How to Insulate a Window AC Unit | Seal Leaks First

Insulating a window AC unit starts with sealing every air gap around the chassis and side curtains, then adding foam or a cover.

For the full breakdown, see our best AC Unit Insulation guide.

Most of the cold air a window air conditioner loses escapes through gaps, not the walls. The manufacturer’s weatherstripping and foam gaskets in the box do the real insulating work, so the job is mostly installation precision: closing the spaces between the unit, the sash, and the sill. From there, rigid foam and an off-season cover add a second layer against drafts. The payoff of a properly sealed unit is lower energy bills, fewer dust and insect intrusions, and a room that actually stays cool.

Why Sealing Air Gaps Beats Adding Insulation

ENERGY STAR’s room air conditioner FAQ states that window units must ship with weather stripping and gasket materials resistant to air and water infiltration and UV degradation. Those materials are the primary insulation, and they only work if placed correctly.

Every contact surface matters: the sill, the movable sash, and the sides where the accordion panels touch the window frame.

Double-hung windows add one more vulnerability. In a partially opened window, the space between the fixed upper sash and the movable lower sash is a major infiltration path unless you seal it with the included foam strip. Home Depot’s installation guide notes that window AC units often ship with a foam insulating strip that should be cut to length and inserted between the upper and lower sashes.

Step-by-Step: Sealing the Unit During Installation

The sealing process happens while you install, not after. GE’s installation instructions specify covering the curtain panels with adhesive-backed foam sheets, installing the window lock bracket, and cutting a foam top window gasket to fit the window width to block outside air and insects.

1. Measure the side curtain area first. Cut adhesive-backed foam sheets to match each accordion panel before attaching them, so the foam lies flat without unsightly bulges.
2. Attach the foam to the curtain panels. Press the adhesive side firmly to the panel surface, smoothing from the center outward to remove air bubbles.
3. Install the window lock bracket. GE specifies a single type B screw for this bracket, with different placement notes for wood versus vinyl windows. The bracket keeps the window from being forced open from outside.
4. Cut the foam top window gasket to the exact window width. Insert it between the top of the glass and the window frame to block outside air and insects from entering above the unit.
5. Seal the sash-to-sash gap. Cut the included foam strip to length and press it between the upper and lower sashes of a double-hung window. Skipping this step leaves a direct path for outdoor air.
6. Use weather stripping at every contact point. Apply gasket material anywhere the AC chassis or side curtains touch the window, including the sill and the sides of the frame.

When the installation is done correctly, you should feel no drafts when you run your hand along every seam between the unit and the window frame.

Off-Season Insulation: The Role of a Cover

ENERGY STAR recommends that off-season instructions suggest an appropriately sized cover for additional insulation and air sealing. A tight-fitting cover is a valuable extra layer, but it is not a substitute for proper installation seals.

Using a cover without first sealing the unit’s gaps is incomplete: the cover traps air, but unsealed gaps still allow drafts and moisture to move through. Seal everything during installation, then add the cover when the unit goes dormant for the season. If you want to compare tested insulation products before buying, our roundup of the best AC unit insulation covers the options that hold up over multiple seasons.

Common Mistakes That Undo Your Insulation Work

Even careful installers skip steps that quietly cost efficiency. The most frequent errors are leaving accordion side panels unsealed, skipping the sash-to-sash seal in double-hung windows, and treating a winter cover as a fix for gaps that were never sealed.

Manufacturer hardware matters, too. Using the specified window lock bracket and screw type keeps the unit secure and the seals compressed.

One final check: after installation, close the window fully against the unit and verify the sash locks. A window that does not seat properly leaves a visible gap and undoes the weather stripping’s work.