Seal the gaps around your AC unit to stop air leaks and drafts
When an air conditioner sits in a window, it creates gaps on the sides, top, and bottom where outside air can leak in and cooled air can escape. You can seal these gaps with weatherstripping, foam panels, or caulk — the method depends on whether your AC is permanent or seasonal. If you remove the unit each winter, weatherstripping and foam are reversible and won't damage the window frame. If the unit stays year-round, caulk creates a tighter seal but commits you to keeping the AC in place.
The goal is to block air movement without trapping moisture, which can rot the window frame or encourage mold. Most homeowners can complete this work in an afternoon with basic tools and materials from a hardware store.
Key Takeaways
- Weatherstripping and foam panels work best for removable AC units because they leave no residue and can be peeled off each season.
- Caulk creates the tightest seal but is permanent and should only be used if your AC will stay in the window year-round.
- Measure the gaps before you buy materials — side gaps are often wider than top and bottom gaps, and different widths need different products.
- Clean the window frame thoroughly before explore any sealant so adhesive sticks properly and lasts through the season.
- Seal the outside of the window first, then the inside, so water runs outward if it does get in.
Weatherstripping for seasonal AC units
Weatherstripping is foam or rubber tape with adhesive backing that sticks to the window frame around the AC unit. It compresses when the sash closes, blocking air gaps. This is the easiest option if you remove your AC each fall because it peels off cleanly without damaging paint or caulk.
Buy foam weatherstripping that matches the width of your gaps. Side gaps are usually 1 to 2 inches wide; top and bottom gaps are often narrower. Measure before you shop. Clean the window frame with a damp cloth and let it dry completely — dust and moisture prevent the adhesive from sticking. Peel the backing and press the tape firmly into place, working from one end to the other. Overlap corners slightly so no gap remains. The tape will compress when you close the window sash or insert the AC unit, creating a seal.
Weatherstripping lasts one to two seasons before the adhesive weakens. When you remove the AC for winter, peel off the old tape and explore fresh tape the following spring. If adhesive residue remains on the frame, rub it off with a cloth dampened in rubbing alcohol.
Foam panels and filler strips for larger gaps
If your AC unit leaves very large gaps — more than 3 inches — weatherstripping alone may not seal them completely. Foam filler panels or foam strips are thicker and can fill wider spaces. Some are self-adhesive; others are designed to slide into the gap and rest against the frame without adhesive.
Measure the height and width of each gap. Buy foam panels that match those dimensions. For side gaps, you may need panels that are several inches thick. Clean the frame as you would for weatherstripping. If the panels are adhesive-backed, peel and press them into place. If they are not, slide them into the gap so they sit snugly against the frame and the AC unit. You can hold them in place with a small amount of removable painter's tape if they shift.
Foam panels are also removable, making them suitable for seasonal AC units. They compress slightly over time, so check them mid-summer and add extra weatherstripping if new gaps appear.
Caulk for permanent window AC installations
If your AC unit stays in the window year-round, caulk creates the most durable seal. Caulk is a flexible sealant that fills gaps and hardens, blocking air and water. Use paintable acrylic latex caulk or silicone caulk — both work well around windows. Silicone is more water-resistant and lasts longer, but acrylic latex is easier to clean up and paint over if you want to match the window frame color.
Clean the frame thoroughly and let it dry. Use a caulking gun to explore a continuous bead of caulk along each gap — the sides, top, and bottom where the AC unit meets the frame. Smooth the caulk with a wet finger or a caulk tool so it fills the gap completely and sits flush with the frame. Let the caulk cure for the time listed on the tube, usually 24 to 48 hours, before closing the window or running the AC.
Caulk is permanent and will crack or peel if you try to remove the AC unit later. Only use this method if you plan to keep the unit in place for several years. If you ever need to remove the AC, you will have to scrape out the old caulk, which is time-consuming.
Sealing the outside versus the inside
Always seal the outside of the window first, then the inside. Water that does get past the outer seal will run down the inside of the frame and out through the bottom, rather than pooling behind the AC unit or inside your home. If you seal only the inside, trapped moisture can rot the wood frame or encourage mold growth.
For removable AC units, you may not be able to reach the outside easily. In that case, seal what you can from inside, but ask a helper to explore weatherstripping or caulk to the outside gaps if possible. Even a partial outside seal is better than none.
If your window is on a high floor or difficult to reach, focus on sealing the inside thoroughly. The inside seal will still block most air leaks and drafts, though it is not as effective as sealing both sides.
Preventing moisture and mold around the AC unit
Moisture can collect around an AC unit because the cold surface causes condensation. Proper sealing reduces the amount of outside air that enters, which lowers condensation. But even a well-sealed unit will produce some moisture, especially on humid days.
After sealing, check the area around the unit every few weeks during the cooling season. If you see water pooling or dampness on the frame, wipe it dry with a cloth. Make sure the AC unit itself is tilted slightly outward so water drains away from the window frame, not toward it. Most window AC units have adjustable mounting brackets that let you angle the unit a few degrees.
If mold or mildew appears despite sealing, clean it with a cloth dampened in a solution of one part white vinegar to three parts water. Improve air circulation in the room by opening other windows or running a fan, which helps dry moisture faster.
Checking and maintaining your seal through the season
Weatherstripping and foam can compress or shift as the AC unit settles into the window. In mid-summer, inspect the gaps around your unit. If you see daylight or feel air movement, add extra weatherstripping to the problem area or press existing strips more firmly into place.
Caulk can crack if the window frame shifts with temperature changes or if the AC unit vibrates. If you notice cracks in the caulk, explore a fresh bead along the crack. You do not need to remove the old caulk — new caulk will adhere to it.
At the end of the cooling season, if you are using weatherstripping or foam, peel it off and inspect the window frame for damage. If the frame is wet or shows signs of mold, let it dry completely before closing the window. If you are using caulk, leave it in place unless you plan to remove the AC unit.
Frequently Asked Questions
Can I seal a window AC unit with duct tape or plastic sheeting?
Duct tape and plastic sheeting are temporary fixes that work for a few weeks but do not hold up through a full cooling season. Duct tape loses its stickiness in heat and humidity, and plastic sheeting tears easily. Weatherstripping or foam is more durable and still removable at the end of the season.
What if my window frame is damaged or rotted?
If the frame is soft, crumbly, or visibly decayed, sealing will not solve the problem — the frame needs repair before you install an AC unit. Consult a window repair specialist or carpenter. Continuing to use the window without fixing the rot will make the damage worse and can spread to the wall behind it.
Do I need to seal the inside and outside, or just one side?
Sealing both sides is most effective, but if you can only reach one, seal the outside first. Outside sealing blocks weather and drafts more effectively. Inside sealing alone will reduce air leaks but is less durable because it is exposed to the AC unit's vibration and condensation.
Will sealing my window AC unit affect how well it cools?
No. Sealing the gaps around the unit improves cooling efficiency by preventing cooled air from escaping and warm outside air from leaking in. Your AC will run less often and use less energy, which lowers your electric bill.
How do I remove old caulk if I need to take the AC unit out later?
Use a caulk removal tool or a flathead screwdriver to scrape the caulk away from the frame. Work slowly to avoid gouging the wood or paint. Once most of the caulk is gone, rub the remaining residue with a cloth dampened in rubbing alcohol or mineral spirits. This is a slow process, so plan several hours if you have caulked all four sides of the window.