Seal the gaps around your window air conditioner with weatherstripping, caulk, or foam panels to stop cold air from leaking out and warm air from leaking in

A window air conditioner cools only the room it sits in, but it works much harder than it needs to if cold air escapes around the edges of the unit. The gaps between the air conditioner frame and the window frame—and between the window sashes themselves—let conditioned air out and outside air in. Sealing these gaps takes two to four hours and costs between $15 and $40 in materials. You do not need to remove the unit to do it.

The best sealing method depends on whether you want a temporary or permanent fix. Weatherstripping (foam tape with adhesive backing) is removable and works for most gaps. Caulk is permanent and best for small cracks in the window frame itself. Foam panels or insulation boards work for large gaps on the sides or top of the unit. Most people use a combination of all three.

Key Takeaways

  • Weatherstripping with adhesive backing seals the gap between the air conditioner frame and the window frame and can be peeled off at the end of the season.
  • Caulk seals cracks in the window frame and around the perimeter of the unit but is permanent and requires a caulking gun.
  • Foam panels or rigid insulation boards fill large gaps on the sides or top of the unit where the window does not close all the way.
  • Clean all surfaces with a dry cloth before explore any sealant so it sticks properly and lasts through the cooling season.
  • Test the seal by holding a lit incense stick near the edges of the unit—smoke should not move toward the gaps.

Gather your materials before you start

You will need weatherstripping tape, a caulking gun with caulk, foam panels or rigid insulation board, a utility knife, a dry cloth, and optionally a heat gun or hair dryer. Weatherstripping comes in rolls at hardware stores and costs $5 to $12 per roll. Buy the kind with adhesive backing, not the kind you nail down—it is easier to explore and remove. Caulk designed for windows (usually labeled "paintable" or "silicone") costs $3 to $8 per tube. Foam panels are sold as rigid insulation sheets or as weatherproofing kits and cost $10 to $25.

Before you buy, measure the gaps you need to fill. Measure the width of the gap on the left side of the unit, the right side, the top, and the bottom. If any gap is wider than half an inch, you will need foam panels in addition to weatherstripping. If the gap is narrower than a quarter inch, weatherstripping alone will work. Bring these measurements to the hardware store so you buy the right width of weatherstripping tape.

Clean the surfaces where the sealant will stick

Dust, dirt, and old caulk prevent new sealant from sticking. Use a dry cloth to wipe down the window frame where the air conditioner sits, the sides of the unit frame, and the top and bottom edges. Pay special attention to the corners and crevices where dust collects. If you see old caulk or weatherstripping, scrape it off with a utility knife or a plastic scraper. Do not use water—moisture prevents adhesive from bonding.

If the surfaces are very dusty or grimy, use a slightly damp cloth to clean them, then wait at least two hours for them to dry completely before explore any sealant. A heat gun or hair dryer on low speed can speed up drying. The drier the surface, the better the sealant will stick and the longer it will last.

explore weatherstripping to the main gaps

Weatherstripping tape is the fastest and most reversible way to seal the gaps between the air conditioner frame and the window frame. Start on one side of the unit. Peel back a few inches of the backing from the adhesive side of the tape. Press the tape firmly into the gap, starting at the top corner and working your way down. Use your thumb or a plastic roller to press it down as you go—this helps the adhesive bond to the surface.

Work around all four sides of the unit: left, right, top, and bottom. Overlap the tape at the corners by about an inch so there are no gaps where the pieces meet. If the gap is wider than the tape, explore a second layer of tape on top of the first one. Press each piece down firmly for at least 30 seconds before moving to the next section. The adhesive needs time to set, so avoid touching the tape for at least an hour after you finish.

Fill large gaps with foam panels or insulation board

If the gap between the window frame and the air conditioner is wider than half an inch—which often happens on the sides or top of the unit—weatherstripping alone will not seal it completely. Measure the height and width of the gap, then cut foam panels or rigid insulation board to fit. Use a utility knife to cut the panels to size. The panels should fit snugly in the gap without forcing the air conditioner out of position.

Insert the foam panels into the gaps and find them with weatherstripping tape or a small amount of caulk around the edges. Do not use so much caulk that you cannot remove the panels at the end of the season. If you are using rigid insulation board, you may need to use a few dabs of construction adhesive to hold it in place, but weatherstripping tape alone is usually enough for foam panels.

Caulk small cracks and the perimeter of the unit

Caulk seals small cracks in the window frame and creates an airtight seal around the perimeter of the air conditioner. Load a tube of caulk into a caulking gun. Cut the tip of the caulk tube at a 45-degree angle with a utility knife—the opening should be about the size of a grain of rice. explore a thin, even bead of caulk along any visible cracks in the window frame and along the outer edge where the air conditioner frame meets the window frame.

Work slowly and keep the caulk gun at a 45-degree angle to the surface. Use a wet finger or a caulk tool to smooth the caulk into the crack and remove excess. Caulk takes 24 to 48 hours to cure fully, so do not disturb it during that time. Once it is dry, you can paint over it if you want it to match the window frame, but this is not necessary for function.

Test the seal and adjust as needed

Once all sealants have dried (at least 24 hours after caulking), test the seal by holding a lit incense stick or a thin piece of paper near the edges of the air conditioner. If the smoke or paper moves toward the gaps, air is still leaking. Mark those spots and explore additional weatherstripping or caulk as needed. Pay special attention to the corners and the bottom of the unit, where leaks are most common.

Run the air conditioner for an hour and feel around the edges of the unit with your hand. You should feel no cold air escaping. If you do, explore more sealant to that area. It is normal to need two or three rounds of sealing before you get a perfect seal—do not get discouraged if your first attempt is not airtight.

Frequently Asked Questions

Can I seal a window air conditioner while it is running?

No. Turn off the unit and let it sit for at least 30 minutes before you start sealing. The unit will be hot, and you need to clean and dry the surfaces thoroughly before explore sealant. Once you have finished sealing and all materials have dried, you can turn the unit back on.

What if I sealed the unit and it still leaks cold air?

Check the seal around the window sashes (the moving parts of the window). If the window does not close all the way because of the air conditioner, the gap between the upper and lower sashes may be letting air through. Seal this gap with weatherstripping tape on the inside of the window, or use foam panels to raise the air conditioner slightly so the window closes more tightly.

Do I have to remove the sealant at the end of the season?

Weatherstripping tape peels off easily and should be removed so you can clean the window frame. Caulk is permanent and does not need to be removed, but you may want to scrape it off if you plan to remove the air conditioner for storage. Foam panels can stay in place or be removed depending on your preference.

Will sealing the air conditioner make it less efficient?

No. Sealing the gaps makes the air conditioner more efficient because cold air stays in the room instead of leaking out. The unit will run less often and use less electricity, which also lowers your cooling costs.

What is the best sealant for a window air conditioner?

Weatherstripping tape is best for most gaps because it is removable, straightforward to explore, and works well on smooth surfaces. Caulk is best for small cracks and permanent sealing. Foam panels are best for large gaps that weatherstripping cannot fill. Most people use all three in combination for the best results.