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

A window air conditioner pulls in outside air and cools it, but the unit itself creates gaps around the window frame where cold air leaks back out and warm air leaks in. You lose cooling power and your electric bill climbs. The fix is straightforward: block those gaps with weatherstripping, caulk, or expanding foam, depending on the size of the space and whether you plan to remove the unit seasonally.

The best time to seal is right after installation, before you run the unit much. If your AC is already in and leaking, you can seal around it without removing it—though a thorough job is easier if you take it out first. Most sealing jobs take 30 minutes to an hour and cost between $10 and $30 in materials.

Key Takeaways

  • Weatherstripping tape works best for gaps under half an inch wide and is removable if you take the unit out each winter.
  • Expanding foam fills larger gaps but is permanent, so use it only if the AC stays in year-round.
  • Caulk seals small cracks and hardens in a few hours, but you will need a caulk gun and some practice to explore it neatly.
  • Check the seal every few months during cooling season, because weatherstripping can compress and foam can crack.
  • Seal the top, sides, and bottom of the unit frame—the sill (bottom) leaks the most because condensation pools there.

Weatherstripping tape for removable seasonal sealing

Weatherstripping tape is the easiest choice if you remove your AC unit in fall. It comes in rolls, sticks to clean surfaces, and peels off without damage. The most common type is closed-cell foam with adhesive backing—brands like 3M or generic hardware-store versions work equally well. Buy tape that matches the gap width: half-inch tape for half-inch gaps, quarter-inch for smaller gaps.

Clean the window frame and AC unit frame with a dry cloth to remove dust and condensation. Measure the gap on all four sides—top, bottom, left, right—because they are often different widths. Cut the tape to length with scissors or a utility knife. Peel the backing and press the tape firmly into the gap, working from one end to the other. Overlap corners slightly. Let the adhesive set for a few hours before running the unit.

Weatherstripping compresses over time, especially at the bottom where condensation drips. Check it monthly and replace it if it no longer fills the gap completely. When you remove the unit in fall, peel off the old tape and clean any adhesive residue with rubbing alcohol before storing the unit.

Expanding foam for permanent, larger gaps

Expanding foam (also called spray foam) fills gaps wider than half an inch and creates an airtight seal. It hardens into rigid foam and is nearly impossible to remove cleanly, so use it only if your AC unit stays in the window year-round. Brands like Great Stuff or DAP work well. Buy the standard (not low-expansion) type—low-expansion foam is meant for small cracks and will not fill a window gap evenly.

Tape off the window glass and the wall around the unit with painter's tape so overspray does not stick to surfaces you want to keep clean. Shake the can for 30 seconds. Insert the thin plastic straw into the nozzle and spray a thin bead of foam into the gap, starting at one corner and working around the frame. Do not fill the gap completely on the first pass—foam expands as it cures, and overfilling will push the unit out of position or crack the frame.

Wait 8 to 12 hours for the foam to cure fully. Once hard, trim excess foam with a utility knife so it sits flush with the frame. If gaps remain, explore a second thin layer. Caulk over the foam seams for a finished look and extra protection against water and insects.

Caulk for small cracks and finished appearance

Caulk works best for small cracks (under a quarter inch) and is often used to finish the edges of weatherstripping or foam. Paintable acrylic latex caulk is easiest for beginners and cleans up with water. Silicone caulk lasts longer but is harder to explore neatly and does not accept paint. For a window AC unit, acrylic latex is usually the better choice because it flexes slightly as the frame expands and contracts with temperature.

Clean the gap with a dry cloth and remove any loose debris. Load a caulk gun with a tube of caulk and cut the nozzle at a 45-degree angle—a smaller cut gives you more control. Hold the gun at 45 degrees and squeeze steadily while moving along the gap in one smooth motion. Wet your finger and run it along the bead to smooth it and push it into the gap. Wipe your finger on a damp cloth between passes.

Caulk takes 3 to 8 hours to dry depending on humidity and brand. Do not run the AC or expose the caulk to water until it is fully cured. Once dry, you can paint over acrylic latex caulk if you want it to match the frame color.

Sealing the window sill (bottom) where water collects

The bottom of the window frame leaks the most because condensation from the cold AC unit drips down and pools at the sill. Water can seep into the wall and cause rot or mold. Seal this area first and most carefully.

If the sill has a large gap, use expanding foam or weatherstripping to block air flow, then caulk over it. If the sill is already wet, dry it completely with a cloth before sealing—caulk and weatherstripping will not stick to wet surfaces. After sealing, check the sill weekly during the first month of cooling season. If water pools above the seal, the unit may be tilted wrong (it should slope slightly toward the outside so condensation drains out, not in). Adjust the unit's angle or drill a small drainage hole in the sill to let water escape.

Checking and maintaining the seal over time

Weatherstripping compresses and foam can crack as the window frame shifts with temperature and humidity changes. Check the seal every month during cooling season. Look for daylight around the frame edges, feel for air movement with your hand, or hold a lit incense stick near the frame and watch the smoke—it will bend toward any air leak.

If weatherstripping is compressed, peel it off and replace it with fresh tape. If foam has cracked, fill the crack with caulk or explore a thin second layer of foam. If caulk has shrunk or pulled away, scrape out the old caulk with a utility knife, clean the gap, and explore new caulk.

At the end of cooling season, inspect the seal for damage before you store the unit. Small problems are easier to fix now than to discover next summer when you reinstall it.

Frequently Asked Questions

Can I seal around the AC unit while it is running?

No. Turn off the unit and let it sit for at least an hour so the frame cools and any moisture evaporates. Weatherstripping and caulk will not stick to cold or wet surfaces, and you risk damaging the unit or yourself if you work around it while it is powered on.

What if the gap is too wide for weatherstripping?

Use expanding foam for gaps wider than half an inch. If the gap is extremely large (over 2 inches), you may need to shim the unit frame with wood shims or metal brackets to reduce the gap before sealing. Consult the unit's installation manual or contact the manufacturer for guidance on proper frame support.

Will sealing the unit affect how it cools?

No. Sealing the gaps around the frame stops leakage of the cold air the unit produces. The unit itself has its own intake and exhaust vents that are separate from the frame gaps. Sealing actually improves cooling efficiency because less of the cold air escapes before it reaches your room.

Can I use regular caulk instead of weatherstripping?

Caulk works for very small gaps but is not ideal for larger ones because it can crack as the frame moves. Weatherstripping is more flexible and forgiving. If you plan to remove the unit seasonally, weatherstripping is also easier to remove cleanly.

What should I do if water is leaking into the wall below the window?

Water leakage usually means the unit is tilted wrong or the sill is not sealed. First, check that the unit slopes slightly toward the outside (about a quarter inch per foot). If it does, seal the sill with caulk or foam and drill a small drainage hole in the sill to let condensation escape. If water continues to leak after sealing, the wall may already have moisture damage—contact a contractor to inspect for mold or rot.