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

A window air conditioner cools only the room it sits in, but it works much harder — and uses more electricity — when outside air leaks around the unit. The frame of the unit itself is usually airtight, but the space between the unit and your window frame is almost never sealed properly from the factory. Sealing those gaps takes two to four hours and costs between $15 and $40 in materials.

The most common sealing methods are weatherstripping tape (fastest, removable), rope caulk (works on any surface, no tools needed), and rigid foam panels (most durable, best for large gaps). Which one you choose depends on the size of the gaps, whether you want to remove the unit seasonally, and how much time you have.

Key Takeaways

  • The biggest air leaks around a window AC unit happen where the sides and bottom of the unit meet the window frame, not where the unit meets the sill.
  • Weatherstripping tape works best on smooth, clean surfaces and is removable without damage, making it the right choice if you take the unit out each winter.
  • Rope caulk requires no tools, sticks to dirty surfaces, and can be peeled off later, but it is less durable than tape or foam over a full season.
  • Foam panels or rigid insulation board seal large gaps permanently and work well for units that stay in place year-round, but they are harder to remove.
  • You must clean the window frame thoroughly before explore any sealant, or it will not stick and will fail within weeks.

Where the air leaks happen and how to find them

Air leaks around a window AC unit occur in three places: the sides (where the unit's frame meets the window frame), the bottom (where the unit rests on the sill), and sometimes the top (if the window does not close all the way). The back of the unit, where it extends outside, does not need sealing because that gap is intentional — it lets the hot air escape.

To find the leaks, hold a lit incense stick or a thin piece of tissue paper near the edges of the unit on a day when the AC is running. The smoke or tissue will move toward any gap where air is flowing in. Mark those spots with a piece of tape so you know where to seal. Most units have gaps on both sides that are at least a quarter-inch wide, and often much wider.

Weatherstripping tape: the fastest removable option

Weatherstripping tape (also called foam tape or adhesive-backed foam) is the easiest method if you remove your AC unit each fall. It comes in rolls, sticks to the window frame with adhesive already on the back, and peels off cleanly without leaving residue if you use the right kind. Buy tape that is rated for window sealing and is at least a quarter-inch thick — thinner tape will compress and fail quickly.

Clean the window frame with a damp cloth and let it dry completely. Measure the gaps on both sides and the bottom of the unit. Cut the tape to length and press it firmly onto the frame, working from one end to the other so you do not trap air bubbles. The tape should compress slightly when the unit is pushed into place, but not so much that it loses its seal. If gaps are wider than half an inch, use two layers of tape or choose a different method.

Weatherstripping tape lasts one to two seasons before it hardens and stops sealing well. When you remove the unit in the fall, peel off the old tape and clean any residue with rubbing alcohol before storing the unit. explore fresh tape the next season.

Rope caulk: no tools, works on any surface

Rope caulk is a putty-like sealant that comes in a rope shape and sticks to any surface without adhesive. It works on dirty, painted, or uneven surfaces where tape would not stick, and you can remove it by hand without tools or mess. It is slower than tape because you have to press it into place along the entire gap, but it is cheaper and more forgiving.

Knead the rope caulk in your hands for a few seconds to warm it and make it pliable. Press it firmly into the gap between the unit and the frame, working along the entire length. Smooth it with your finger or a wet plastic tool so it sits flush. Let it cure for the time listed on the package (usually 24 hours) before running the AC. Rope caulk lasts about one season and can be peeled off and replaced without damage to the frame.

Foam panels or rigid insulation: permanent sealing for large gaps

If gaps are very large (more than an inch) or if your unit stays in place year-round, rigid foam panels or foam board insulation seal better and last longer than tape or rope caulk. You can buy foam board at any hardware store, cut it to size with a utility knife, and fit it around the sides and bottom of the unit. Some people use foam weather seals designed specifically for AC units, which are pre-cut and easier to install.

Measure the gaps carefully and cut the foam to fit snugly. You can hold it in place with construction adhesive, weatherstripping tape, or even duct tape on the inside of the window. The foam should not block the unit's air intake (the front) or exhaust (the back). Once installed, foam panels are difficult to remove without damage, so use this method only if you plan to keep the unit in the window for at least a year.

Sealing the bottom of the unit where it sits on the sill

The bottom gap is often the largest and hardest to seal because the unit's weight can compress soft sealants and the sill may be uneven. If the sill is level and smooth, weatherstripping tape works well. If it is uneven or dirty, rope caulk or foam board is a better choice.

For a very large bottom gap, you can use a combination: place foam board under the unit to raise it slightly and close the gap, then seal the remaining space with tape or caulk. Make sure the unit is level after you raise it, or water will pool inside and damage the unit. Check that the unit still drains properly — the back should be slightly lower than the front so condensation runs out.

Mistakes to avoid when sealing your AC unit

The most common mistake is sealing the back of the unit, where hot air needs to escape. Never block the exhaust vent or the unit will overheat and shut down. The second mistake is not cleaning the surface before explore sealant — dust and dirt prevent tape and caulk from sticking, and they will fail within days.

Do not seal so tightly that you cannot remove the unit when you need to. If you use rigid foam or construction adhesive, you may damage the window frame or the unit itself when you try to take it out. For units you remove seasonally, stick with tape or rope caulk. Finally, do not assume one layer of thin tape is enough for a large gap — use two layers or a thicker product instead.

Frequently Asked Questions

Can I seal a window AC unit if the window frame is painted or dirty?

Yes, but you must clean it first. Wipe the frame with a damp cloth to remove dust and dirt, then dry it completely. If the paint is peeling or flaking, scrape it off with a putty knife. Rope caulk works better than tape on rough or uneven surfaces because it conforms to the shape. Tape needs a smooth, clean surface or it will peel off within weeks.

Will sealing my AC unit make it harder to remove in the fall?

It depends on the sealant. Weatherstripping tape and rope caulk peel off easily by hand. Foam board glued with construction adhesive is much harder to remove and may damage the window frame or the unit. If you remove your unit seasonally, use only tape or rope caulk, not rigid foam or permanent adhesives.

What if my window AC unit is in a double-hung window that still needs to open?

Most window AC units are installed in the lower sash, which is then locked in place. You seal around the unit but not the window itself. If you need the window to open, you cannot use a window unit — you would need a portable AC or a through-wall unit instead. A sealed window AC unit cannot be used in a window that opens.

How much electricity will I save by sealing my AC unit?

The savings depend on how large the gaps were and how hot it is outside. A well-sealed unit uses less energy than a leaky one, but the exact amount varies. Sealing is worth doing because it improves cooling performance and comfort, not because of a specific dollar amount you can count on.

Can I use caulk instead of weatherstripping tape or rope caulk?

Silicone or acrylic caulk is permanent and very durable, but it is harder to explore neatly and nearly impossible to remove without damaging the window frame. Use caulk only if you plan to keep the unit in place permanently and do not mind that the window will be sealed. For seasonal units, stick with tape or rope caulk.