Don't let AC side panels hold you back. Set up your AC the right way, no matter the size of your window.
What to Do When Your AC’s Side Panels Won’t Fit in Your Window
Published on Jul. 21, 2025
Summer is officially here, and the hot weather isn’t going anywhere anytime soon. If you use window AC units, it’s time to drag them out of storage, clean the filters and wrestle them back into your windows. But what if this summer you’ve moved into a new place and your window AC doesn’t fit into the new window frame. Or maybe you bought a new AC and the side panels just don’t span the gap in a big window. Now what?
That’s the problem I faced when I moved into my new home and the bedroom windows were too wide for my AC. I thought at first I’d have to buy a new one, but with a little creativity — and some plexiglass — I made it work. Ahead, we’ll explain different options to fit an air conditioner if the side panels don’t fit your window, with tips from Micah Sherman, Virtual HVAC Expert at Frontdoor.
What Are AC Side Panels?
Attached to the sides of the AC unit are the side panels—plastic, extendable pieces that bridge the gap between the unit and the window frame. Side panels serve several important purposes. They keep bugs, rain and dust out of your home, and help prevent the cold air generated by the AC from escaping outdoors. These panels come included with the window unit and are designed to fit that specific model.
What To Do If My AC Side Panels Won’t Fit In My Window
Only use one panel
If the window is too narrow to accommodate both side panels, remove one for a better fit. Position the AC unit closer to one side of the window instead of centering it, then extend the remaining panel to cover the gap. You can also remove both panels if your AC still won’t fit and seal the gaps with weatherstripping or tape.
Use custom panels
If your panels aren’t long enough to bridge the window gap, you can make custom panels from a variety of materials including:
- Plywood
- Rigid foam board
- Egg crate foam
- Plexiglass
- Cardboard
Whatever material you choose, “measure and cut to size and then tape it into the gaps of the window to ensure no outdoor air is leaking into the room,” Sherman says. Be sure to remove the existing side panels first before installing your custom replacements.
Use a larger AC
If your window opening is a foot or wider than your AC unit with the side panels fully extended, consider upgrading to a larger model. A better-fitting air conditioner will reduce the size of the side gaps and minimize the need for extra sealing.
Seal Gaps with Foam or Weatherstripping
If the panels almost fit but leave small gaps, use adhesive-backed foam insulation, weatherstripping or sealing tape to close the spaces. This works best for minor adjustments.
How Do You Plug Up Space Around an AC?
Seal any remaining gaps between the side panels and window with weatherstripping, painter’s tape or sealing tape. Flex Seal Flood Protection Tape is a sealing product designed to be easily removed without leaving any residue behind, making it a good option for temporary installations.
Alternatives to AC Side Panels
In Sherman’s opinion, “the best alternative would be foam board and tape,” adding, “This provides a little insulation against the outdoor temps, and if cut properly, can seal the gap in the window very effectively.” Other options include cutting custom panels from plywood, cardboard or egg crate foam. Plexiglass allows for increased visibility and daylight in the room, but can be tricky to cut without cracking it. Learn how to cut plexiglass.
FAQ
Can you mount a window unit AC with one side panel?
Side panels do not provide any structural or safety support on a window air conditioner. The bracket that fits around the unit is what holds it securely in place, so it’s perfectly safe to install an AC using only one side panel.
About the Expert
- Micah Sherman is a Virtual HVAC Expert at Frontdoor with over 12 year of industry experience.
Related
