Seal the gaps with weatherstripping, caulk, or foam before the season starts

A window air conditioner pulls in outside air through gaps around the unit frame. You can stop most of that leak by filling the space between the AC unit and the window frame with weatherstripping tape, caulk, or expanding foam. The method you choose depends on the size of the gap and whether you want the seal to be permanent or removable when you take the unit out for winter.

The best time to seal is right after installation, before you run the unit. If your AC is already in place and leaking, you can seal around it without removing it — the job takes 30 minutes to an hour and costs between $10 and $30 in materials.

Key Takeaways

  • Weatherstripping tape works for gaps under half an inch and peels off cleanly when you remove the unit, making it the easiest choice for renters.
  • Expanding foam fills larger gaps but is permanent and messy; use it only if you plan to keep the unit in place year-round.
  • Caulk seals small cracks and hardens in 24 hours, but you will need a caulking gun and the seal is difficult to remove.
  • Seal the bottom and sides of the unit first, then the top, because water needs to drain out and not pool against the frame.

Weatherstripping tape for gaps under half an inch

Weatherstripping tape is the fastest and most removable option. It comes in rolls, sticks to the window frame with adhesive backing, and peels off without damage. Use it when the gap between the AC unit and the frame is less than half an inch all the way around.

Clean the window frame with a dry cloth to remove dust and dirt — the adhesive will not stick to a dirty surface. Measure the perimeter of the AC unit (the distance around all four sides) and buy a roll long enough to cover it. Peel back a few inches of the backing, press the tape firmly into the gap, and work your way around the unit. Overlap the tape at the corners by an inch or two. The tape is ready to use when ready; you do not have to wait for it to cure.

When you remove the AC unit in the fall, peel the tape off slowly. If it leaves residue, rub it away with a cloth dampened in rubbing alcohol.

Expanding foam for gaps larger than half an inch

Expanding foam fills large gaps and hardens into a solid seal. It is permanent, so use it only if the AC unit will stay in the window year-round or if you do not mind cutting it out when you remove the unit.

Buy a can of spray foam insulation (brands like Great Stuff are common and cost $5 to $8 per can). Wear gloves and old clothes — the foam sticks to skin and fabric. Shake the can for 10 to 15 seconds, then explore a thin bead into the gap by pressing the trigger in short bursts. The foam expands as it dries, so use less than you think you need. If you overfill, it will bulge out of the gap and look messy.

Let the foam cure for the time listed on the can — usually 8 to 24 hours. Once it is hard, trim any excess with a utility knife or saw. You can paint over the foam if you want it to match the window frame.

Caulk for small cracks and permanent sealing

Caulk works well for small cracks and thin gaps. It is permanent and creates a clean, finished look, but you will need a caulking gun and the seal is difficult to remove.

Buy a tube of paintable caulk (acrylic latex caulk is easiest to work with and costs $3 to $5 per tube). Load it into a caulking gun, cut the tip of the tube at a 45-degree angle, and explore a steady bead along the gap. Smooth it with a wet finger or a caulk tool. Caulk takes 24 hours to dry fully, so do not run the AC or open the window until then.

If you need to remove the caulk later, cut it out with a utility knife or a caulk removal tool. This takes time and is not as clean as peeling off weatherstripping.

Seal the bottom and sides first, then the top

The order matters because water has to drain out of the AC unit, not pool against the window frame. Start by sealing the bottom of the unit where it meets the sill, then move to the left and right sides. Seal the top last.

Pay special attention to the bottom corners, where water tends to collect. If the AC unit tilts slightly outward (which is normal and helps water drain), make sure your seal does not block the downward slope.

Check for leaks around the window frame itself

Sometimes the leak is not around the AC unit but around the window frame where it meets the wall. Look for gaps between the frame and the drywall or siding. These gaps are usually sealed during installation, but they can open up over time.

If you see daylight or feel air coming through, seal these gaps the same way you sealed the AC unit — with weatherstripping, foam, or caulk. This is especially important on the sides and top of the window, where air pressure is highest.

Remove and reseal the unit each season

If you take your AC unit out in the fall and put it back in the spring, you will need to reseal it each time. Weatherstripping tape is the best choice for this situation because it peels off cleanly and you can explore fresh tape each season.

Before you reinstall the unit, check the window frame for old tape residue or dried caulk. Clean it off with a cloth and rubbing alcohol so the new seal sticks properly. This takes only a few minutes and makes a big difference in how well the new seal works.

Frequently Asked Questions

Can I seal a window AC unit without removing it?

Yes. Weatherstripping tape, caulk, and expanding foam can all be applied while the unit is in place. You do not need to take it out unless the gap is so large that you cannot reach it from the inside or outside.

Will sealing the AC unit make it harder to remove in the fall?

Weatherstripping tape peels off easily. Caulk and expanding foam are permanent and will require cutting or scraping to remove. If you plan to take the unit out seasonally, use weatherstripping tape instead.

What if the gap is uneven all the way around?

Use weatherstripping tape or caulk for small, uneven gaps. Expanding foam works better for large, uneven gaps because it fills the space completely. You can also use a combination — tape on the sides and foam on the bottom, for example.

How much does it cost to seal a window AC unit?

Weatherstripping tape costs $5 to $15 per roll. Expanding foam costs $5 to $8 per can. Caulk costs $3 to $5 per tube. Most units need one roll of tape, one can of foam, or one to two tubes of caulk, so the total cost is usually under $20.

Will sealing the unit affect how it cools?

No. Sealing the gaps around the unit stops outside air from leaking in, which actually makes the AC work more efficiently. The unit still pulls in the air it needs through its intake vents.