How to Make a Pet Door

Make Your Own Cat Door
After spending quite some time researching online and in stores, I've decided to build my own pet door, for my cat. The problem I'm having with all of the current offerings is that they are not energy efficient, and leak lots of cold air into our basement where the cat comes and goes. I'm not so worried about the blast of air when she comes in and out. I have more concern about the constant leakage around the door seals. It's kind of like a leaky faucet, where the single drips really add up over a few hours. (Update: Click here to see my recent review, and installion of a Freedom Pet Pass, Pet Door.)

How to Build a Pet Door, Prototype Only
Yesterday I started to brainstorm ideas on how to replace our current old pet door with something that: I could build, is cheap, and highly energy efficient.

Those are lofty goals when you consider that there are hundreds of pet door designs on the market, many of which claim to be "weather resistant" or draft stopper types. Well I figured I'd take a stab at it anyway, as I couldn't find any such plans online for making your own pet doors.

This is the old door. It was installed by the previous home owner. Yeah, it's not a expensive model, and I'm sure a new flapper would have helped it, but not much. The slots on the sides (to block off the pet door when not in use) are on both side and on the inside and out. Those let huge amounts of cold air in, and the flaps never stay closed around the edges. (Note: click on photos to enlarge them)

Here's the original old cat door, showing 36 degrees F. Not a scientific measurement, but good enough for a general idea. What you also can't feel, is the solid draft of cold air coming in from all sides and from the flap of the cat door. The outside temperature was 21 degrees F.

This is the after shot for the first prototype. As you can see we are now at 51 degrees, which is an increase from the original old cat door by 15 degrees! Not to mention it is now weather sealed from cold drafts. I could only feel a slight breeze at the bottom where the weather stripping wasn't meeting well due to the uneven surface of the rigid foam insulation it was stuck to. The outside temperature was 21 degrees F.

Here's the temperature in the middle of the basement, at the same height as the pet door. It's at about 60 degrees F. The outside temperature last night was 21 degrees F.

Keep in mind I had the idea yesterday, and last night in a few hours I built the first prototype. Although the initial results are very promising, there are design issues that I will improve with the next prototype. More on that to come. But I figured I share what I've done so far.

How to Build a Pet Door

Here I'm cutting out the piece that will be the door flapper part. I'm using some 2" Dow rigid foam insulation that I had left over from a previous weatherization project.

I bought 2 pieces of Lexan Sheets from HD to adhere to the front and back of the door. I've heard that cats won't enter a door that they can't see though, thus the window. The plexiglass/Lexan sheets add most of the weight to the door, so on my next model I'll make the window the same size, but have less Lexan overhanging onto the insulation. And also less silicon caulk (that's what I used to glue and weather seal the window). I'll have to look up the insulation R-value for dead air space, but I know it's a good thermal barrier, and the rigid foam has an R-value of 14.

I've removed the old cat door and what's left is some plywood that I'll use for now, as it's there, and I'm still in the testing phase. I'll make a good backer for the next piece, shown below.

This piece will go into the window covering all of the plywood there now. It's also the 2" think stuff and will give R-14 insulating to the whole pane area. Attached to it is some weather stripping I had around. This was not my original pan, as what I really want to test is some refrigerator sealing material, which has an internal magnetic which would keep the door really closed. HD didn't have any and I was eager to get to it, thus the weather stripping for now. I'd have to do some experimenting to be sure the cat could still open it with the refrigerator seal. I'll work with it on the next prototype.

Here's the piece (shown above), now placed in the window with some silicon caulk behind it, to attach it with the plywood.

Now it's time to put the cat door on. I'm still playing with the best way to hang the door. Right now I have a few eyebolts coming out from the plywood backing and through the backing insulation. The cat door just has 2 nails, one in each side, so that it can swing in the eyebolts.

Here's the profile, of that simple, but not final design. But you can see that the door is lying quite flat to the backing. So far so good.

Obviously I'll be refining this area for strength and for fine tuning the offset of the cat door from the backing insulation.

While it did a fairly good job closing flush on it's own, a breeze could open it, so I added 2 magnets to the bottom of the cat window, that grab each other when it closes. Seems to work well. I don't have a picture of that installed. The picture above is a package of about 8 magnets.

Here's the final so far, with the cat looking quite dismayed. She doesn't know what to make of it so far. This photo is from this morning and she didn't go in it by herself yet, but I had to get to work, so she may have figured it out. She did go through when I opened the door for her. The door is light enough for her to open. She might not like the metallic look. Future models will include making it more attractive to cats and wives.

