove.