Over 30 years ago, when builders first began installing rigid foam wall sheathing, they had to figure out their own methods of fastening flanged windows. In 1982, when I sheathed my house with 1-inch-thick EPS, I installed “picture frames” of 1-inch lumber around each window rough opening. As it turned out, Joe Lstiburek was also building a foam-sheathed house in 1982, but he used a different approach.
“Back then, we weren’t using OSB or plywood sheathing — just XPS foam sheathing, usually Dow blue board, with diagonal metal strapping for bracing,” Lstiburek told me recently. “At first, we would run a horizontal 1-inch thick board under the bottom flange of the window, to help support the weight of the window. Under the other three flanges, there was nothing but foam. We attached the windows to the studs with screws through the foam. We built thousands of houses this way, using foam up to 1½ inch thick. We never experienced any problems.”
Eventually, Lstiburek realized he didn’t need to install any wood under the lower flange. “In the late 1980s, I took that practice with me to the U.S.,” said Lstiburek. “Then we stopped installing the horizontal board underneath — except for wide windows. If the window was wider than 4 feet, we’d still install a board under the bottom flange. But most of the windows we installed were basically hung from the flange fasteners.”
What about thicker foam?
I asked Lstiburek how he attaches flanged windows on walls with very thick foam. “I don’t have enough of a track record with 2-inch foam, so when we go above 1½-inch-thick foam, I recommend using side straps,” Lstiburek told me. (Side straps are also called masonry clips.) “You attach the windows at the sides of the rough opening with the straps. The side straps work…