How to Seal Window AC Unit? | Stop Leaks & Drafts

Most window AC leaks, drafts, and insect invasions trace back to one failure: skipping the foam. Sealing a window AC properly takes about fifteen minutes with the materials in the box. It blocks hot outside air, keeps cool air inside, and stops bugs. Here is the step order that works, based on ENERGY STAR and major manufacturer requirements.

What Does Sealing a Window AC Actually Involve?

The seal closes every contact point between the unit, the window sill, the movable sash, and the side curtain assemblies.

On a double-hung window, that means four zones: the gap between the fixed and movable sashes, the space between the AC and the window frame sides, the sill where the unit rests, and the top where the lower sash meets the unit’s top edge. Midea’s guide states it directly: use the included foam to seal gaps between the window and the AC, and use the window sash seal foam between the windows. GE’s instructions specify weather strips on the top, bottom, and sides, plus a foam top window gasket cut to width.

The Right Way to Seal a Window AC in 7 Steps

These steps follow manufacturer-documented sequence to keep the adhesive working and the seal continuous.

1. Clean the window frame and sill with a damp cloth and let it dry completely. Adhesive-backed foam only bonds to clean, dry surfaces.
2. Cut the foam weather seal to the exact window width. Amana’s instructions specify cutting the strip to width before installing. Trim with scissors, cutting slightly long to fill tightly.
3. Insert the seal foam between the glass and the sashes. For double-hung windows, Frigidaire’s guide says to insert foam between the upper and lower sashes, pressing firmly into the gap.
4. Apply weather strips to the top, bottom, and side edges where the unit and accordion side curtains contact the frame—the most common draft entry points.
5. Seal any remaining gaps with leftover foam or a suitable plastic sealer for holes or cracks, per Amana’s instructions. Run your hand along every joint and feel for airflow.
6. Close and lock the window firmly against the top of the unit, which anchors the AC, compresses the foam, and improves the seal.
7. Install a sash lock or window-lock bracket to secure the window and reduce the risk of shifting or outside lifting.

Done right, you should see no daylight and feel no air moving around the edges.

Common Mistakes That Ruin the Seal

Most failed seals come from five errors. Leaving gaps around the accordion side panels is most common—the foam there gets skipped because it sits behind the curtain. Failing to center the unit leaves uneven gaps. Skipping surface cleaning means the adhesive lifts within days. Using the wrong material for the window type leaves gaps foam cannot bridge. Omitting the lock bracket leaves the window loose enough to shift the unit.

If the window is not sized for the kit, the supplied foam may need trimming—check the model’s installation guide. Before buying seal materials, our tested roundup of window AC seals covers which kits handle which window styles.

Sealing a Portable AC Window Kit

Portable air conditioners use a different system but the same principle: the kit seals the window opening around the exhaust hose, with everything aligned from the center outward.

1. Clean the window surfaces thoroughly so the adhesive tape or Velcro bonds properly.
2. Apply the adhesive tape or seal material to the window frame according to the kit’s layout.
3. Align the panel from the center outward to avoid uneven gaps.
4. Route the exhaust hose through the designated opening and fully close the zipper or closure mechanism tightly around it so hot air cannot enter.

Portable window seals are model-specific in shape and orientation. One manufacturer guide notes the zipper side must face the correct direction and the hose must pass through the designated opening. Double-check your manual before trimming anything.