I agree with bumping the temp up a few degrees while y'all are away... the exact temp is relative (80 in Michigan really is different from 80 in South Louisiana for instance
)
I also wouldn't allow them use of a doggy door while no-one is home. While I don't always go outside with mine, I do watch them from our picture window with a full view of our fenced in back yard. I can't imagine allowing them access to outside with no adult supervision