How to Fill the Gap Over an Air Conditioner in a Casement Window


An air conditioner will cool a room much better if the window around the unit has been properly sealed. Sealing the gaps can be difficult when a regular air conditioner is in a casement window. When the side-sliding casement window is closed on the unit, all of the space above the air-conditioning unit remains open. This can be sealed in an attractive and economical way using an acrylic sheet of glass such as Plexiglas. The acrylic glass will allow the window above the unit to let in sunlight, and it is easier to manage than plastic.

Things You'll Need

  • Tape measure
  • Acrylic glass sheet
  • Duct tape
  • Clear window caulk
  • Measure the height and width of the open window space above the air conditioner with a tape measure.

  • Purchase the acrylic glass sheet in the size necessary.

  • Place the acrylic glass sheet in the open window space. Fasten the sheet in the window using a strip of duct tape across the top of the acrylic glass and the window frame.

  • Caulk around the bottom and sides of the acrylic glass sheet and allow the caulking to dry. Remove the duct tape and caulk around the top of the acrylic sheet. Make sure all the cracks are caulked to prevent air leaks and water damage.

Tips & Warnings

  • Do not put any of the air conditioner's weight on the acrylic sheet because it is not meant to hold weight.

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  • Photo Credit window air conditioner image by Aaron Kohr from
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