Seal the gaps with weatherstripping, caulk, or foam panels before cold air escapes

A window air conditioner works only if the cold air it produces stays inside your room. Gaps around the unit let that air leak straight outside, forcing the AC to run longer and harder while your electric bill climbs. Sealing those gaps takes two to four hours and costs between $15 and $40 in materials—far less than the wasted energy over a summer.

The three main gaps to seal are the sides of the unit (where it meets the window frame), the top (between the upper window sash and the AC body), and the bottom (where the window sill meets the unit). Each gap calls for a different material depending on whether it's small, medium, or large, and whether you want a removable seal or a permanent one.

Key Takeaways

  • The sides of the AC unit are sealed with adhesive-backed weatherstripping, which compresses as you close the window and costs about $5 to $10 per unit.
  • The top gap is sealed with foam panels or rigid foam board cut to size, which you can remove when you take the AC out for the season.
  • The bottom gap is sealed with caulk or weatherstripping, depending on whether the sill is wood, metal, or plastic.
  • Measure every gap before you buy materials, because window sizes vary and buying the wrong width wastes money and leaves air leaks.
  • Seal the unit after installation but before you turn it on, so you can see exactly where air is escaping.

Measure the gaps around your AC unit

Before you buy any materials, measure the width and height of each gap. Use a ruler or measuring tape and write down three numbers: the width of the left gap, the width of the right gap, and the height of the top gap. The bottom gap is usually sealed with caulk, which comes in one standard width, so you only need to know its length.

Measure at three points along each gap—top, middle, and bottom—because window frames are often slightly uneven. If the measurements differ, use the largest number when you order materials. A gap that is 1 inch wide at the top and 1.25 inches at the bottom means you need weatherstripping rated for 1.25 inches.

Take a photo of your window and AC unit with your phone so you have a reference when you're at the hardware store. This saves a second trip if you forget a measurement or realize you need a different material.

Seal the side gaps with adhesive-backed weatherstripping

The left and right sides of the AC unit are the easiest gaps to seal. Buy adhesive-backed foam weatherstripping in a width that matches your gap—common sizes are 1/2 inch, 3/4 inch, and 1 inch. The adhesive backing sticks to the window frame, and the foam compresses when you close the window, creating an airtight seal.

Clean the window frame with a dry cloth to remove dust and debris. The adhesive will not stick to a dirty surface. Peel back 2 to 3 inches of the backing and press the weatherstripping firmly onto the frame, starting at the top of the gap. Work your way down, pressing as you go. When you reach the bottom, close the window slowly to compress the foam and check that the AC unit closes smoothly. If it binds or feels stuck, you used weatherstripping that is too thick—peel it off and try the next size down.

Repeat on the other side. The entire job takes about 15 minutes. Weatherstripping lasts one to two seasons before the adhesive weakens, so plan to replace it each year.

Seal the top gap with foam board or rigid foam panels

The gap between the top of the AC unit and the upper window sash is usually too large for weatherstripping alone. Instead, use rigid foam board or foam insulation panels, which you can cut to size with a utility knife. These materials are removable, so you can take them out when you uninstall the AC for winter.

Measure the width of the window opening and the height of the gap. Buy a sheet of 1-inch-thick foam board (also called foam core or XPS foam) from a hardware store. Lay it on a flat surface and mark your cut lines with a pencil. Use a straight edge and a utility knife to cut the foam to size. Make three or four passes with the knife rather than trying to cut through in one stroke—this gives you a cleaner edge and is safer.

Fit the foam panel into the gap above the AC unit. It should sit snugly but not so tight that it prevents the window from closing. If it is too tall, trim it with the utility knife. Once it fits, you can leave it loose (it will stay in place from friction) or find it with a small amount of removable adhesive tape on the back. Do not use permanent caulk or construction adhesive, because you will want to remove this panel when the AC comes out.

