How to Install a Cat Door In a Window Sash

Finished Car Door In Window Inside View

I’m a great believer in pet doors! But I also abhor the thought of cutting holes in doors or walls to install them. I think it potentially decreases the value of your home. Those holes can be fixed, but repairs will be costly. My solution is to install our cats’ door in a window sash. Installing a cat door is fairly easy. But if you want to install a cat door in a window sash, it can be a bit more complex. I’ll cover the step by step instructions on how to install a cat door in a window sash without destroying your window!

Should I Buy or Build a Window Sash Cat Door?

There are a variety of window sash cat doors that you can buy. These devices are designed to fit in your window. You would raise the sash, install and secure one of these, and then lower the sash on to the cat door. However, they are expensive. I have not seen any of these that are less than $100. Some of them are quite a bit more. Which is why I chose to build one myself.

What Cat Door Should I use?

If you are going to install a cat door in a window, you’ll need to buy a cat door. I’m pretty picky about what cat doors we used. The features I look for are that I can lock it in either direction. This comes in handy if you’re trying to get the cats in before a storm or if you want to keep them out for any particular reason. I also look for cat doors that have a magnetic closure. The magnet doesn’t actually hold the door closed, but assists in stopping it in the closed position after a cat leaves or enter. I found a decent one online for $15.

Build and Install a Cat Door in a Window Sash

Start with a piece of shelving from your local hardware store. I used a piece of melamine shelving from the HD. The board I bought is 3/4″ thick, 15 3/4″ wide and 4 foot long. I’ll have some left over, but I’m sure I’ll find a use for the leftover piece down the road. I also bought some 3/4″ thick weather stripping. Total cost for this was a bit over $15. Everything else I used was from my collection of screws, bolts and other materials left over from past projects. This is probably all you would need to install a cat door in a window.

Measure Before You Cut

We’ll need to cut a piece of the board to the correct length. I want the edges of the board to match the profile of the sash in the window. This way it will mimic how the sash seats against the sides, giving you a weather tight seal. You’ll have to take a few measurement for this. First measure from the outside edge to the outside edge across the window. The picture below shows the points you will be measuring. Make note of the distance between the outside and inside edge of the side rail on your window frame. We’ll be cutting a rabbet edge to that depth on the sides of the board.

Cat Door Window Sash Measurement
Cat Door Window Sash Measurement
Cat Door Slide Rail Measurement for Rabbet Cut
Cat Door Slide Rail Measurement for Rabbet Cut

Next, measure the distance from the inside of the window to the edge where the sash slides on. This will be the depth that we’ll need to leave behind on the board when we cut a Rabbet cut on the end of the board. The picture above depicts this measurement. Cut the Melamine board to the longest length across the window that you previously measured. If you are using a circular saw, cut the board with the saw on the side facing the outside of the house. This will keep the melamine from chipping on the side facing the inside.

Cutting a Rabbet on the Sides of the Board

Cat Door Router Fence Adjusted to Depth pf Rabbet Cut
Cat Door Router Fence Adjusted to Depth pf Rabbet Cut

My preference to do this is to use a table saw. you can use a regular blade and make several passes, or you can use a dado blade on the saw to do it in one pass. Unfortunately, my table saw is out on loan. So as an alternative, I used a router table with a straight plain bit to route the rabbet into the sides of the board. The picture below shows the depth of the cut that will be made on the edges of the board.

For those of you that may not be familiar with some of the woodworking phrases such as Rabbet and Dado, here’s an excellent resource that is a good Primer for Carpentry Joints.

Cutting out a Hole in the Board for the Cat Door

Measure and Mark the Hole for the Cat Door

In order to center the door on the board, I first marked the exact center of the board. From here, I was able to measure out and mark half of the width and length in all for directions. This gave me the reference marks to center the outside frame on. One I centered that frame, I used a sharpie to draw a cut line around it. I used the outside frame for this as it is the bigger of the two frames. The pictures below will give you a general idea of how I did this

Centered Cat Door on Board
Centered Cat Door on Board
Cat Door in Window - Board Marked and Ready to Cut
Cat Door in Window - Board Marked and Ready to Cut

Before marking the board, check the jigsaw blade you will be using. If it cuts on the down stroke, place your reference marks on the side facing the inside of the house. Conversely, if it cuts on the upstroke, mark the side of the board facing the outside of the house. Once again, this is to keep the side facing the inside of the house with a clean cut edge. It probably doesn’t matter since the edges of the hole will be covered by the cat door, but it’s still a good practice.

Cut Out the Hole for the Cat Door

This step is fairly straightforward. Drill a hole in one corner of the are that was marked off. Make sure the hole you drill is large enough that the jigsaw blade you will be using will fit inside of it. If your jigsaw blade is narrow enough, you can saw around the entire profile you marked to cut out the hole. Alternatively, you can drill a hole in all four corners of the cutout so that you can easily turn the jigsaw around. Once you have the hole cut out, you can test fit both sides of the cat door.

