The most popular, flexible air retarders (also called housewrap) are composed of polyolefins, which include polyethylene and polypropylene. These air retarder materials typically are fiber-reinforced as spunbonded, woven, or laminated products. Standard gypsum panels, cement board, no. 15 felt, industry building wrap, and other common construction materials also can serve as air barriers if installed properly (but not as vapor retarders)10 (Fig. 4.3).