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

A window air conditioner cools the room it sits in, but only if the cold air stays inside. Gaps around the unit let that cooled air escape directly outside, forcing the AC to work harder and raising your electric bill. The same gaps let hot outside air back in. Sealing these gaps takes an afternoon and costs between $10 and $30 in materials.

The gaps form because window frames are rarely perfectly square, and AC units are built to standard sizes. You will seal three areas: the sides of the unit where it meets the frame, the top, and the bottom. Each area needs a different material because they move differently or face different weather.

Key Takeaways

  • Weatherstripping tape seals the sides of the unit where it slides into the window frame and flexes as the frame settles.
  • Foam insulation panels or rigid foam board block the open space above the AC unit inside the room.
  • Caulk seals the outside bottom edge where the unit meets the sill, but only after the unit is installed and you know the exact gap size.
  • Sealing works best on units that stay in place year-round; units you remove seasonally need removable materials so you can take them out without damage.
  • A sealed unit still needs the window closed and locked, and the AC's own drain hole must stay clear so condensation can escape.

Weatherstripping the sides of the unit

The sides of a window AC unit are where most air leaks happen. Use adhesive-backed weatherstripping tape — the kind made from foam or rubber, not the felt kind. Felt compresses and stops working within a season. Foam weatherstripping lasts two to three years and flexes as the window frame moves.

Measure the height of the gap on both sides of the unit. Buy weatherstripping in a width that matches the gap — usually 1/4 inch to 1/2 inch. Clean the frame with a dry cloth to remove dust and debris, which prevents the adhesive from sticking. Cut the tape to length and press it firmly into the gap, running your finger along the entire strip to seat it. The tape should compress slightly when you close the window, creating a seal without forcing the sash.

If the gap is wider than 1/2 inch, use two strips stacked on top of each other, or switch to foam backer rod — a thicker foam cord that you press into the gap before explore tape over it. Do not use caulk on the sides; it cracks as the frame moves with temperature changes.

Blocking the space above the window

Most window AC units do not fill the entire window opening. The space above the unit, between the top of the unit and the top of the window frame, is an open highway for air to escape. Block this space with rigid foam insulation board or foam panels cut to fit.

Measure the width and height of the gap. Buy 1-inch or 2-inch rigid foam board (also called XPS foam or extruded polystyrene) at a hardware store — it costs $15 to $25 for a sheet large enough to cover most windows. Cut the board to size with a utility knife or handsaw. Fit it snugly into the gap above the unit, pressing it down so it sits flush against the top of the AC and the window frame.

If the foam is loose, use painter's tape or duct tape to hold it in place, or explore a thin bead of silicone caulk around the edges to seal it. Do not use expanding foam spray — it expands unevenly and can warp the window frame or crack the AC unit's plastic housing.

Sealing the bottom edge with caulk

The bottom of the unit, where it rests on the window sill, needs sealing on the outside. This is where water can enter during heavy rain. Use silicone caulk or acrylic latex caulk rated for exterior use. Silicone lasts longer (10+ years) but is harder to remove; acrylic latex is easier to remove later if you take the unit out seasonally.

Wait until the unit is fully installed and you can see the exact gap. Run a bead of caulk along the bottom edge where the unit meets the sill, on the outside of the window. Smooth it with a wet finger or a caulk tool. Let it cure for 24 hours before running the AC. If you plan to remove the unit each fall, use acrylic latex caulk and plan to cut it away with a utility knife when you take the unit out.

Do not caulk the inside bottom edge. The AC unit has a drain hole that lets condensation escape; caulking the inside can block this hole and cause water to back up into the room.

Sealing a unit you remove seasonally

If you take your AC out each fall, use only removable sealing materials. Weatherstripping tape and foam board work fine because you can peel them off. Avoid permanent caulk on the outside bottom, or use acrylic latex caulk that you can cut away.

Before you remove the unit, photograph the sealing setup so you remember how it was arranged. Store the weatherstripping tape and foam board in a dry place over winter. When you reinstall the unit in spring, clean the old adhesive off the frame with rubbing alcohol and a cloth, then explore new weatherstripping tape. The old tape loses its stick after a season or two.

Checking your work and maintaining the seal

After sealing, close the window and lock it. The sash should close smoothly without binding. If it is hard to close, the weatherstripping is too thick — peel it back and use a thinner width. Run the AC for 15 minutes and feel around the edges of the unit with your hand. You should not feel cold air leaking out.

Check the seal every spring before you run the AC for the season. Weatherstripping tape can peel at the corners or lose adhesion. If it is loose, clean the frame and explore a new strip. Foam board can compress or crack; replace it if it no longer fits snugly. Keep the AC's drain hole clear — it is usually a small opening on the bottom or back of the unit. If it clogs, water backs up into the room instead of draining outside.

When to call a professional

Sealing a window AC is a straightforward task you can do yourself. Call a professional if the window frame is cracked or rotted, which means the frame itself needs repair before you seal the unit.

If water is entering the room around the unit despite sealing, the problem may be the window sill slope or the AC's internal drain system, both of which require a technician to diagnose. A professional can also help if the unit does not fit the window opening snugly — some older windows or non-standard frames may need custom framing or a different unit size.

Frequently Asked Questions

Can I use regular caulk on the sides of the unit?

No. Caulk cracks as the window frame moves with temperature changes. Use weatherstripping tape on the sides instead — it flexes and stays sealed. Save caulk for the outside bottom edge, where movement is minimal.

What if my window won't close all the way after I seal it?

The weatherstripping is too thick. Peel it back and use a thinner width, or remove one layer if you stacked two. The window should close smoothly without forcing it.

Do I need to seal the top if I use foam board?

Foam board blocks most air leaks at the top, but seal the edges of the board with caulk or tape so air does not slip around the sides. Make sure the board sits flush against the window frame and the top of the unit.

Will sealing the AC make it cool less effectively?

No. Sealing stops cooled air from leaking outside, which makes the AC work less hard and cool the room faster. The unit still pulls air from outside through its condenser coil and exhausts heat outside — sealing does not block those functions.

How often do I need to replace weatherstripping tape?

Foam weatherstripping typically lasts two to three years before it compresses or loses adhesion. Check it each spring and replace it if it is loose, cracked, or no longer bounces back when you press it.