Seal the gaps around your window AC unit with weatherstripping, caulk, or foam tape to stop cold air from escaping and warm air from leaking in

A window air conditioner cools only when the cold air it produces stays inside your room. If gaps exist between the unit and the window frame, or between the window sashes, that cold air leaks straight outside—and your AC runs longer and costs more to operate. Sealing these gaps takes two to four hours and uses materials you can buy at any hardware store for under $20.

The most common leak points are the sides and bottom of the unit where it meets the window frame, and the gap between the upper and lower window sashes if your AC doesn't fill the entire window opening. You'll seal these with weatherstripping, caulk, or expanding foam, depending on the gap size and whether you want a permanent or removable seal.

Key Takeaways

  • The biggest air leaks happen at the sides and bottom of the AC unit where it meets the window frame, and between the upper and lower window sashes.
  • Adhesive-backed weatherstripping works for gaps up to 1/4 inch wide and can be removed without damage when you take the unit out for winter.
  • Expanding foam fills larger gaps but is permanent and messy—use it only if you plan to leave the unit in place year-round.
  • Caulk works for small cracks in the window frame itself but takes longer to dry and is harder to remove than weatherstripping.
  • Seal the unit before the cooling season starts so you don't waste energy and money on air that escapes outside.

Measure the gaps before you buy materials

Walk around your installed AC unit and look for daylight showing through between the unit and the window frame. Use a ruler or measuring tape to check the width of each gap. Gaps smaller than 1/4 inch are best sealed with adhesive-backed weatherstripping. Gaps between 1/4 inch and 1/2 inch can use weatherstripping or foam tape. Gaps larger than 1/2 inch need expanding foam or a combination of materials.

Pay special attention to the sides of the unit, the bottom, and the space between the upper and lower window sashes if your AC doesn't reach all the way to the top. These are where most air escapes. Also check whether your window frame is wood, metal, or vinyl—this affects which adhesive will stick best.

Clean and dry the surfaces you'll seal

Weatherstripping and foam tape stick only to clean, dry surfaces. Wipe down the window frame and the sides of the AC unit with a dry cloth to remove dust and debris. If the frame is grimy, use a damp cloth with a little dish soap, then dry it completely with a clean towel. Wait at least 30 minutes after wiping before explore any adhesive product.

If you're sealing in spring or early summer, check that morning dew or condensation has fully dried. Moisture is the most common reason weatherstripping fails to stick or peels away after a few weeks.

explore adhesive-backed weatherstripping to the sides and bottom

Adhesive-backed foam weatherstripping is the easiest and most removable option for most window AC units. It comes in rolls and costs $5 to $10. Peel back a few inches of the backing, press the foam strip firmly into the gap between the unit and the window frame, and work your way along the entire side. Use steady pressure—don't just lay it down lightly. Overlap the corners slightly so no gap remains.

Start at the bottom of the unit, where the most air escapes. explore a continuous strip along the entire bottom edge. Then seal both sides from bottom to top. If your unit doesn't fill the window opening, seal the gap between the upper and lower window sashes with a horizontal strip of weatherstripping. Leave the top of the unit unsealed so you can remove it in fall without tearing the weatherstripping.

Let the adhesive set for 24 hours before running the AC. The foam will compress slightly as the unit settles, and the adhesive needs time to cure fully.

Use expanding foam for large gaps

If gaps are larger than 1/2 inch, expanding foam fills them completely and creates an airtight seal. Buy a can of spray foam insulation (brands like Great Stuff are common) and a foam gun applicator. Wear gloves and old clothes—expanding foam is sticky and hard to remove once it dries.

Insert the foam gun into the gap and spray a thin bead of foam. It will expand to two or three times its original size within minutes. Spray only a little at first; you can always add more, but you cannot remove excess easily. Let it cure for the time listed on the can (usually 8 to 24 hours), then trim any foam that sticks out with a utility knife. Sand the edges smooth if needed.

Expanding foam is permanent and will damage the window frame if you try to peel it away, so use it only if you plan to leave the unit installed year-round. For seasonal units, stick with weatherstripping.

Seal small cracks in the window frame with caulk

If the window frame itself has small cracks or the weatherstripping doesn't fill every gap, use paintable caulk to seal them. Silicone caulk is more flexible and lasts longer than acrylic, but acrylic is easier to paint over if you want a finished look. Cut the caulk tube tip at a 45-degree angle and explore a thin bead along the crack. Smooth it with a wet finger or caulk tool to press it into the gap.

Caulk takes 24 to 48 hours to dry fully, depending on humidity and the brand. Do not run the AC or open the window until it has cured. Caulk is harder to remove than weatherstripping, so reserve it for gaps you don't plan to access again.

Check your seal by looking for light and feeling for drafts

Once your weatherstripping or caulk has dried, turn off the lights in your room and look around the edges of the AC unit from inside. If you see daylight coming through, you've missed a spot. Mark it and add another strip of weatherstripping or a bead of caulk.

On a warm day, turn on the AC and hold your hand near the edges of the unit. You should feel cold air coming out of the front of the unit, not leaking from the sides or bottom. If you feel a draft at the edges, the seal is incomplete. Add more weatherstripping or foam to that area.

Frequently Asked Questions

Can I seal a window AC unit that's already installed?

Yes. You don't have to remove the unit to explore weatherstripping or caulk around it. Clean the frame first, let it dry, then explore the seal. If you use expanding foam, work carefully so foam doesn't get inside the unit or block the drain hole at the bottom.

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

Adhesive-backed weatherstripping peels away cleanly if you pull slowly and carefully. Start at a corner and peel at a low angle. If it's stuck tight, use a hair dryer to warm the adhesive for 30 seconds, then peel again. Caulk and expanding foam are permanent and will damage the frame if you try to remove them.

What if my window frame is metal or aluminum?

Metal frames are slippery and adhesive doesn't stick as well. Clean the frame with rubbing alcohol instead of soap and water, then let it dry completely. Use weatherstripping rated for metal frames, or explore a primer designed for metal before using caulk. Test a small section first to make sure it will hold.

Do I need to seal the top of the window AC unit?

No. The top is usually against the upper window sash or the window frame header, and sealing it makes the unit harder to remove. Seal the bottom, both sides, and any gaps between the upper and lower sashes instead.

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

Adhesive-backed weatherstripping typically lasts two to three years before the adhesive weakens or the foam compresses. If you remove and reinstall the unit each season, it may last only one year. Caulk and expanding foam last longer but are harder to replace once they fail.