How to Make a Window Air Conditioner Fit a Large Window

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So you bought an air conditioner and are all ready to install the unit in your window, but you soon discover that the window is too large to hold the unit, even with the side panel accordion-style spacers. Fortunately, there are a variety of materials you can use to fill in the gaps during air conditioner installation.


When fitting air conditioners that are too small for a large window, your goal is to fill in the gaps on the sides of the air conditioner to maximize the unit's effectiveness and keep out unwanted insects and small animals. Whether you use the accordion-style curtains will depend on the method you use to fill in the gaps.

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Install Insulated Side Panels

Hardware stores sell insulated side panels to use when installing air conditioners that are too small for windows. These side panels can be used with or without the accordion-style spacers and are easy to cut and fit. If you use them with the spacers, it will create more insulation. You will need side panels, which can be cut. A standard size is 9 x 18 inches.


This method is ideal for blocking minor warm air leaks and keeping out pests. If you live in a cold climate, this method is not ideal if you plan on keeping the air conditioner in place permanently. The insulation will not adequately block the transfer of cold from the outside to the inside.

  1. Measure the gap on both sides.

  2. Purchase the correct size side panels.

  3. Using heavy-duty scissors, cut the panels so that they fit within the open space.

  4. Seal the panels with caulk or weatherstripping tape.

  5. If you use caulk, wait until it dries to run the air conditioner.


Use Spray Foam Sealant

Spray foam sealant can be used to seal the open area surrounding an air conditioner. The material expands to fit the space and won't lose shape once dry. In addition to insulating, spray foam sealant is waterproof. It's easy to cut off excess spray foam insulation once it dries. You can use it with or without the side panels.


While this method does a good job of sealing the air conditioner, it may be difficult to use it to fill in large holes since it comes out in a wet foam before drying. You will likely have to use the foam in stages, letting it dry between applications.

  1. Cover the surrounding area to protect against overspray.

  2. Put on safety goggles and work gloves.

  3. Keep the sealant away from open flames when working as it is flammable.

  4. Spray the sealant into the opening, beginning on the window ledge, so that the sealant has something to which it can adhere. Move upward from there. Continue until you've filled in the space. Avoid overfilling the space. The sealant doubles in size when it dries.

  5. Once the sealant has cured (dried), cut off excess with a serrated insulation knife.



Opt for Plywood

Plywood is an option for fitting an air conditioner that is too small for a window. The plywood is weatherproof and pestproof. Once installed, it can also be painted to match the colors of your interior and exterior. That will give the installation a more finished look. For best results, use plywood that is 1/2 inch thick.


As a more permanent solution, plywood can be difficult to remove when you need to change out the air conditioning unit. You may have to repair the surrounding area, such as the window frame, after removing the unit.

  1. Measure the gaps on both sides.

  2. Purchase 1/2-inch-thick plywood that is large enough to cover the openings.

  3. Mark the plywood where you need to cut it to fit the opening.

  4. Cut the plywood with a circular saw.

  5. Fit the plywood into the openings.

  6. Seal the edges of the plywood with spray foam. Once it dries, cut off the excess.



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