A room. A window. A cat. A human.
Cat wins. As always.

A friend found herself basically freezing to death and being woken up, having to leave the window open several times a night to let her cat in and out. I thought I could come up with a better solution and this Instructable describes it. Although it’s extremely custom for the geometry of the window some of the ideas might be transferrable and I also have some tips about making things and point out some of my favorite tools.

The problem and constraints:

The volume of (cold) air flowing in through the open window surprised me. I think within a minute or two the room would get freezing cold. There must be a big air pressure difference due to the building and surroundings (maybe because it’s facing a river).

I thought of different ways to leave the window open, close off all of the gap except a space to let the cat through. Some of the constraints—it should:

- be able to be easily put up and taken down every morning/evening

- require no modifications to the rented apartment / historical building (from the 1500’s plus or minus); e.g. no mounts screwed in

- be hardly visible from from the outside

- be weather/water resistant

- be very light since the person using it is on the petite side

- store under the bed -> modular

- be robust enough withstand not just the normal huge airflow but larger gusts of wind

- look not too too ugly

After mulling over several ideas I chose to go with styrofoam as the main structural component and do a kind of carwash portal to let the cat through but keep the air flow to a minimum.

Step 1: Measure Twice, Cut Once.

This Instructable is going to be mostly a visual story--lots of pictures with annotations, not a huge amount of text outside of the pictures. NOTE: I just noticed that the majority of my annotations did not get saved! :( I probably won't have time to redo many of them--it was a lot of time. Sorry.

I was only able to go over a couple times to take measurements so I tried to cover everything. It was important to not make too many assumptions, like the window is square to the surface below it or that a hinge on the left side of the window had the same gap from it to the frame as another same-shaped hinge on the right side, since it's an old building an nothing necessarily was going to be square or flat.

I took a lot more measurements than shown. I did not do much in the way of sketching, I relied on looking back at the pictures to design into the space.

Step 2: Mind the Gap

The main problem here involved finding a way to seal the edges, as they were step shaped. I decided on a V-shaped groove covered with foam, which when compressed should not allow much air or cold through it.

I considered polyurethane foam to form fit everything, covering the surface with cling wrap first to protect it, then spraying the foam, cutting to shape and using that to really custom fit everything but it seemed to be a lot more work than the V-groove solution.

I don't have a lot of pictures of this step but basically I took a large plate of styrofoam from the building section of the hardware store, called Jackodur I believe, and cut a rectangular piece out. The width (which I later found out I figured wrong) was the width of the gap the window would open plus the height of the V-groove (where foram will be taped in). The height equals the height of the open window minus the vertical height of the space left for the cat to get through (look at the final pictures of the gizmo installed, if this is confusing).

I cut the blue polyurethane foam on the band saw, after extending the table out by clamping some aluminum sheet onto it. Bandsaw found by watching the local classifieds for a few months--$20 and it works great for small stuff!

Step 3: Rolling Out the Red Carpet

This section deals with the 'landing pad', the bottom part that the cat will walk in/out on and which will block cold air flowing through the gap under the window.

Some considerations on the floor part of the design:

- should be water resistant

- eliminate the little sound cats make when they jump on something, that little thud, to try to reduce the chance of being woken up by it

- give the cat a good grip/feeling since I think they like that (from experience)

- not too expensive (this constraint applied to everything actually)

I don't remember now the choices I considered but I think I settled fairly quickly on carpet and was happy to find entryway carpet--rubberized bottom and made for wet-shoe traffic--at the hardware store. I could have maybe used that green astroturf carpet but it feels really plasticky and the one I got felt soft.

See pics for extensive comments about each of the steps involved in making the bottom part of the cat portal gizmo.

I did not glue down the top part yet since that could wait and I wanted to make sure everything was going to work before attaching it since in the worst case I'd have less to tear apart and rebuild if there was a problem...which there was. :(

Step 4: Pirate's Portal

The cat's basically a little pirate, roaming free to do whatever it wants, so I saw this pirate theme fabric at the cloth shop and got it.

This opening where the cat came in and out was the most complicated part. It had to block wind & rain. Be cat-friendly, not too hard to push through, and soft but not so soft that it got blown open by airflow. Be removable/modular for potential cleaning. Be adjustable to get a good fit and minimal cold air flow through.

I somehow had the idea of a carwash type arrangement, with pieces of fluffy fabric hanging down, arranged into multiple layers to act as a baffle for the wind. When I did a test at home with my own window on a first version of this part (the first few pictures) I felt it needed something more to block the wind so I added leather flaps which could be moved (attached with velcro) either one on the inside, one outside, or both on inside/outside as real-world usage showed one configuration or the other to be more effective. The velcro attachment also meant that something else could be attached later instead, e.g. a heavier or lighter material. I liked leather though since it's tough, soft (it was a kind of suede) and looked nice.

Step 5: Getting a Grip

How to hold the whole thing during installation? It needed some kind of handle to hold it up while securing it in place.

I tried 3D printing one first but it didn't feel solid enough. I did print with a low fill amount but even if I'd printed it completely solid I didn't feel like it would feel stiff enough and hold up to hundreds of uses, maybe being dropped etc. plus I think the print time was going to be exceedingly long if I remember correctly. I could have printed with a partial infill then injected epoxy into it but I think the solution I finally used was easier and stronger.

I took part of the 3D design but used a piece of aluminum as the core of the handle.

Step 6: A Fix and Afixing

I made a design error in the main purple styrofoam plate width and had to fix it.

Then I had to cut a new piece of carpet to fit the modified base part. This step covers those two issues.

Step 7: Topping It Off

This project started to take too much time and I didn't have a great idea for the top so I decided to take the first doable, simple-ish solution I could think of. Keep in mind the top had to be insulating, modular for storage, not heavy, fit a custom space, block as much cold air flow as possible, etc. I had several other nicer ideas but they all required more time/effort.

I thought it'd be ok to use thinner styrofoam plate on top since the cold air should ? mostly be going down rather than up and so not as much insulation would be needed on top. I suppose there could be an unusual airflow pattern that would push the cold air up rather than down but from what I felt when testing the airflow in the beginning that wasn't the case at that time.

Step 8: Icing on the Cake

The friend said she didn't care about having a fancy cover but I kind of wanted it to look at least halfway reasonable when installed. I tried to keep with the wintry/arctic theme and chose a fake pelt-like fluffy fabric, which not only would look decent but also help to stop airflow through any cracks around the edges.

Step 9: Et Voilà

It's not perfect. I made no attempt to cover surfaces that would be little seen, the bottom part and the part facing the walkway outside. This was a side project and I tried to get it done quickly.

The very first time we installed it the cat was able to go through and I believe now it's being used regularly.

And They Lived Happily Ever After

...well, at least the cat did.