That depends on the flap in the pet door you buy.

The examples to the right show a very popular 'large' size flap that has been used by two dogs--one about 75 lbs and one about 60--for a period of around 3 years.

The edges of the flap are severely 'potato chipping' and, consequently, let in large amounts of outside air. Here in San Luis Obispo, California, that doesn't matter too much and replacing the flap hasn't been a high priority. (Obviously, cleaning the flap hasn't been either!).

What if you happen to be in a less climate-friendly area?

Some manufacturers include fuzzy weather-strip down the sides of the frame which is an improvement. Unfortunately, the fuzz still passes air freely.

Some will also include an occasional magnet on the side which helps also.

However,  our new 'Endura Flap (pat. pending) pet doors are very best solution to this problem:

•   Continuous magnetic contact top to bottom.
   
•  Unique 'bellows' on both flap edges permit temperature change expansion and contraction without breaking the seal.
   
•  Pivot rod allows the flap to swing without flexing so the flap material doesn't flex and fail.