My self caught garters are all from wet meadows and ditches.
They spend quite a lot of time in their water dishes or damp hides (plastic box filled w sphagnum moss.)
I am coming to believe that the smaller snakes need a damp, high humidity area as well as dry warm and cool areas.
I believe larger snakes tolerate low humidity, but all would be happier with a higher humidity than found in an average home.