How To Seal Window Air Conditioner Jun 2026

Pull them out until they meet the window track.

Most window units come with expandable side curtains (often called accordions). A review of user experiences shows that these are often the weakest link in the seal.

The AC sits on the sill, but the sill itself often has a gap where the AC’s bottom channel meets the wood. how to seal window air conditioner

It blocks ants, flies, and mosquitoes from crawling through the window frame.

Sealing a window air conditioner is a necessary maintenance task that pays for itself in energy savings. While the manufacturer-provided foam strips offer a baseline solution, the yields the best results. Pull them out until they meet the window track

| Material | Rating | Review | | :--- | :--- | :--- | | | ⭐⭐ | Cheap and squishy. Good for blocking drafts but fails quickly when wet. Often included with the AC. | | Closed-Cell Foam Tape | ⭐⭐⭐⭐ | Denser and more durable than open-cell. Better for compression seals and water resistance. Highly recommended. | | Rubber Weatherstripping | ⭐⭐⭐⭐ | Excellent for large gaps. Durable and long-lasting, but can be visible. | | Silicone Caulk | ⭐⭐⭐⭐⭐ | The gold standard for waterproofing, but use sparingly if you intend to remove the unit seasonally. | | Duct Tape | ⭐ | Avoid. It leaves a sticky residue, peels off in the heat, and looks unsightly. Use foil tape instead. |

When you open a double-hung window to install an AC, a large gap is created between the top of the lower sash and the glass of the upper sash. This is where most cooling loss happens. The AC sits on the sill, but the

Use a large foam block (often provided) to plug the gap between the top of the AC unit and the glass of the window.