Advice from people who have outside dogs?

I have a 2 year old male black lab and a german shepherd mix 5 month old puppy female. They both live outside and each have their own dog. Their houses are those great big wooden ones with shingles and they're lined in cedar chips and have a carpet square as a home made doggie door (hey! it works!) But winter's hitting hard this year, its been getting down to the 20's at night and next week supposed to get in the low teens! Is this enough protection? I know they're dogs/wild animals and can endure but geez! We live on 250 acres and they have full run but always stick around the house and use their houses. Should I go buy a crate and bring them in at night? tips?