Cats may have nice, thick coats of fur, but that does not mean you can leave them in a non-insulated outdoor cat enclosure during the winter. Cats require warmth and can easily get hypothermia if their body temperature drops too low. The average body temperature of a cat is 38.9 degrees C. To maintain that temperature, cats are naturally drawn to warm places. Maintain an environment within the outdoor enclosure of your cat shelter that meets the need of your feline friends.
Cover much of the walls, but still leave a gap for the cats to look out. Use plywood, vinyl, outdoor blankets or foam boards.
Line the floor with carpeting, which will provide insulation and comfort.
Place heated pet beds or pads in those places where the cats like to sleep. Use electrical heated pet beds that are water-resistant and have chew-proof cords. Non-electrical, thermo-reflective beds are also available.
Add brooder lamps. Use one 250-watt brooder lamp for every 8 square feet of space. Hang or clamp the lamps in places where the cats cannot reach them, as cats like to snuggle near warm things and could burn themselves on the bulbs.