I did the same thing when we bought our current home in our detached garage. I live in No Utah, so it is also very cold here too. I built a room inside of the garage (4 new walls inside of the existing garage walls). I put down styrofoam on the cement floor and covered it with soundboard and then put lanoleum tiles over that. The roof of the herp room is very well insulated, as are the walls, and the room holds heat too well. I have to use an air conditioner (mounted in the wall to an external air source) almost year round because the heat from the cage and rack heating heat up the room very well. I used an external door with weather stripping and have a deadbolt and a normal lock on the door. The garage gets pretty cold in the winter, (half of the garage is used for the mice and rats) but the room stays nice and toasty. It has been the best thing for me and my wife, and now there are no more complaints on the reptiles and rodents stinking up the house.
Good luck,
Justin

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