Depending on how much you want to spend you can get doors that are simple and straight forward, or ones that open and lock with an electroni "KEY" on your pets collar - this keeps out any unwanted critters. Make certain you dont pt it where someone can reach through it to pen a door knob. We had one when we lived in Tahoe where it can snow 8 feet in 4 hours, and it did not increase the electricity bill. You can buy them with an extra wood frame around it so you can "lock" out anything including any cold by putting a board make to fit into the U shape, the board overlaps the door itself. I suggest you do research on line and check out ratings of et doors to see which will be the best fit for you, Good Luck

I had one with a hard panel that was "lockable" with a lever that let the cat go just in, just out, both, or neither. I thought the kitties would get stuck with the hard flap but they were fine.

The soft models have an insert you slide in to prevent use. We had one of those once and had to start using it when a raccoon began using it. We were in Michigan and I think we saved energy with it even though we didn't get a high end insulated model. If you have to hold the door open for someone to decide if they really want out, you know what I mean.

It's easy to install if it's going in a door and you know how to cut a hole in a door. The hole is the only hard part.

My cat caught on to the cat door and loved it! So did the raccoons --> found 3 of them sleeping on the living room couch one morning. Locked the cat door.

Yes, everything said above. All the doors I've seen have some kind of "lock" feature. If you can arrange it so that you have a two door system--say a door into the garage, then a door into the house, you will save on heat loss and more importantly, keep out the "bad kitties" and raccoons. You'd be surprised how fast other cats figure out how to get into your house and get to the cat food (keep the bowls faaaar away from the cat door). Otherwise I just love cat doors!

I took out my basement window, brought it along with a pet door to the glass/window repair shop. They replaced the glass with a plexiglass material and installed the door in that for me. The cats jump up on a couple of bins to reach it. The door leads underneath a deck (which I screened in) and then to a kennel. Even with the barriers I did get a squirrel in the house once. Other than him, the cats themselves have carried in a few little critters (moles, chipmunk and a mouse) not always dead! My door locks in various ways (you can lock things from coming in, going out, or both). I've used all the options at various times depending on the situation. I think it's better to have 2 doors (like one to go into the garage and then another to go outside from the garage) if you have that option to keep the wild animals from taking up shop in your house. It's easier to get rid of them from the garage. If it has to go directly to the outdoors, I would put some type of tunnel on the outside to try to deter incoming critters a little better.

If you live in an area with cold winters, a door with 2 flaps will make a world of difference in preventing heat loss. However, some of my cats never figured out how to use the double doors --- they seemed afraid of getting caught in the small area between the doors. No problem with the dogs, though.