How to Insulate Window Air Conditioner | Seal The Leaks

Sealing a window AC means blocking air leaks with the foam, panels, and weatherstripping that ship with the unit, plus a top window gasket.

Window air conditioners come with the basic ingredients for a tight seal: adhesive-backed foam panels for the side curtains, a foam top gasket, and weatherstripping strips. Cut and placed correctly, these strips stop warm outdoor air from sneaking in around the sash, block insects, and help the unit cool more steadily. The step-by-step routine below follows the installation patterns GE and Frigidaire spell out in their official instructions, so the order matches what the manuals expect.

If you’d rather buy a complete kit instead of cutting panels to size, our tested roundup of the best air conditioner insulation options covers the ready-made upgrades that work with most window units.

What Ships In The Box And Why It Matters

Every ENERGY STAR-qualified window room air conditioner must ship with weather stripping or gasket materials that resist air and water infiltration, and the side curtains themselves must fit tightly and meet a minimum R-value of 1. That kit is your primary defense against drafts.

Before you reach for caulk or spray foam, use what came with the unit. Most kits include:

• Adhesive-backed foam sheets that cover the side curtain panels.
• A foam top window gasket that seals the gap between the sash and the glass.
• Weatherstripping strips for the contact points where the unit meets the sill and frame.

Loose side curtains defeat the whole system. If the panels aren’t seated fully, the seal breaks and the unit may not meet ENERGY STAR performance requirements.

How To Seal The Side Curtains First

Start with the side curtains because they cover the largest air gaps on either side of the unit.

1. Measure the inner curtain width using the ruler or template supplied in the kit.
2. Mark the patterned side of the foam panel to that same width, then cut it straight across with scissors or a utility knife.
3. Peel the backing and press the panel onto the curtain frame so the adhesive side contacts the metal or plastic.
4. Slide the panel into the frame until it seats completely, then repeat on the opposite side. Trim the foam again if it bunches or won’t slide fully.

GE’s guidance says to cover the curtain panels with adhesive-backed foam sheets cut to fit, then apply them to the side curtains and frames. The foam creates a soft compression seal when the curtain pushes against the window track.

Sealing The Top Gap And The Contact Points

The top of the window is the second-biggest leak point, and it’s the one most people skip.

Cut the foam top window gasket to match the width of the window opening, then insert it between the glass and the window frame. Frigidaire’s instructions put that same foam seal between the upper and lower sashes, which blocks both outside air and insects from entering through the top gap. If the gap is wider than the foam, fold a second strip in half to build thickness before tucking it in.

Finish by pressing weatherstripping along every contact surface where the unit or the side curtains meet the window sill, the sash, and the vertical tracks. Frigidaire’s instruction set says to use suitable weather stripping to minimize air leaks between the room air conditioner and the window opening. Work the strip into the seams with a flat tool so it seats below the surface instead of bulging.

What Not To Do And Off-Season Care

Skip the spray foam and caulk around a window unit. The unit needs to come out for cleaning and winter storage, and permanent sealants make removal destructive. Stick with the foam and weatherstripping that compress and release.

One common misunderstanding: ENERGY STAR recommends an appropriately sized cover for off-season protection, but only for through-the-wall units that stay installed when not in use. That guidance is not the same as insulating an operating window unit. If you’re covering a window AC for the winter, remove the unit, store it, and seal the window itself instead. If removal isn’t possible, a fitted outdoor cover beats leaving the unit exposed, but you’ll still lose heat through the unit’s body.

Check the seal twice a year before the cooling season. Foam degrades with UV exposure, and a cracked panel lets the drafts straight back in. Replacing a worn gasket is a five-minute job that costs a few dollars.