4" diameter side nail mount fan and fixture box. Fiberglass construction with a flexible polyurethane flange to form an energy efficient vapor seal. Does not distort with high temperatures, shatter resistant with low temperatures. Can help stop warm and cool air leaks. 35 lb fan rated, 50 lb fixture rated. 1/2" offset. UL (Underwriter's Laboratories) listed. 4" W x 2-3/8" D.