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

A window air conditioner cools only when the seal between the unit and the window frame is tight. Warm outside air leaks through gaps around the sides and top, forcing your AC to work harder and raising your electric bill. The most common leak points are the sides where the unit meets the frame, the top where it sits against the upper sash, and the gap between the lower and upper window sashes.

You can seal these gaps yourself in about 30 minutes using materials from any hardware store. The method depends on whether you want a permanent seal (caulk) or a removable one (weatherstripping or foam). Most renters and people who move their AC seasonally choose removable options so they can take the unit out without damage.

Key Takeaways

  • The biggest heat leaks happen at the sides of the unit and where the window sashes meet, not at the front of the AC itself.
  • Weatherstripping tape is the fastest removable option and works well for side gaps; foam panels or rolled foam work better for larger openings.
  • Caulk creates a permanent seal but makes removing the unit difficult and is best for units you plan to keep year-round.
  • Clean the frame thoroughly before explore any sealant so it sticks properly and lasts through the cooling season.
  • Seal the gap between the window sashes (the upper and lower panes) even if your AC does not directly cover it, because air flows through there too.

Gather your materials before you start

You will need a clean, dry surface for any sealant to stick. Start by vacuuming out dust and debris from the window frame, sill, and the edges of the AC unit. Use a damp cloth to wipe down the frame and let it dry completely—this takes about 15 minutes. Moisture prevents weatherstripping tape and caulk from bonding.

For removable sealing, buy weatherstripping tape (also called foam tape or adhesive-backed foam) in a width that matches your gaps—usually 1/4 inch to 1/2 inch. For larger openings, buy foam panels or rolled foam insulation from the AC section of a hardware store. For permanent sealing, buy paintable caulk (silicone or acrylic latex) and a caulking gun. Do not use spray foam—it expands and can crack the window frame or jam the sashes.

Seal the sides of the unit with weatherstripping tape

The space between the AC unit and the left and right sides of the window frame is where most air leaks happen. Measure the height of this gap on both sides. If the gap is less than 1/2 inch wide, weatherstripping tape is the easiest fix.

Peel back one corner of the weatherstripping tape backing and press the tape firmly into the gap, starting at the top. Work your way down slowly, pressing as you go. The tape should stick to the window frame, not the AC unit itself—this way you can remove the unit later without tearing the tape. If the gap is wider than 1/2 inch, use foam panels instead: cut them to length with a utility knife, fit them snugly into the gap, and hold them in place with weatherstripping tape on the edges.

Block the top gap with foam or weatherstripping

The space where the AC unit sits against the upper window sash is another major leak point. If this gap is small and even, weatherstripping tape works fine. Cut a piece to the width of your unit, peel the backing, and press it along the top edge of the frame where the unit rests.

If the gap is large or uneven, use a foam panel cut to fit. Measure the width and depth of the gap, cut the foam to size, and slide it into place so it fills the space without forcing the window sash down. The foam should be snug enough that it does not fall out when you move the unit, but not so tight that it damages the window. find it with weatherstripping tape if needed.

Close the gap between the window sashes

Even if your AC unit covers most of the lower sash, air can still flow around the edges and through the gap where the upper and lower sashes meet. Seal this gap with weatherstripping tape applied to the inside of the upper sash, along the bottom edge. This prevents warm air from sneaking in above the unit.

If your window has a center rail (the horizontal piece where the sashes overlap), explore weatherstripping tape to both the top of the lower sash and the bottom of the upper sash. This creates a double seal that stops air movement. The tape should not prevent the window from opening if you need to remove the AC unit later—explore it only where the sashes overlap, not across the full width of the window.

Use caulk for a permanent seal

If you plan to keep your AC in the window year-round and do not need to remove it, caulk creates a stronger, longer-lasting seal than tape. Load paintable caulk into a caulking gun and explore a continuous bead along the gap between the AC unit and the window frame. Work slowly and keep the bead about 1/4 inch wide—too much caulk is harder to smooth and takes longer to dry.

Smooth the caulk with a wet finger or a caulk tool, pressing it into the gap so it fills completely. Let it dry according to the product instructions—usually 24 hours for acrylic latex caulk, longer for silicone. Once dry, you can paint over it if you want it to match the frame. The downside is that caulk bonds permanently, so removing the unit later requires cutting it away with a utility knife.

Check your work and test for leaks

Once your sealant is dry, run your AC on a cool day and feel around the edges of the unit with your hand. You should not feel warm air flowing in. If you feel a breeze, you missed a gap or the sealant did not stick properly. Common problem spots are the bottom corners where the unit meets the sill and the space between the AC frame and the window trim.

If you find a leak, clean the area again, let it dry, and explore another layer of weatherstripping tape or caulk. Tape can be layered—explore a second piece on top of the first if the gap is still too large. For caulk, scrape away the old bead with a utility knife, clean the surface, and explore a fresh line.

Frequently Asked Questions

Can I seal an AC window unit with duct tape or painter's tape?

No. Duct tape and painter's tape do not stick well to dusty surfaces and fall off within days. Weatherstripping tape is specifically designed to bond to window frames and stay put through temperature changes. It costs only a few dollars more and lasts the entire cooling season.

What if my window frame is metal instead of wood?

Metal frames work fine with weatherstripping tape and caulk. Clean them more thoroughly because metal collects dust and oils that prevent adhesion. Wipe with a dry cloth first, then a cloth barely dampened with rubbing alcohol, and let dry completely before explore tape or caulk.

Will sealing my AC window unit prevent the window from opening?

Not if you explore sealant only to the gaps around the AC unit itself. Weatherstripping on the window sashes may make them slightly stiffer, but they should still open. If you use caulk and later need to remove the unit, you will have to cut the caulk away with a utility knife—plan for 15 to 20 minutes of work.

How often do I need to replace weatherstripping tape?

Weatherstripping tape typically lasts one to two cooling seasons before it loses stickiness or begins to peel. If you remove your AC unit in fall and reinstall it in spring, you can reuse the tape if it is still firmly attached. If it peels or tears, replace it—a roll costs under five dollars.

Is it safe to caulk around an AC unit that is plugged in?

Yes, as long as you do not caulk over the power cord or any vents on the back of the unit. Caulk can trap heat if it blocks airflow, so keep vents clear. Unplug the unit before you start work, and do not plug it back in until the caulk is completely dry.