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Window Too Wide For Air Conditioner (Easy & Fast Fixes)

One of the most frustrating situations you can find yourself in after buying a brand new AC unit is realizing that you didn’t take your window’s measurements correctly. Now, you’re stuck with a window too wide for the air conditioner with tons of space and gaps that kills insulation.

But don’t worry—not all hope is lost. Buying a bigger air conditioner isn’t the only fix for a crisis like this. It’s pretty easy to fill in the gaps and spaces between your unit and your window’s edges. Today, we’re going to take you through five simple ways to do it.

Why your aircon should fit your window properly

Many people get so excited about buying a new air conditioner for the summer months that they completely forget one of the most important installation tasks—measuring the window properly. When this happens, you’ll be stuck with a window too wide for your air conditioner.

This is a bad situation to be in because gaps between your air conditioning unit and window frame will lead to subpar room insulation.

An air-conditioned room requires good thermal insulation to maintain that cool or warm temperature throughout the day. One way to maintain good airflow is by ensuring there are no spaces or gaps for the cool air to escape. That’s why we close all windows when running the AC.

The first step in ensuring that your window air conditioner will have good insulation is measuring your window width before you even buy a unit. That way, you can be sure that it’s the right size for your AC.

Plus, having so many spaces allows pests and small animals to sneak into your home. So, you need to seal these spaces as soon as possible.

How to fix a window too wide for the air conditioner

How to fix a window too wide for the air conditioner

It’s a lot better to have a window too wide for the air conditioner than a small window where your unit will never fit. At least because the window sill is too wide, all you have to do is seal the gaps by filling them in. It’s quite a simple task if you think about it.

The best part is that there are many ways to fix this problem. Here are five ways to do so:

1. Assemble some insulated side panels

One of the best ways to make your AC fit your wide window and boost insulation in your room is by installing insulated side panels. These accordion panels are made of foam and are incredibly easy to cut and fit into the space between your unit and window frame.

Simply measure the gaps you need to fill in and cut your panels in that specific size. Then, push them into the spaces and seal them with caulk, duct tape, or weather stripping. When you’re done, you shouldn’t have any air leaks.

You can get these panels in your local hardware store. Watch this video for a demo on how to cut and install side panels for your window AC units: 

2. Push fiberglass board insulation into the gaps

Another foam-like material you can use for filling in the spaces is fiberglass board insulation.

What’s awesome about this alternative is that it has a very high r-value, aka the capacity of a material to resist heat flow. Fiberglass is known to slow down the spread of temperature changes, whether it’s hot or cool air. So, insulation will be top-notch.

Another excellent thing about fiberglass is that it’s a breeze to cut, so you can easily get the right thickness for your window frame. When you have the fiberglass board in the size you want, just slide it into the gaps in your window.

Make sure that the board isn’t overly snug in the window space. If it’s extremely tight and squished, it will actually reduce insulation.

Here’s the proper way to insert fiberglass board insulation into your window’s gaps:

3. Fill in the spaces with plywood

If plywood is more accessible to you than the first two materials, you can use that. Hardware stores usually cut plywood for you, so just tell them how thick you need it to be to cover up all the space between your AC and window frame. Use a sealant to lock in the wooden planks, and you’re done.

The downside is that plywood has no insulation power, so you’ll likely have issues with maintaining the temperature you want in your room. One quick solution for that is by adding a layer of foil over the plywood to get at least a little bit of insulation back.

4. Lock out air with spray foam sealant

Spray foam sealant is an expanding material usually meant for sealing leaks and holes. But it also does an excellent job of filling in the space between your AC and window.

Before spraying your walls, remember to cover the area with a layer of foam or cardboard so that the foam doesn’t touch the walls. When you’re ready, spray in any gaps to fill them in. It could take about 24 hours for the foam to harden.

Foam sealant is waterproof, so it’s perfect for those who live in rainy and humid climates. That said, it’s probably the option on this list that looks the least visually appealing, which is a huge con.

5. Let good old cardboard do the trick

Let good old cardboard do the trick

If you’re looking for a temporary fix to buy you time to buy materials like fiberglass or spray foam sealant, you might want to consider blocking the gaps in your window with good old cardboard.

Sure, they’re not as sturdy as the other materials on this list. But it’s a great band-aid solution if you need to cover up the gaps fast until you think of a more permanent game plan.

Lock the cardboard in place with some duct tape and perhaps a layer of foil for extra insulation, and you can buy yourself time before planning out your next steps.


If you ever find yourself in the precarious situation of having to install an air conditioner by a window that is way too large for it, don’t panic.

There are five different things you can do to alleviate the situation. The good news is they’re all easy and accessible, even for people who aren’t necessarily fans of housework.

Whether you go with insulated panels, fiberglass, plywood, spray foam sealant, and even cardboard as your go-to solution for this dilemma, fixing the issue of a window too wide for the air conditioner will be a piece of cake.

Have any questions or other suggestions for how to fix this problem? Share it down below!

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