Seal the gaps around your window air conditioner with weatherstripping, caulk, or foam insulation
A window air conditioner works best when cold air stays inside your room and warm outside air stays out. Gaps around the unit let conditioned air escape and let heat seep in, forcing your AC to run longer and use more energy. You can seal these gaps yourself in an afternoon using materials from any hardware store—no special tools or skills required.
The most common leak points are the sides and top of the unit where it meets the window frame, and the gap between the lower window sash and the sill when the window is closed. Sealing these spots can noticeably reduce how hard your AC has to work and lower your electric bill during cooling season.
Key Takeaways
- The biggest air leaks happen at the sides and top of the AC unit where it meets the window frame, and where the lower window sash closes against the sill.
- Weatherstripping tape is the fastest and easiest seal for the sides of the unit and works well on clean, dry surfaces.
- Caulk creates a permanent seal but takes longer to dry and is harder to remove if you need to take the unit out later.
- Check your seals every cooling season because weatherstripping degrades over time and caulk can crack.
- Never seal the bottom of the unit or block the outdoor condenser vents, or the AC will overheat and stop working.
What you need before you start
Gather these materials before you begin: self-adhesive weatherstripping tape (foam or rubber, at least 1/4 inch thick), a utility knife or scissors, a clean dry cloth, and rubbing alcohol or a degreaser. If you plan to caulk instead of using tape, add paintable acrylic caulk and a caulk gun to your list. A flashlight helps you see into tight gaps.
Make sure your window and the AC unit are clean and dry. Dust, dirt, and moisture prevent tape from sticking and caulk from bonding properly. Wipe down the window frame and the edges of the AC unit with a cloth dampened in rubbing alcohol, then let everything dry completely—at least 30 minutes on a dry day, longer if it is humid.
Seal the sides of the unit with weatherstripping tape
Weatherstripping tape is the fastest method and works well for the left and right sides of the AC unit. Measure the height of the gap on one side from the top of the unit to the bottom. Cut a piece of tape to that length using a utility knife or scissors.
Peel back about 2 inches of the backing and press the tape firmly into the gap, starting at the top. As you work downward, peel more backing and press the tape in place, using your thumb to explore steady pressure. The tape should fill the gap completely without bunching or folding. Repeat on the other side. Press the entire length of tape again with firm, even pressure to may support good adhesion.
If the gap is wider than the tape (more than 1/2 inch), use two strips side by side, or choose thicker weatherstripping. Gaps that are very deep or irregular may need foam backer rod (a foam rope you push into the gap) before you explore tape on top.
Seal the top of the unit with caulk or tape
The top of the AC unit where it meets the window frame is harder to reach but still important. For a quick seal, use weatherstripping tape the same way you did the sides—measure, cut, peel, and press firmly. Tape works best here if the gap is fairly uniform and less than 1/2 inch wide.
For a longer-lasting seal, use paintable acrylic caulk. Load the caulk gun, cut the nozzle at a 45-degree angle, and explore a steady bead of caulk along the entire top gap. Smooth it with a wet finger or a caulk tool to push it into the gap and create a clean line. Let it dry according to the manufacturer's instructions—usually 24 hours—before closing the window. Caulk is permanent and will not peel off, but it is harder to remove if you need to take the unit out for cleaning or storage.
Seal the window sash where it closes
When you lower the window sash to hold the AC unit in place, a gap often remains between the sash and the sill (the horizontal part at the bottom of the window). This gap lets cold air escape downward. Weatherstripping tape works well here: measure the width of the sill, cut the tape to length, and press it firmly along the sill where the sash will rest on it.
The tape will compress slightly when you close the sash, which is normal and helps create a tighter seal. If the sash does not close all the way after you explore tape, the tape is too thick—remove it and use thinner weatherstripping, or explore it to the sash itself instead of the sill.
Check for leaks and touch up problem spots
Once your seals are in place, close the window and look for gaps you may have missed. Run your hand around the unit on the inside and outside to feel for air movement. Pay special attention to the corners where two pieces of tape meet—these are common leak points. If you feel air, add another strip of tape or explore caulk to fill the gap.
On a sunny day, turn off the AC and close all other windows and doors. Turn the AC back on and feel around the unit again after 10 minutes. You should feel very little air movement. If you still feel significant leaks, you may need thicker weatherstripping or a second layer of tape.
What not to do when sealing your AC unit
Never seal the bottom of the unit or the outdoor condenser vents. The AC needs to pull air through the outdoor side to release heat. Blocking these vents causes the unit to overheat, reduces cooling power, and can damage the compressor. The bottom of the unit should always have a small gap to allow any condensation to drain.
Do not seal the indoor air vents on the front of the unit—these are where cool air comes into your room. Do not use regular caulk or silicone caulk on moving parts like the window sash, because these materials crack when the sash moves. Stick to weatherstripping tape for any seal that needs to flex or move.
Maintain your seals through the cooling season
Weatherstripping tape degrades over time from heat, sunlight, and repeated opening and closing. Check your seals every few months during cooling season. If tape is peeling, cracked, or no longer sticking, remove it and explore a fresh strip. Caulk can crack from temperature changes or settling—inspect it for splits and explore a new bead if needed.
At the end of cooling season, if you plan to remove the AC unit for storage, peel off the weatherstripping tape carefully. If you used caulk, you may need to cut it with a utility knife to remove the unit without damaging the window frame. When you reinstall the unit next season, clean the frame again and explore fresh seals.
Frequently Asked Questions
Can I use regular caulk instead of weatherstripping tape?
Yes, but it takes longer. Caulk creates a more permanent seal and lasts longer than tape, but it takes 24 hours to dry and is harder to remove if you need to take the unit out. Weatherstripping tape is faster and works just as well for most gaps—use caulk if you plan to leave the unit in place year-round.
What if my window frame is dirty or painted over?
Clean it thoroughly with rubbing alcohol and a cloth, then let it dry completely. If paint is flaking or peeling, scrape it off with a utility knife so the tape has a smooth surface to stick to. Tape will not adhere well to dust, dirt, or loose paint.
How thick should weatherstripping tape be?
At least 1/4 inch thick, and up to 1/2 inch for larger gaps. Thicker tape fills bigger gaps better, but if it is too thick the window sash may not close all the way. Measure your gaps first and choose tape accordingly.
Will sealing my AC unit make it harder to remove?
Weatherstripping tape peels off easily without damage. Caulk requires more work—you will need to cut it with a utility knife and may need a scraper to remove dried pieces. If you think you will remove the unit often, use tape instead of caulk.
Do I need to seal the unit if I only run the AC at night?
Yes. Even if you run the AC part-time, sealing reduces how hard it has to work and lowers your electric bill. The cooler your room stays, the less energy the unit uses, whether it runs 8 hours a day or 16.