I have a pet door that my 3 little girls (doggies) go out during the day and night. The problem is that during the winter I am losing heat but hate to close off the door. If I do, I know they will soil the floor or carpet in the house. I have been thinking that if I make some sort of box that would butt up against the pet door (a kind of tunnel) it might keep the cold air from escaping outside as they go in and out.
If the pet door is fit pretty loosely you could snug it up with weather stripping. Also, it sounds like your critters are house trained, so I expect if you sealed the door up they would ask to be let out when they need to go. Housebroken dogs don't like to go in the house any more than we like them to. Of course with three of them, you may lose as much heat opening the door for them as you're losing with the doggie door.
We had the same problem and did the box thing on the outside. We built it so they go into the tunnel and out on the opposite end to make sort of an S. We put a carpet flap on the outside entrance door. In the inside we also put a plastic storage box in front of the dog door and store their dogfood in it. The whole thing works pretty well.
Heat only rises, it never sinks.
You can get flexible ducting (like oversize dryer hose) big enough for the pets, or make a chute out of wood, make a hole in the wall with it's lowest point just a bit higher than the diameter of the ducting. Place the ducting from the floor up to the hole and down again on the outside. Heat gets trapped at the highest point and since it won't go down on the outside, it forms an air lock.
If the ground on the outside is lower than the inside floor, you can have the hole at floor level and the duct just going down to the outside ground from there.
If you have an upstairs and open windows up there, then of course the air lock trick does not work. In that case you need to hang a carpet or horse blanket into the tunnel from one end to the other, attached at the ends so that it lays loosely on the floor of the tunnel. Pulling a treat or toy on a string through the tunnel will teach the pets how to burrow under the blanket or carpet to get to the other side. Most pets are adventurous and smart enough that they catch on instantly.
my daughter solved her problem by placing a stiff carpet piece on the outside and then putting one on the inside of the doggie door also/her doggies lift up the carpet with their heads and go thru,then the one on the outside falls back into place when their out.
Installing a cat door to the garage allows the litter boxes to be kept out there cutting down on the litter scatter in your house. This is a page about installing a cat door.