How To Fill Gap Above A Window Air Conditioner?

Are you planning to install your AC on your window but notice that there's a gap between the top of the AC and the window's underside? What can you do to fill the gap? We've got the answers for you in this article. We asked the experts, and here's what they told us you should do.

You can use the following materials to fill the gap between the AC and the window. The suitable material will depend on the gap size and your window type.

  • Weatherstripping
  • Rope caulk
  • Spray foam sealant
  • Wood and spray foam insulation
  • Acrylic glass

Keep on reading to learn how you can use each of these materials to seal the gap in your window AC. This article also discusses whether your window AC should be tilted and gives you tips on how to protect your AC unit in the winter. Let's get down to business!

A old window unit air conditioner still being used by people whom don't have central air, How To Fill Gap Above A Window Air Conditioner?

How do I fill the space above my window air conditioner?

If you need supplemental cooling in one of your rooms, a window air conditioner can do the trick. It is easy enough to install, and it won't take up precious floor space in the room.

As the name implies, window ACs are designed to fit in your windows. However, not all windows are the same. So, it would really be possible that the AC unit that you buy won't fit perfectly with your particular window.

Window air conditioning units are available in different sizes, so it's best to measure your window where you plan to install it and choose the AC unit with the size (and cooling capacity) closest to it. If there's a gap above, just follow these steps to seal it to prevent cool air from escaping.

Window air conditioner perfectly fit through the window

How to Seal an Air Conditioner Window Gap

Here are the different materials that you can use to cover the gap between your AC unit and window case. 



If the gap isn't too wide, you can put weatherstripping to fill it. Here's how to do it.

Click this link to find this weatherstripping on Amazon.

  1. Wipe the AC unit and bottom sash of your window to remove any dirt and dust that can get in the way of proper adhesion.
  2. Measure and cut the weatherstrip according to the length that you need.
  3. Take off the protective paper to expose the adhesive and stick it against the window's bottom sash. Position the weatherstrip a little to the front of the gap to ensure a good seal.

Rope Caulk

This is also recommended for filling small gaps not bigger than 1/8".

Find this rope caulk on Amazon.

  1. Clean the top of the AC unit and the bottom side of the window.
  2. Cut chunks of rope caulk and fit them inside the gap.
  3. Make sure you insert caulk deep into the gap up to the front to make a dense and airtight seal.
  4. Remove any excess caulk.

Spray foam sealant

spray foam

If you have a bigger gap, you can use spray foam sealant to fill it up. It will expand as it dries and cover the space between the AC and the window.

Click this link to find this spray foam sealant on Amazon.

  1. Cover the rest of the AC unit, the lower half of the window frame, and the surrounding wall with newspaper to avoid putting spray foam on their surfaces.
  2. Put on your safety goggles and gloves.
  3. Spray the foam sealant on the top of the AC unit and wait for it to dry.
  4. Repeat Step 4 if there's still a gap.
  5. Remove the excess spray foam sealant.

Plywood and Spray Foam Insulation

Sheets of pine plywood are stacked on top of each other

For wider gaps, you can use wood to close off the space between the AC and the window.

  1. Cover the AC, wall, and window with paper to protect them from the spray foam and paint.
  2. Using a circular saw, cut a 1/2" thick piece of plywood according to the size that you need.
  3. Fit the wood inside the gap.
  4. Spray foam insulation below, above, and on the sides of the plywood to seal any gaps.
  5. Let it dry.
  6. Remove any excess spray foam insulation.
  7. Paint the wood to match the color of your window frame.

Acrylic Glass

Sawcut Clear Acrylic Sheet glass

If you have windows that slide to the sides, you will have a huge gap between the top of the AC unit and the underside of the window frame. You can use acrylic glass for a more durable and aesthetic solution in this case.

Check out these acrylic glass sheets on Amazon.

  1. Get the height and width of the gap between the AC and window frame.
  2. Buy an acrylic glass sheet according to the measurements that you got.
  3. Use duct tape to temporarily hold the glass sheet on the window frame's underside.
  4. Caulk the sides and bottom of the acrylic glass sheet and wait for it to cure.
  5. Remove the duct tape on the top. Caulk this side and give it time to cure.
  6. Ensure that there are no gaps no matter how tiny they are.

Make sure you seal that gap to insulate your room, optimize cooling, and reduce energy costs. It'll also keep water, moisture, dust, dirt, and insects from entering through the window and causing damage to your room.

Should a window air conditioner be tilted?

Maybe you've gotten advice from friends to make sure that you tilt your window AC when you install it, or you've seen other units propped this way. This is because most window ACs, especially older models, do require you to tilt the outside portion at a slightly lower position than the window.

The outward tilting position is important for proper water drainage. As the AC unit processes the air to make it cooler, one of its byproducts is water.

It accumulates in the drip pan unless it is released outside through the drain pipe. With the AC tilted at an angle, it will facilitate better water drainage as it will naturally flow down toward the ground below.

The ideal outward tilt should be an angle between 1 degree to 2.2 degrees depending on how wide the AC unit is. The wider the AC is, the smaller the angle.

To achieve this, you have to raise the indoor portion of the AC by about 1/2". You can put a piece of wood, durable plastic, or thick cardboard underneath the window AC.

You shouldn't tilt it too much because some components of the AC unit might not work properly. This includes the evaporator and condenser coil. For them to function properly, the AC must almost be level. If you tilt way down, the downward pull of gravity will affect their performance.

How do I protect my window air conditioner in the winter?

Window Air Conditioner in a Barred Cage with Snow in New York City during the Winter

One of the advantages of window AC units is that they are somewhat portable. This means you can remove them from their location to transfer them somewhere else. It helps big time when the temperature begins to drop.

Since you won't be using your AC, it is best to store it away for the winter. This will help you protect your AC from being exposed to snow and harsh weather that could potentially damage its exterior and interior components.

This is how you remove the window AC and get it ready for storage.

  1. Unplug the unit.
  2. Put towels below the window since there might be some water left inside the AC.
  3. Remove the AC unit from the window. It's best if you get someone to help you with this since it can be quite heavy.
  4. Put it outside for cleaning. Make sure it is in an upright position. Use a water hose to clean the body.
  5. Remove the filter and clean it with soapy water. Rinse thoroughly when done.
  6. Remove the front panel of the window AC to clean the condenser and water pan. You can also use soapy water for this then rinse using your spray hose.
  7. Do the same with the exterior case of your AC.
  8. Wipe it with a clean and dry microfiber cloth.
  9. Wait for the whole unit to dry before you put it inside its box for storage.
  10. Store in a cool and dry place. Make sure it's in a level and upright position to protect its parts. 

For those who can't remove their window AC anymore, you can buy a winterizing cover to put on your AC. This will help protect it from the cold weather.

Just make sure that it fits perfectly over your window AC to prevent the entry of unwanted particles. Remember to remove the cover when you're about to use your AC again.

Find this window AC winterizing cover on Amazon.

Final Thoughts

A old window unit air conditioner still being used by people whom don't have central air

Choose the right material to effectively seal the gap between your AC unit and window frame. It's easy to do, and you'll reap its benefits for a long time.

If you enjoyed reading this article, you might also want to visit the following posts:

Should You Add A Humidifier To Your HVAC?

Electric Furnace Blowing Cold Air – What To Do?

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