After achieving a good seal, however, insulation is important. A single-walled flap is not satisfactory for the same reason that single-pane glass is not: It just doesn't insulate very well. While single-pane glass is completely replaced these days by dual-pane glass, most pet doors sold on the market today provide no more than a ¹/₈" thickness of vinyl between the living area of the home and the outside weather. 

The Endura Flap consists of a sealed unit that is slightly thicker than a dual-pane glass unit and provides comparable insulation qualities.

The Endura Flap is divided by horizontal barriers which serve to reduce the effect of these air currents.

A trade Off - We could have filled the flap interior with insulation for better insulation performance. We didn't for the same reason that dual pane windows aren't filled with insulation:  It's better if you can see through.