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How to Seal an AC Window Unit

Larry Simmons

Installing an air conditioning unit in a window is a quick and inexpensive method of providing single-room cooling. It does have a slight drawback in that the installation can leave small openings around the unit that can allow the transfer of hot and cold air, ruining the energy efficiency of the window.

Seal your AC unit to prevent the loss of cool air.

To prevent this, you should effectively seal the unit into the window opening. This way you can maintain a greater degree of control over the room’s temperature without the loss of efficiency that can raise your energy bills.

  1. Check the fit of the air conditioner in the window. Make sure that it’s seated solidly on the window sill without any obvious gaps around the unit.

  2. Examine the accordion-style window extensions on the side of the air conditioner that run from the unit to the sides of the window frame. Look for any cracks or tears in the material making up the extensions, and check that they’re properly secured to the sides of the window frame. Replace the extensions if they're damaged. Measure the gaps between the air conditioner and the window frame and purchase new extensions from an air conditioner dealer authorized to carry your brand.

  3. Remove the damaged window extensions from the side of the window frame by unscrewing them from the frame and the unit. Attach the new extensions to the window unit and stretch them outwards to the side of the window frame with the air conditioning unit centered in the frame. Secure the extensions to the side of the frame with the included screws.

  4. Place a bead of caulk on the outside of the window running along the air conditioner edge to cut off the flow of air along the edges of the unit. Smooth the caulk down using a wet fingertip.

  5. Place a strip of air conditioner foam seal along the bottom of the window sash, covering the bottom completely. Lower the sash over the air conditioner unit so that the foam creates a tight seal, cutting off air movement.

  6. Fill the area between the top bar of the open window sash and the windowpane with foam weather stripping tape to prevent any air from escaping through the joint between window sashes.