A slatted rack will do nothing for ventilation in the winter. I do use a an un-slatted shim under my lowest box, year-round.
Ventilation in the winter is all about a) preventing the (relatively) warm, moisture- laden air rising up from the bees' cluster within the hive from making contact with a surface above that is much colder resulting in condensation raining back down on the bees.
Quilt boxes work superbly for this purpose. Warmer moist air passes up through the fabric floor in the QB, through the shavings and OUT of the hive, without condensing. Problem solved. That QBs also may provide some modest thermal protection is a secondary effect.
I insulate, all around my hives (2" between adjacent hives and four inches on exposed surfaces.) I use a QB. I put some insulation under my lid, but mostly to keep it raised a bit so the ventilation holes above the QB are unimpeded in anyway (See QB above. for reason I need that ventilation.)
I also do some more extreme insulation, which is probably overkill even for my cold climate, so I'm not promoting that. But 2-4" of foam insulation is not something that will in any way impede "ventilation" because it isn't tightly sealed to the boxes, like you might caulk foam panels in house construction. It just operates as a radiant barrier.
The cost of insulating a hive is minimal considering what the cost of replacement bees is. The panels take some trouble to craft the first year, but can be re-used indefintely, and once they are cut into the right sizes to cover hive walls they are not diffcult to store during the off season. Mice don't hurt them, mildew doesn't faze them etc., so they can spend the summer in your attic, barn, shed, basement, etc.
I do four things to make sure my bees winter well: year-round mite control; more-than-enough winter food (supplemented as needed); quilt boxes and insulation. I must be doing something right because I have never lost a wintering colony.
Enj.