I got me two 4 month old kitties and will have them spayed next month. I'd like to put them a cat door in our detached garage. We keep it heated to about 45 in the winter, so it doesn't freeze things. I have never had a cat so I don't know anything about the doors. I tried looking on line but it seems like there are so many things it is over whelming. Some of the doors seemed too small for a cat to actually use? Do they seal really tight or would I lose a lot of heat out of it? Last year we had -20F wind chill temps here, so I need to get something pretty efficient for them.

I have a larger dog door and the cats use it easily. Mine is not insulated and located on a heated/cooled back porch. The flap is held closed by a magnetic strip in the bottom.

It always surprises me when I walk in from the sweltering heat to find how pleasant it is. The same is true in winter although we rarely see the lows that you have.

I would suggest you place it away from prevailing winds to prevent more heat loss.

Good luck with it and your kitties.

Are you going to be keeping the cats in the garage full time (other than when they go out through the kitty door) ?

Ditto to the one with magnetic strip - that my sister has seems to work well and cats use it readily.

If you have any concerns about other animals using the door (from feral cats to raccoons), there's a clever innovation available. Some doors have locks, and the key (magnetic, I think) is on the animal's collar. You'd need to use one of the breakaway type collars for their safety (in case the collar gets hung up on something). If they are staying outside or in the garage, you might need a way to keep other animals out of their food.

You might want to add a "nest" or two that they can use for additional warmth inside the garage... there are a lot of designs out there, but a cardboard box or two with an old blanket or something so they're not sleeping right on the concrete floor would probably do the trick.

All good suggestions about the kitty door - but I don't understand why someone would get 2 kittens and then keep them in a detached garage

Sandy, there are lots of reasons why people can't have animals inside. Being a wanted outside kitty is better than being an unwanted feral. Yes, indoor kitties generally have longer lives, but that doesn't mean all cats must be indoor pets, and it sounds like these two will be well taken care of. We keep our cats inside, but there's too much traffic in suburbia to do otherwise. I know this is a "hot button" topic.

I checked on Amazon to see if I could find you an example of the type of door I was talking about and found one that I thought was an even better use of technology... the door is keyed to the cat's microchip! Having the vet 'chip your kitties when they are spayed is easy, and a microchip means they can be ID'd and returned to you if they wander off. https://www.amazon.com/dp/B07DWTLGPG/

As for door size, the size needed is going to vary depending on the size of the cat, and you don't really have a way to know that yet, so I'd go with a larger one.

My sister did have a possum come in once

We had a kitty door for years; there were a LOT of instances where one cat or another would catch some little critter (squirrel, chipmunk, flying squirrel, bird, baby bunny, you name it), bring it back inside with them, and then lose interest in their still-alive catch. Most memorable was a chipmunk that spent a good part of one night running back and forth across the headboard of our bed and up and down the mini blinds on the windows . (I think that was when we decided to get rid of the kitty door.)

I've had many cats, my own were always indoor cats. But I also spent a lot of time on a farm with a lot of outdoor cats as well. There's just no way I see a kitty door ending up being a good thing, for sooooo many reasons. I had one of my indoor cats bring me a piece of chicken my idiot husband left out and drop it on my chest to wake me (while I was imagining what it was and could barely bring myself to open my eyes up slowly). I can only imagine the gifts a cat would be able to bring from the outdoors and see it ending up exactly as Weedwhacker described!

Not so much of a problem if the cat brings a gift inside the garage, at least. I did have a possum jump into bed with me when I was visiting a friend, and we decided he'd managed to come in through the dryer vent, which the cats used as a cat door. I thought the ID chip "key" was genius.

okay sorry guys, life has been happening and I've not visited for a few days.
I got the cats primarily to control rodents on my property and the neighbors property next door, we have moles, and voles very badly and I have spent hundreds of dollars and tried many wives tales and nothing has worked, so cats.

I am allergic to cats very much so. They cannot stay in the house long term.
Our detached garage is a nice building and is heated in the winter. Yes we will make them a specific spot to sleep-they will not have to lay on the concrete floor or anything like that.

The cats will be fed, vetted and cared for.
I have never put collars on cats, (we have cared for many strays over the years, as for whatever reason we seem to be a popular dumping ground I guess). I am afraid of cats getting hurt on collars.

The cats will be fixed, so no babies
I got a small cat door with the magnetic flap, that sounded like a good way to reduce energy waste and probably keep out most unwanted critters. I did read about the ones that scan your cats collar, but they had many bad reviews as people said they were glitchy, didn't work reliably, or quit altogther after a short time. They do rely on batteries, and many reviewers said the doors had a significant pause when reading the tag before opening and it makes a beeping sound and a noise when the latch disengages to open. These things caused their skittish cats to be scared of the door and they never could get their cats to use it. So I decided against that. One of the cats is very skittish, I have still not been able to handle her, although I don't think she is scared of me any longer, she just doesn't like being handled at this point. She is shy of any unknown noise or quick movements. But is coming out more every day to play. I am keeping them in the utility room/hallway/laundry rooms for the time being.

The cats will not be confined to the garage, they will be free to come and go at any time they please-thus the kitty door. I don't really care if there is an occasional gift brought in the garage, that won't be a big deal. I use the garage daily, so I'll be monitoring anything going on.

@critterologist that opossum sounds like he made himself at home with ease, I wonder if he made friends w your friends cat and he slept in that bed regularly!