Garter snakes aren't as sensitive to high temperatures as some types of geckos. House temperatures that go up into to the 80s on occasion will not harm garter snakes. A few years ago I kept Red-sided Garters and Easterns and had the house temperatures go into the 90s some of the time, will no harm to the snakes.

Here's an Eastern Garter I found while herping north-central Ohio the other day:

Third Eye