How To Seal A Window AC Unit

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Updated September 16, 2022

When the summer heat is in full swing, there’s no better way to cool off than slouching in front of an AC unit. However, even the leading air conditioner can lose efficiency if it’s not sealed correctly. Air leakage is a common error, but not knowing how to seal gaps around a window air conditioner can increase electric bills and overall summertime discomfort.


  • Properly sealing your window unit is imperative to lowering energy bills and increasing effectiveness.
  • Put weather stripping around all edges of the AC’s interior perimeter for a proper seal.
  • For an extra-strong seal, you can use caulk or spray foam to seal the outside perimeter of the AC.

Insider Tip

Some areas require AC owners to install mounting brackets. Check this with your building superintendent or local officials before installing.

Sealing is one of the necessary steps in knowing how to make your window AC colder, so below, we’ll provide a step-by-step guide on how to do this. And if the summertime has ended and you are looking to put away your AC unit in prep for winter weather, we have a great guide on how to store a window AC unit.

  • Difficulty: Easy
  • Time Required: 20 Minutes
  • Total Steps: 7
  • Tools Needed: Tape Measure, Pencil

Sealing Gaps Around a Window Air Conditioner

Like knowing how to straighten air conditioner fins or how to reset your air conditioner, creating an airtight seal can dramatically improve the efficiency of your air conditioner unit. And the best news is that removing air leaks is a cinch and can be accomplished by even the most ignorant HVAC souls.

We also have a great article on how to use a portable air conditioner for those who are considering getting rid of their window air conditioning units. And if you are ready to toss them, we have another guide on how to scrap an AC unit.


Clear the window frame of any debris or obstacles preventing your AC window unit from fitting squarely on the frame.


Measure the length of the window frame and mark the dead center.


Place a piece of weather stripping on the bottom of the window frame beneath the AC unit.


Put in your AC so that it sits on the center mark.


Extend the side panels, and secure them with screws.


Apply weather stripping around the top, bottom, and sides of the unit. When applying, carefully place each strip and press them down firmly to seal any gaps.


Never use an extension cord or plug your window AC unit into a power strip. A window AC unit’s electrical current is too much for these connections to handle and sometimes leads to overheating and fires.


Once the interior is sealed, you can use caulk to seal the edges of the outside art of your unit if you want to maximize your seal.

STAT: HVAC experts say 10-25% of a home unit’s air escapes through leaks in window panes and other leaks. (source)

How to Seal Gaps Around a Window Air Conditioner FAQs

Why is the AC blowing air that isn't cold?

When an AC unit isn't blowing cold air, this usually points to a problem with the compressor or a refrigerant leak. You’ll need to contact an AC technician for assistance.

Why is my AC unit suddenly loud?

If your AC unit suddenly becomes loud, this is likely because some debris has fallen inside it or the fan blade is damaged.

Can bugs come through a portable air conditioner?

Insects can enter through a portable air conditioner vent if the seal has cracks or leaks.
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