An air conditioner cannot function efficiently if air is leaking in and out around your window unit. This will obviously make it harder for the AC to keep your living space cool which will in turn rack up your energy bills before you know it. Therefore, it's important that this space is properly sealed. Don't worry; the process is easy to do and requires little time and only 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. So, before your install, measure the air conditioner and the window opening where you'd like to place it. The unit needs to fit in the opening without too much extra around the sides. Purchase an air conditioner 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 outside. Re-install it using the same mounting brackets. Make sure the unit is level, using shims if need be.
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 to help seal it. However, 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. Check for this by holding a lighter just above the AC, under the window, . If there's even the slightest flickering, you'll know there's a leak.
To fix this, purchase some air conditioner foam seal at your local hardware store if there isn't any with your unit. Then, 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 attached to the sides to fill the open space. If this 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. This won't look pretty, but it's a cost-effective way to fix a tiny problem.
Step 4 - Caulk Outside
Be sure to caulk around the outside of the unit to seal the space around it and reduce the chance of air flowing in and out. For best results, caulk during dry, cool weather that's at least 40 degrees Fahrenheit. This increases the likelihood of the caulking sealing correctly.
Step 5 - 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.
These solutions should allow you to fix most, if not all, problems with your window air conditioner's seal.