How To Fill Gap Above Window Air Conditioner?

Did you recently install a new window air conditioner and you noticed that there’s a gap above it? Perhaps you want to know the best way to fill that gap. Well, you came to the right place because we researched this question and have the answer for you.

The ideal way to fill the gap above your window air conditioner is by using the foam strip of your AC. If the foam strip that came with your air conditioner is not enough, or if your AC does not have a foam strip, use a polyethylene foam pipe insulation to fill the gap.

We have detailed instructions on how to install a window air conditioner that includes filling the gap above it. Read through the instructions in the sections below to see if you missed anything when you installed your window AC.

Read on!

A window air conditioning unit outside an old brick apartment building above a garden with yellow sunflowers during summer in Astoria Queens New York, How To Fill Gap Above Window Air Conditioner?

How to install a window air conditioner?

A simple window air conditioning unit seen from the outside on a brick urban residential building

We’ll start our instructions by listing some of the materials that you will need during the installation. We will also include some helpful tips, then we’ll jump into the steps to install your window AC.

Before we proceed, this guide assumes that your air conditioner’s output is rated for the size of the room where you will install it. Most AC boxes have a table on the side where you can find sample sizes of rooms and the recommended AC to use.

What are the tools you need to install a window air conditioner?

Outdoor window air conditioning units on an old New York City brick apartment building with green plants along a sidewalk

  • Phillips screwdriver
  • Cardboard cutter
  • Measuring tape
  • Polyethylene foam pipe insulation
  • Window AC panel insulation

What important things should you prepare before installation?

Here are some important things to prepare before you start the installation of your window AC.

Prepare a dedicated power supply

It is a good idea to have a dedicated power supply for your air conditioner. This will ensure that the circuit that your AC will use will not be overstressed or overloaded.

Your breaker will get tripped when a circuit gets overloaded.

Have an electrician install a dedicated power outlet for your window air conditioner to prevent tripping your breaker each time you plug another electrical appliance on the same circuit. You can use the same dedicated outlet for a space heater during winter too.

Establish a tilt for your AC

Air conditioners should have a slight downward incline going out of your window. This is to ensure that the flow of the excess water will be towards the outside of your window and not into your room.

Manufacturers can have different recommendations when it comes to the angle of the incline. Some recommend adding height to the windowsill to make it higher than the storm window instead of providing an angle.

Check the manufacturer’s installation manual to get the recommended angle or height.

Install a piece of wood on the windowsill that will give you the required angle or height. Make sure that you can still close the window (during winter if you plan to uninstall the AC) after you install the wood.

Clean the window

A woman clean a window pane with a rag and soap suds. Cleaning with a detergent. Hands in pink protective gloves washing glass on the windows of the house with a spray bottle, home routine concept.

Clean the window where you will install the air conditioner. This will prevent your AC from sucking in dirt and dust from the surrounding window.

Sucking in dirt and dust will make the air filters dirty. Dirt and dust will clog your air conditioner, meaning it will have to work harder to cool the room.

Check the window's structural integrity

Your AC likely came with a vertical metal bar on top. The purpose of this bar is to get caught by your window once you close it on top of your air conditioner. This metal bar will prevent your AC from falling out of the window.

However, you need to check whether your window is strong enough to support the weight of your air conditioner. Most new AC models are lighter than older models. However, it is still a good idea to check if your window is sturdy enough to support the weight—especially in older houses.

How to install the side panels on the AC?

Your air conditioner will come with two side panels that you need to install on both sides of the air conditioner. They look like a plastic accordion that you can pull to stretch and seal the rest of the width of your window.

Some window ACs have these side panels already attached out-of-the-box. If your unit does not have the side panels pre-installed, here are the simple steps to install it:

  1. Insert the side panels on the top and bottom guides. Make sure that the panels are facing the right way when you insert them.
  2. Screw the side panels to the side of your AC using your screwdriver.
  3. Do the same for the side panels on the other side of the air conditioner.

How to secure the AC to your window?

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

Here are the steps to install and secure your window air conditioner.

  1. Raise your window and place a prop to keep it open during the installation.
  2. Mark the middle of your window with a light pencil.
  3. Determine the middle of your AC. Make a light mark on top with a pencil.
  4. Install the foam (that came with the air conditioner) on the underside of the window. This will create a seal on the top of your air conditioner when you close the window.
  5. Place your air conditioner on the windowsill and align it with the mark on the center.
  6. Keep the AC in place and remove the prop on your window so that you can slowly close it. Align the center mark on the window with the center mark on top of your air conditioner. Check that the metal frame on top of the AC overlaps the bottom of the window when you close it.
  7. Screw the L-bracket on top of the window to keep it closed. Keeping the window closed will keep your air conditioner in place and not fall outside.
  8. Screw in the L-bracket on the side of your AC and on the windowsill. Do the same thing for both sides of the air conditioner. This L-bracket will keep the AC from sliding into your room.
  9. Extend the left side panel outward until it reaches the left side of the window frame.
  10. Screw the left side panel to secure it in that position.
  11. Repeat steps 8 & 9 to fasten the side panel on the right.

What should you do after installing the AC?

And old styled air conditioner on the wall.

Here are some of the post-installation activities that you should do after completing the installation of your window AC.

Plug in your air conditioner

Plug your AC into the dedicated power outlet that we discussed earlier.

Some air conditioner plugs are thick and heavy. Make sure that the wall socket will be able to support the weight and that the plug will not fall off in the middle of using the AC.

Turn it on and set it to your desired temperature

Most modern air conditioners have a remote control. Set the temperature using the remote control to verify that the remote control and the AC are working.

Remove drain cover

Some ACs have a cover on the drain valve at the back. Remove this cover so that your air conditioner will be able to drain excess water.

It is normal for your AC to produce water as it dehumidifies the room while cooling it at the same time. Some of this water is used by the air conditioner, while any excess water needs to get drained out.

Install insulation on the gap on the window on top of the AC.

Your air conditioner should come with an insulation strip that you can use to seal the gap on your partially open window. However, some windows have a wider gap between them, and the insulation strip might not be enough to seal this gap.

Use the polyethylene foam pipe insulation to fill the gap between your windows. The hollow interior will allow it to be squeezed into the gap. It will then press against the window to create a seal once installed.

The M-D Building Products pipe insulation & tape is available on Amazon. Check it out through this link.

What to do if the side panel doesn’t reach the side of the window?

Some windows are wider than normal, and there will be a gap on the side of your AC if the side panels do not reach the side of the window.

How to seal the gap between the window and the side panel?

If the gap is small—less than a quarter of an inch—you can use a weather seal to close the gap. However, if the gap is too wide for the weather seal to close, use a window AC insulation panel.

These flat foam panels have adhesive sides that you can install on the side and even on top of your air conditioner.

The AnyAir AMIP window air conditioner foam insulating panels in a pack of two are available on Amazon. Check it out through this link.


a professional electrician is cleaning the window air conditioner on the roof top of a house with his high pressure water gun

There are several ways to seal the gap above the air conditioner based on its width.

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