So let's say your attic has 200 square feet over which the ceiling is at least 5 feet high. Your window area must be at least 8% of 200 square feet (16 square feet). Meanwhile, the 4% openable area requirement means you need 8 square feet (4% of 200, that is) providing access to the outside. http://www.epdmcoatings.com/liquid-rubber.html