How to Insulate a Cold Air Leak Around the Air Conditioner

Dismount the air conditioner from the window and set it aside.

Open the window where the unit will be installed. Measure the distances of the windowsill and the window sash. Cut strips of weather seal that are the same lengths as the windowsill and sash. Peel the backs off the weather strips and press the windowsill strip against the windowsill and the window sash strip against the window sash.

Insert the air conditioner back into the window and close the open window down into the window unit’s top guard rail.

Install L brackets on the top of the open window to prevent it from opening. Place the L brackets in the corners up against the window frame sides and the top of the open window. Drill holes through the brackets into the window frame. Insert screws into the holes and thread screws through the brackets and into the window frame.

Measure the top of the open window and cut a strip of weather seal the exact same length. Peel the backing off the strip and insert it between the stationary window and the top of the open window to prevent air from seeping through the opening.

Measure the distance between the air conditioner unit and the open space created between it and the window frame on both sides. Cut strips to fit along the top, bottom and edge of the window frame on each side. Peel the backing off the strips and press the strips against the frame, sill and sash. Pull the accordion panels out from the sides of the air conditioner and situate them over the tops of the seal.

Thread screws through the accordion panel holes and into the frame to fasten them down and prevent air from leaking through.