Introduction: Pet Door Window Insert From XPS Foam and Double-Pane Glass

This cat/dog door insert for a double-hung window is made entirely out of Extruded polystyrene (XPS) foam. When properly installed it provides much better thermal insulation than plastic or aluminum inserts. Obviously there is no intrusion security with this design. I am renting and this insert had to be easy to remove with no permanent modifications to the window.

Foam is easy to work with, and you could probably adapt this approach to other types of windows or sliding doors as well (for very large inserts I'd probably use a plywood core, but 3 layers of foam are plenty strong for this project). You don't need to add glass windows, if you just want a simple, strong, opaque insert.

I made two different versions:

  1. The "Prototype" (just below): By simply gluing 3 sheets thick (or could have used thicker foam if I had it), painting and then cutting/trimming to size. This worked great but has a rough appearance since the glass is glued on to the outside.
  2. "Version 2" (further down): By completely shaping and painting the 3 sheets individually and then sandwiching them together with the glass.


  • A cat door flap of your choice (although you could probably make your own as well)
  • Foam sheets (XPS or whatever you are comfortable with)
  • Foam Glue (eg: 3M 77 Spray Glue)
  • Cutting tools (a sharp razor and/or hot wire cutter)
  • Sandpaper (for smoothing, especially useful if you aren't using a hot wire cutter)
  • Paint
  • Weatherstripping


  • Glass panes (4 picture frames from a dollar store)
  • Caulk for glass panes
  • Backer rod (or any foam) for top of window sash

Cost breakdown

  • $20 4'x8'x0.5' XPS Foam sheet (enough for multiple projects)
  • $5 Weatherstripping and backer rod (price may vary depending on size)
  • $20-$300 for the cat flap (more if you want a high tech flap)
  • $4-10 for the glass pieces, if using (dollar store, Walmart, Home Depot, etc).

(I'm not counting shop supplies like spray glue and caulk)

Step 1: The Prototype

This is the first one I made, and it is probably an easier project. I'm including it here just in case.

Short summary is:

  1. Glue 3 pieces of 1/2" foam together with spray glue (cut them larger than needed). You could also start with 1.5" thick foam (more expensive) and not bother with glue.
  2. Cut it down to fit the window (tall enough for the cat flap). You can try to get a perfect fit, or cut it slightly small and apply weatherstripping.
  3. Cut an opening for the flap.
  4. Paint (I used different paint for interior and exterior surfaces)
  5. Done... or add windows (I added mine a year later). I recessed the glass into the foam so that it's flush, but the caulk holding the glass is still visible and the cuts were not too clean.

Step 2: Version 2: Design

I wanted an insert that would be low-profile and would fit the window perfectly. I also wanted the glass edges to be hidden. To do this, I would need to hide the glass edges between the layers of foam during assembly. This means that every layer needs to be fully shaped first.

Step 3: Shaping the Layers

I have 3 layers in this design, and their cumulative thickness is exactly right - front layer is inside the room, and the other two are as thick as the window sash resting on top of them:

  1. Inside layer is the tallest because its upper lip is in front of the window sash
  2. Middle layer has smaller openings for windows and cat flap (it's the "tunnel")
  3. Outer layer is exactly the same as the inside layer but without the upper lip (so it's 1/2" lower).

Once I cut the cat flap openings I used the cat flap to hold the layers together during all the other fitting and measuring.

The dimensions for this insert are 29.75"x11.5" (29.75 is the window opening width)

With a "small" cat door flap, that left me just enough room for two 8x10 glass panes (4 pieces of glass) that I got from a dollar store in the form of picture frames.

  • For beveled layers (outside and inside), I cut at a 45 degree angle with the outer dimensions matching the glass.
  • On the other side of these sheets of foam, I simply cut a 1/16" (approximately) square recess groove for the glass to fit.
  • The inner layer glass "tunnels" match the size of the beveled openings in the other layers. (see diagram above)
  • Note the groove in the bottom of the middle layer, it fits over a lip in my window frame.
  • My cat flap happened to be made for thin walls, so I installed it "around" the inner foam layer and recessed through the other layers. If I had a long tunnel type cat flap I probably would have installed it in a protruding way like on the prototype above, but I really like the recessed look, so it worked out.

Embarrassing Note: Although I made a hot wire foam cutter for this project, I couldn't manage to make perfectly smooth cuts, so you can see imperfections in the beveled cutouts. In the end, it's not that noticeable.

Step 4: Assembly

  1. Paint the parts separately! Make sure that no unpainted surfaces are visible when assembled, since this will be glued and cannot be taken apart.
  2. Make sure that everything looks good when held together, and the assembly fits your window reasonably well.
  3. Clean the glass and keep it clean. You won't get another chance to clean the interior surfaces.
  4. Apply caulk to the glass pane recessed grooves.
  5. Spray the glue on all the mating surfaces. It'll be tricky because the middle layer needs to be sprayed on both sides. You'll have at least 10 minutes before the glue dries.
  6. Start with the interior layer laying face down. Place the glass in the grooves.
  7. Attach the middle layer (you did mark them so you know which side is which, right?). Use the cat flap as an alignment guide. Once the surfaces touch, you're done. Press to make sure that the glue worked
  8. Leave the remaining layer down and place its glass in. Don't lift it or the glass will fall out.
  9. Place the other glued layers on top of it.

Step 5: Installation

Now it's just a matter of strategically attaching weatherstripping to the foam to ensure a good fit. I have weatherstripping all around and have absolutely no leaks (except for the cat flap itself, and for that I made a foam plug you can see in the photo, I insert it when the door is locked).

If the insert won't fit, you can always shave (or sand) off a little to make this easier.

Finally, you now probably have a massive air leak from the top of the window sash (because it's partially open). I used a caulk backer rod (5/8" foam rod) to fix this.

Step 6: Convincing Your Cat to Use the New Door

Accessibility (ramp)

If your cat is anything like my lazy little control freak, you'll need to provide a convenient platform to step on from both sides of the window. I made this little ladder from a modular closet organizer (the cube-shaped kind, search for "Cube Grid Wire Storage Shelves") with a sheet of... yes, XPS foam on top. There is a heavy rock in there to keep it from tipping.

It may also be necessary to tape the flap open for some time before expecting the cat to push it open.


  • Me: Hey cat, come use this new cat door!
  • Cat: I have a better idea: How about you open the human door for me, slave?
  • Me: Hey cat, look, I'm even holding it open for you!
  • Cat: Don't care.
  • Me: Look, it's the outside!
  • Cat: Oh look, I see something else that is way more interesting. Yes, definitely.

(That's where I am so far, even though this cat has used a cat flap for years).

Step 7: Variation: Recessed Door

This door is installed in a second floor window (first floor for the Europeans), and the very agile cat that uses it stands on a narrow window ledge when entering. In order to give him more runway I recessed the cat flap into the room. The entire assembly is still made out of foam, but, obviously, it involves a bunch of compound angles when gluing. Otherwise, it is the same design.

An interesting side effect of this design is that the recessed section can act as a funnel in strong winds, causing the flap to flip open. If you have strong winds coming from the target window's direction, this may not be the design for you.

Step 8: Variation: Odor Eliminator (exhaust)

This isn't a cat door, but it is the same technology and is still cat related. I have a temporary cat situation that involves a litter box in the living room. The best solution to the odor problem I could come up with was to place the litter box by the window and have a continuous low-speed exhaust fan pull the air out. It's shaped like a kitchen exhaust hood but, since I don't want to heat the outdoors in winter, it's powered by a very low CFM silent 80mm computer fan via a DC adapter.

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