And Here's What You've all Been Waiting For!
I can hear you know. "Okay, great, so you can build a pet door, but how does the cat go from inside to outside, this door only swings one way!?"

You are right and that's why I'm going to put an "out" door just like it, in the pane to the right of this cat door. Yeah, I'm doubling materials and doubling draft possibilities, but I figure when my design is final and airtight, it really won't matter how many doors there are.

Also, I know there are pet doors out there that are draft resistant, with 2 sets of inline doors, but I decided for a do it yourselfer, with true energy efficiency in mind, that trying to make a pet door all things in one design, is not possible. So in order to make it air tight, insulated, usable and cheap to make, I'm not complicating the design with the need to make one door swing both ways.

This a quick post to show a prototype and is not close to final, so don't use this as your design to make your own cat door just yet. Stay tuned. I'll put more detail in when I have more time, and when I have the next model up and running. I also have to address driving rain that could come in the cat door design as it is now. Not bad results though for a first prototype and a night of fun work.

Prefab Weatherized Pet Doors (These are not endorsements)
Moore Pet Supplies: Insulated Pet Doors
Secure Pets: Petsafe Extreme Weather Pet Doors
Dog Beds & Crates: All Weather Energy Efficient Pet Door (Patio Pacific Inc.): Endura Flalp Pet Doors
Hale Pet Doors: Energy Efficient Pet Doors
Dog Door Store: Ruff Weather Pet Door
SmartPet Premium Products: Aluminum and Lexan Pet Doors
Freedom Pet Pass: Energy Efficient Pet Doors
Pet Super Store: Pet Doors

Legal Note: This design and prototype is not based on any other ideas I have seen, and is my intellectual property. Other variations of this idea include: a one piece window mounted design that can fit between a window sash and window sill. Also this design could be used for dog doors and doggy doors that are mounted in doors or walls, with side by side in and out doors. The original idea as I see it, is to have two doors (one leading in, and one to exit) vs. the traditional pet door which tries to achieve both entry and exit in one design. Also the use of an air sealed flapper of substantial width (2 inches) is unique to this design. The integration of rigid insulation foam board is also unique as a stand-alone design. © 1/2009.

My Standard Disclaimer: This is what I did. This does not mean this is the best way, the right way, to building code, or even safe for your needs. So you are on your own with your project. I make no promises about the information presented here. I'm just a do-it-yourselfer, not a professional at all, sharing my story. So if something goes wrong with your project, you are on your own. Good luck, and have fun!

All content and photos, copyright 2009, Dover Projects.

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  1. Nice door. How would you install a lock?

  2. Hi,

    My name is Brian, about five years ago I approached the leaky pet door problem which resulted in the development of the Freedom Pet Pass Dog Door. I have more recently developed a cat or small pet flap using the same patent pending design that is proven draft-free and energy efficient as well. If you don't need a security door for your application I can offer you a flap that works both ways and mounts to your frame. I can be reached at 989-667-3763. Thank you for listing our website on your blog.

  3. Hi Brian,

    Quite an honor to receive your note. I may have to take you up on that because I've yet to build the "out" door" and the cat although now using the "in" door just fine, would like something to go out as well sooner, rather than later!


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  5. Update, My cat door is still holding up really well. I'm going to have to make version 2.0 of this thing!

  6. Thank you, Peter!
    The idea is very simple and promising.
    Funny that I have not found anything - at first glance at least - in our, "Russian" areal.
    We are going to the country for a week with two our cats and I wanted to "improve" two doors leading out (it is still not hot near Moscow). But now, with your hint, I see that it is better to "improve" one window, its narrow part. Thank you again!

  7. Hey Igor!

    The cat door only goes one way, and guess what. That's all it needs to do as the door is fat enough that the cat can open it by digging it's claws in the fat foam side of the door (takes a bit for them to learn that though, maybe). The only repair I have to do, is the nails I used for the quick and dirty hinges need some reinforcing in the foam, as they have become a bit loose and thus the door doesn't always close just right, and thus the magnet doesn't always meet up perfectly. The door is starting to hang a little low. So perhaps glue in the nails with some epoxy glue.

  8. So does your last comment mean that the cat can go both ways thru a one way door. How do you make it strong enough to close but weak enough to have the cat push thru?


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