Seal the bottom gap with caulk or weatherstripping

The bottom gap—between the window sill and the bottom of the AC unit—can be sealed with either caulk or weatherstripping, depending on the material of your sill. If the sill is wood, use paintable caulk. If it is metal or plastic, use weatherstripping or silicone caulk.

For caulk: Clean the sill with a dry cloth. Load a caulking gun with paintable caulk and run a bead along the entire length of the gap. Smooth it with a wet finger or a caulk tool, pressing it into the gap as you go. Let it dry for 24 hours before you turn on the AC. Paintable caulk can be painted over if you want it to match your sill, but it is not removable—use this method only if you plan to keep the AC in place year-round.

For weatherstripping: Cut adhesive-backed weatherstripping to the length of the sill. Clean the sill, peel the backing, and press the weatherstripping firmly along the gap. This method is removable and takes only 5 minutes, but it is less durable than caulk and may need replacing after one season.

Check for remaining air leaks after sealing

Once all gaps are sealed, turn on the AC and let it run for 10 minutes. Hold your hand around the edges of the unit and feel for cold air escaping. Pay special attention to the corners, where two sealed surfaces meet—these spots often have small gaps that are straightforward to miss.

If you feel air leaking from a corner, use a small piece of weatherstripping or a dab of caulk to fill it. If air is leaking from under the weatherstripping on the sides, the foam may not be compressed enough—close the window more firmly or add a second layer of thinner weatherstripping on top of the first.

Check the seal again after one week. Weatherstripping sometimes shifts slightly as the window settles, and you may need to press it down again or add a small piece of tape to hold it in place.

Common mistakes that leave air leaks

The most common mistake is buying weatherstripping that is too thin for the gap. A 1/2-inch strip in a 3/4-inch gap will compress but will not fill the space, leaving a channel for air to escape. Always measure the largest point of the gap and round up to the next standard size.

The second mistake is not cleaning the window frame before explore adhesive weatherstripping. Dust, paint chips, and old caulk prevent the adhesive from sticking. Wipe the frame with a dry cloth and let it sit for a few minutes before you peel and press the weatherstripping.

The third mistake is sealing the top gap with weatherstripping instead of foam board. Weatherstripping is designed to compress between two moving surfaces (like a window sash), not to fill a large static gap. It will compress too much and leave an air leak. Use foam board for the top gap every time.

A fourth mistake is using permanent caulk on the bottom gap if you plan to remove the AC seasonally. Once caulk dries, it is nearly impossible to remove without damaging the sill. Use weatherstripping or removable caulk if you take the unit out in fall.

Frequently Asked Questions

Can I seal a window AC unit that is already installed?

Yes. Turn off the AC, let it cool to room temperature, and then seal around it. You do not have to remove the unit. Measure the gaps carefully because the unit may have settled slightly after installation, and the gaps may be different on the left and right sides.

What if my window frame is metal instead of wood?

Adhesive-backed weatherstripping sticks to metal, but the bond is weaker than on wood or plastic. Clean the metal thoroughly with a dry cloth and let it sit for 5 minutes before you explore the weatherstripping. Press firmly and let the adhesive set for 24 hours before you close the window. If the weatherstripping peels off, use silicone caulk instead—it bonds better to metal.

Do I need to seal the AC unit if I only use it at night?

Sealing still saves energy even if you run the AC for only a few hours. Cold air that leaks outside forces the unit to work harder to cool the room, raising your electric bill. Sealing takes a few hours and costs $15 to $40, so the savings add up quickly over a summer.

Can I use weatherstripping on the top gap instead of foam board?

No. The top gap is too large and does not have a moving surface to compress the weatherstripping. Weatherstripping will compress too much and leave an air leak. Use rigid foam board or foam insulation panels for the top gap.

How long does the seal last?

Adhesive-backed weatherstripping lasts one to two seasons before the adhesive weakens. Caulk lasts three to five years if it is paintable caulk on a wood sill, or longer if it is silicone caulk on metal or plastic. Foam board lasts as long as you keep it in place and does not degrade from temperature changes.