I assume you are in a safe neighborhood because any pet door big enough for a Lab would be big enough for a burglar. When we first moved into our current house with my old black Lab, Jake, the back door had a pet door. Jake being an inside couch potato only used it to come in, never to go out. I would put him out and before I could sit back down he would be back up on the couch. For security reasons we installed a new door.
From my past experience I would avoid any screen door unless your dog is exceptionally, and I do mean exceptionally, well trained. Any screen I have ever had could not hold up against a dog that wants to be let in. ;D
We put a pet door in our patio screen door. The screen is a bit too flimsy to support the pet door so the screen will likely have to be replaced every year or two. Also the pet door itself is quite flimsy and I'm not sure if it will last longer than the screen. I purchased it at one of the large home hardware stores for approx $35 after our pup went through the screen door - unfortunately it wasn't open at the time.
Another word of warning about pet doors: The dog can bring in undesirable things and take out desirable things. My brother has a pet door and their lab mix is always taking out everything from blankets, clothes, pet beds, etc.
While a person can fit thru a Lab sized door, I think most individuals would look at the size of the opening...assume a big dog lives there and pass the house.
Due to our floorplan (and not wanting to cut a hole in my block wall), my dogs have to go thru 2 doggy doors to get outside. One door is in the laundry door to the garage and the 2nd is from the garage to the outside. Both are solid wood doors. They were very simple to install.
My inlaws bought a cabin that had doggy doors already installed for a beagle. The doors are on the main door and the screen door so the dog actually passes thru both doors at the same time when the screen door is closed. As I said, these are beagle sized doors but Murray and Essy have no problem using them. They have to duck a little to go thru but they make it. Now I wouldn't recommend purchasing a Beagle size door for a Lab if it was the door the dog had to use on a daily basis but this size works fine for the week we are at the cabin and they have yet to tear the screen door off the hinges.
I've installed pet doors in 2 previous home for a previous Lab but never in a door.
The first one I installed between the studs in the wall between the kitchen and garage (right next to the people door to the garage). When Bess wanted out, she'd leave the kitchen through her door into the garage. Once there, she had a 10 foot long path to the outside wall of the garage, made a right angle turn and went another 10 feet to a pet door mounted (between the studs) to the outside. On this 20 foot path, I laid burlap bags and washable cotton throw rugs. These trapped or absorbed most of the mud or sand, etc., before she entered the house. Periodically I'd wash these but even more frequently just take them out and shake them. I'd NEVER use a single doggie door to the outside -- way too much dirt comes right in.
That worked so well that in the next house (built into the side of a hill) where the living space was above 2 garages, separated by a front door and hallway, Bess had to go through 4 DDs to make a trip outside.
Puff [YF, AKC field line (from competing HT/FT breeder) 62 lbs, dob: 8-'01]
In these parts Doggie doors "invite" all sort of undesirables to come in, rats, cats skunks & racoons. One of my favorite stories is my next door neighbor's adventure with a possum that entered her doggie door and "visited" her visiting mother-in-law at 3AM.
Apparently it was a real 3 ring circus! From next door it sounded like a riot.
We have a pet door that fits perfectly in our screen door. It also comes with an infrared key that Frodo wears on his collar to unlock it. That way the cat can't get out and no other animal can get in. It's made by Staywell. We love it.