The Ideal "Ultra-Flex" Pet Door
incorporates a three-part flap consisting of very strong polycarbonate blocks with flexible hinges between them. The polycarbonate blocks are hollow so they improve the insulation value. There is plastic weather strip around the flap to provide for a good seal.
The frame is ABS plastic which has the advantage of not conducting heat in very cold climates and not corroding in salt air near the ocean.
Ideal has released an upgraded version of this pet door--the Premium "Draft Stopper"-- that is identical except that the frame is white painted metal and the flap includes 8 magnets for increased resistance to wind.