Last year I installed the windoor to the front porch, which gives the cat's access to the enclosed front porch whenever they so choose. It works well for us, due to giving us easy control over when they have access and when they do not just by removing the unit from the window.
Theo loves hanging out on the enclosed back porch, so a solution was needed to allow the cats controlled access to the back porch without letting the cold air in. The answer is the Small Ruff-Weather Pet Door. It has two flexible flaps and an air space in the middle between the flaps. Each flap has magnets on the bottom, so they always seal up fairly well and do a great job at keeping the cold air from getting in. I installed it in the wall near the back door to the mudroom.
Nahum navigates the new door.
Theo squeezing through. The door is a little small for big Theo, but he manages.
Buddy, working his way through the door. You can see the first flap on his head
while he is about to go through the second flap.
This new cat door has been in and working now for a little while, and I am very impressed. It does a great job keeping the cold air outside. Installation was a bit of a challenge, but that may have been more due to installing it in an old house.