Cutting Out Hole For Cat Door With Jigsaw
Cutting Out Hole For Cat Door With Jigsaw

Most of the cat doors I have installed come in two sections. Typically, the piece that mounts facing inside has the swinging door plus any locks or other mechanisms. The piece that goes on the outside is cosmetic. It’s sole purpose is to cover up the outside of the hole as well as providing a barrier to the outside weather. The inside piece that goes into the hole is usually a bit smaller than the outside one as is the case with this door I am installing. The inside panel basically slides into the outside panel through the hole. 

Trimming the Outside Panel when Installing a Cat Door in a Window

Cutting off Excess on Cat Door with Bandsaw
Cutting off Excess on Cat Door with Bandsaw

The first thing you will notice is that if you try to connect the two panels through the hole you just made, you wont be able to get them to sit flush. These units are made for walls and doors which can range from an inch to several inches. So they have a lot of extra plastic to bridge those gaps. Since we are mounting these to a 3/4″board, we’ll have to trim some of the access off. The cat door I bought had several cut marks on them to make this task easier. I used a band-saw to cut through the cut mark closest to the panel. (See Picture)

Although I used a band-saw, There are other ways to trim down the outside piece. A small trim saw will work, as will heavy-duty shears or even a box cutter. The plastic on these is fairly soft, and the section that needs to be cut is fairly thin, so you shouldn’t have too much trouble cutting it down. But, if you have to use a box cutter, Please be careful!

Cat Door Outside Panel Too Large
Cat Door Outside Panel Too Large
Cat Door Outside Panel Cut to Size
Cat Door Outside Panel Cut to Size

Installing the Cat Door in the Board

Now we are ready to install the cat door into the board. The cat door kit came with several wood screws. This kit is intended for thicker mounting surfaces (Walls and doors). Since I am installing the cat door on a relatively thin board, the mounting screws on the inside and outside panels will collide. to avoid that, I will be using long thin bolts and nuts on the outside to secure the two panels.

Drilling through the Panels and Board

Begin by mounting the inside panel. Next, mount the outside panel so that it interconnects with the inside panel. Next, using a drill bit just large enough to allow the bolts to pass freely, drill through the panels and the board. You’ll need to make sure that the drill is going in straight so that the hole you are drilling goes through the screw holes on both panels.

Next, install the long bolts from the inside out. You should be able to push these all the way through. I threaded the nuts on to the bolts on the outside and pushed them far enough into the recessed hole so that the plastic of the panel would hold the nut steady. After that, I simply tightened the bolts until the nuts were recessed into the holes in the panel. In my case, I used bolts that I had laying around which were a bit long, so I cut them so that they were close to flush with the panel.

Cat Door On Board with Long Screws
Cat Door On Board with Long Screws

New Cat Door in the Window

Cat Door Rabbet Cut Fit to Window Slide
Cat Door Rabbet Cut Fit to Window Slide

Now to Install the cat door in a window. What I do is first install a piece of molding on the bottom of the existing sash. This sash is wood, so I used brass wood screws to mount it. I am not particularly worried about making holes in the sash. Should I want to restore this, it is an easy process to fill the holes and paint over them. If you have a plastic sash, you’ll have to decide on the best mounting method. Two-sided 3M tape or glue (that can be removed) may be an option. The molding should be long enough to span across the entire sash.

You’ll want at least 3/4″ of the molding to hang below the sash. You will be attaching your new cat door to the sash with the overhanging molding. Raise the sash so it is out of the way. Next, slide the board with the cat door in it so that the rabbets you cut engage the slides on the dies of the window. Slide the cat door sash down as if you were closing a window. Next, slide the window sash down so that the excess molding overlaps the new cat door.

Cat Door Molding Installed on Window Sash
Cat Door Molding Installed on Window Sash
Attaching Cat Door to Window Sash
Attaching Cat Door to Window Sash
Secure the Cat Door in Place

Once this is in position, it’s a simple matter of drilling two holed through the molding and into the cat door board. Secure the molding with the board with wood screws and you’ll be almost done.

The last and final step is to close all of the gaps with weather stripping. I used 3/4″ thick weather stripping. I sealed the gaps between the two existing panes of glass, top, and bottom. Since I had a good bit left over, I also ran strips down the sides of the new cat door. I did this on the outside. Hopefully, that will minimize any drafts.

Cat Door Weather Stripping Install
Cat Door Weather Stripping Install
Finished Car Door In Window Inside View
Finished Car Door In Window Inside View
Cat Door Installed in Window Outside View
Cat Door Installed in Window Outside View

You’ve finished your install of a cat door in a window. More importantly, You can easily remove it at any time without leaving any large holes in your walls or doors! If you’ve enjoyed this article or have any tips you would like to share, please email me with this contact form.

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