How to Seal Space Around an Air Conditioner Unit

It's important to seal the space around a window air conditioner unit to prevent cool air from escaping and reduce the chance of hot outside air seeping into your room or home. The process is easy to do and requires little time and a few, inexpensive materials.

Step 1 - Install the Air Conditioner Correctly

Prevention is the best way to reduce air leaks and the amount of sealant you'll need to repair cracks or seal space. Measure the air conditioner and the window opening where you'd like to place the air conditioner. The unit needs to fit in the opening without too much space around the sides. Place the air conditioner in a window that fits this criterion.

If your air conditioner is already installed, as in a rental unit, see if you can move it to a better suited window if there's currently too much space around the unit. Re-install it using the same mounting brackets.

Make sure the unit is level. Use shims if need be to level it.

Step 2 - Check for Leaks around the Top of the Unit

Since the majority of portable air conditioners are installed in an open window, the window needs to be lowered onto the top of the unit. Even if it looks as though the window is tightly closed, there usually is some space between the unit and the window where air is leaking out.

To check if there's a leak, make sure the unit is off and hold a lighter just above it under the window. If there's the slightest flickering, you'll know there's a leak.

To fix this leak, purchase some air conditioner foam seal at your local hardware store if there isn't any with your unit. Lift the window and place the seal under the lip of the window. Re-close the window tightly.

Step 3- Examine the Sash

Most window air conditioners come with an accordion-style flap that's attached to the sides to fill the open space not taken up by the unit. If the flap looks yellowed and worn or if there's fine cracks in its surface, you'll need to replace it. Pick up a replacement sash at a local home hardware store. If there's a single crack, you may be able to seal it temporarily with duct tape. The duct tape seal won't look pretty,, but it's a cost-effective way to fix a leak.

Step 3 - Caulk Outside

Be sure to caulk around the outside of the unit to seal the space around it and reduce the chance of leaks. For best results, caulk during dry cool weather that's at least 40F. This increases the likelihood of the caulking sealing correctly.

Step 4 - Spray Foam

As a last resort, you can use spray foam where your lighter indicates there are leaks or where you see cracks. While spray foam is very effective at sealing space and stopping leaks, it is very difficult to remove and should only be used with units that aren't taken down during cooler weather.