I have no heating and one shed is insulated the other not.I don't find any difference in the condition of the animals in the two sheds.The non insulated is colder but the insulated is damper.I have a few that look a bit out of condition ,but nothing more.I never clean the nest area in cold weather, its fatal.I have bought a full bale of horse hay instead of the usual petshop stuff and a full bale of kennel bedding which is the same as the soft tissue hamster bedding but for dogs.I have stuffed the cages with bedding.I think the feeding is probably crucial.I'm feeding wild bird mainly but have also bought a sack of go cat and some plain mixer biscuits.It's cost a lot alas.In mild conditions I feed my mice every other day but in these extremes its not possible.I find rats don't cope so well,mine are all in the house now and will not be going out again,they've moved in for good.I wonder if it would be worth feeding the ones whose water bottles freeze some moist food instead Ian.I have given up with my ferret water bottles,I just make sure they get plenty of moist food and dry only once a week.Im going to check out Kellys cream cracker idea as well.