How to Seal Space Around an Air Conditioner Unit

shiny silver tape sealing window air conditioner
  • 1-2 hours
  • Beginner
  • 50-75

If you don't know how to seal space around an air conditioner unit, there's a really good chance that you are paying higher utility bills than you should. Creating a tight, leak-free seal is essential for any AC unit, and it's a relatively easy way to make your home a little more energy-efficient.

Energy experts talk about creating a secure envelope around the home, which just means that all the air leaks are sealed so that the outside world can't get in and the heating and air you pay for don't seep out. This is why you need to know how to seal space around an air conditioner unit.

Seal Space Around an Air Conditioner Unit

Lots of air conditioners have their own expansion panels that are designed to create a snug fit around the unit. However, this is not at all a tight seal, and air leaks still happen even in units with this design.

Even the smallest gaps are undesirable because they will cause you to lose energy, which translates directly into a higher utility bill. Not only that, these gaps can let bugs and debris from the outside world into your home.

In all designs, you need to seal up the cracks around the AC unit whether there are expanding panels or other features designed to create a snug fit. Nothing can replace a well-done seal, and that's why you need to learn how to create one.

Install the Air Conditioner Correctly

Prevention is the best way to reduce air leaks and reduce the amount of sealant you'll need to seal the unit. So before you install the AC, 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 space and get as close as possible to make sealing the unit much easier later on.

If this is not possible because you’re working with a pre-existing unit, it’s okay. There are multiple ways to seal an air conditioner unit, even when there are large gaps around it.

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 small lit candle just above the AC, under the window. Hold your breath and move the candle close to the top of the AC unit, moving it very slowly and deliberately from one side to the other.

If there's even the slightest flickering in the flame and you're moving steadily and slowly, you'll know there's an air leak.

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 are 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.

Caulk Outside

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.

Caulk will not dry properly in moist conditions or in very cold conditions. You want to place the caulk all the way around the unit on all four sides.

Allow the caulk to dry for about an hour, and add more if necessary. As the caulk dries and settles, it may end up not filling spaces entirely.

Any cracks around the unit that are smaller than about 1/8 of an inch can be sealed with caulk. Larger cracks, however, will need to be addressed differently.


Most of the time, you can use weatherstripping to seal AC units and block all air leaks. Adhesive-backed weatherstripping is very easy to work with because it's a lot like using any strong tape.

First, clean the area where you will be placing the weatherstripping. Use plain soap and water to get rid of all debris and grime.

You need to clean the area first so that the weatherstripping will stick and create a proper seal. Place the weatherstripping all the way around the unit.

Work slowly and with precision, treating the weatherstripping just like tape. Apply it carefully, pressing the weatherstripping down as you go.


Weatherstripping and caulk can be used to seal all gaps around the air conditioner that are smaller than one inch. But if you have larger gaps of one inch or more, you will need to use insulation to fill up those cracks.

The easiest option for this type of seal is spray foam, which can be literally sprayed directly into the crack you need to seal up. Spray foam will expand as it dries and hardens in place and truly fill the cracks in a very tight seal.

The seal is so tight, in fact, that it will be extremely difficult to remove in the future. If you plan to seasonally remove your air conditioner for the winter and then put it back for the summer, spray foam is not an option for you.

You should use spray foam only if you don't intend to remove your air conditioner unit any time soon, unless you feel like doing a whole lot of work to remove the unit and the foam.

Alternatively, use a foam board. This can be cut to size and stuffed in the cracks. It is not as effective as spray foam, but this is often the best way to seal up a large gap.

You can always apply caulk around the edges of the foam to create a tight seal and fill the spaces the stiffer foam can’t quite fill.

Insulation Kits

In the past, people have used duct tape, cardboard, and all sorts of homemade solutions to seal cracks around air conditioners. One newer option is a window insulation kit.

These kits come with several vinyl panels that you cut to size yourself to create a sleek surround for any air conditioner. It does not matter how large the gaps are because the panels come in large enough sizes, and you can cut them down to the precise measurements you need.

The vinyl panels can be a little tricky to work with but the end result is a more built-in, finished look than what you will get with many other sealing options. This is a good solution if you have an air conditioner that has large gaps anywhere around it.

Why Energy Efficiency Matters

An air conditioner cannot function efficiently if air is leaking in and out around your window unit.

Spaces around the unit will make it harder for the AC to keep your living space cool, which will jack up your energy bills. This is because the unit has to do more work and keep chugging out more cool air to compensate for all the cool air that is leaking out of your home around the cracks.

You don't want to pay any more on your utility bill than you have to. The extra work the AC unit is putting in is also putting extra wear and tear on the unit, which means it will need more frequent maintenance, and it won’t last you as long as a unit that is installed in a more energy-efficient way.

The good news is, sealing your air conditioner unit is a fairly straightforward DIY that you can do all on your own in most cases. It’s affordable and fairly simple to seal these gaps and cracks.

Air Conditioner Unit Seal FAQs

What type of caulk should you use to seal an air conditioner unit?

Caulk is often included in lists of DIY materials, and it often appears in just about any home improvement project you might be taking on. But there are actually several different types of caulk and sealants that perform similarly to which should you choose?

Latex caulk is a good do-it-all type of choice. This material is very easy to apply and it bonds to both wood and metal well.

You can also paint over latex caulk, which is a nice feature if you want to make the caulk blend into your house.

Rubber caulk works very well on masonry like brick or stone, and it's water resistant once it has dried.

Polyurethane is really the jack of all trades. This caulk can bond with just about anything and it's very durable.

However, polyurethane does fade in sunlight so you will probably want to paint it to give it a more finished look.

Silicone caulk is one of the best-known and most-used types of caulk. This caulking material is waterproof, it's highly flexible, and it has some natural mildew resistance.

How do you remove an air conditioner unit that has been sealed with caulk?

Once the caulk seal is in place, it can be difficult to remove, though not as difficult as removing foam insulation. Some types of caulk can only be removed with special chemical formulas, which you can find at home improvement stores.

Silicone caulk, however, can be removed with a few common tools and some elbow grease. To remove silicone caulk, start by heating it up with a hair dryer on a medium setting for about half a minute.

Working gently and with care, use a utility knife to score the caulk. Run the blade along the caulk line, touching only the caulk and not the AC unit or the window frame around it.

Pull away the caulk using a pair of pliers, removing as much of it as you can. Loosen the last remaining caulk with a putting knife or scraper, working along the crack where the caulk was applied.

This should loosen the caulk well enough that you can remove the AC unit. The rest of the caulk can be gently scraped away with the putty knife.

Use a sponge soaked in mineral spirits to remove the silicone caulk residue. You need to remove all traces of caulk before re-install the air conditioner.

How do you seal the cracks around a small air conditioner?

If you have a small air conditioner that does not really fit into any window with large gaps all around it, don't despair. You can seal the spaces around an AC unit even when the spaces are big.

Foam board insulation and vinyl window kits come in large pieces that can be cut to size to fill even large spaces around an air conditioner.

Further Reading

3 DIY Air Conditioners

Air Conditioner Parts 101

How to Cover a Window Air Conditioner for Winter

How to Install a Window Air Conditioner

How to Protect a Window-Mounted Air Conditioner

How to Replace an Air Conditioner Filter

How to Service and Maintain Your Air Conditioner

How to Stop a Portable Air Conditioner From Leaking

How to Vent a Portable Air Conditioner

How Well Does a Windowless Air Conditioner Work?

Installing a Window Air Conditioner in a Tilt Window

Replacing Freon in an Air Conditioner

Troubleshooting Common Air Conditioner Blower Problems

What Is a Propane Air Conditioner?