building sciences Photo 3 (top): Membrane Shrinkage. Notice the roofing membrane being pulled away from the cant strip. Photo 4 (bottom): Tear in Membrane. Membrane shrinkage results in the membrane pulling apart. of rigid thermal insulation on the top of this air control layer—in two layers at least with the joints off-set horizontally and vertically.§ This insulation should be screwed down to the metal deck. Then on top of the rigid thermal insulation there should be a coverboard. This coverboard is also screwed down to the metal deck. Finally, a roof membrane is fully adhered to the coverboard. The function of the coverboard is twofold. First, it is a hygric buffer that reduces roof membrane blistering. A discussion §To more fully appreciate the need to offset the rigid insulation joints horizontally and vertically and to wrap the perimeter of the roof assembly insulation check out ASHRAE Journal, August 2009, “Complex 3-D Airflow Networks” or http://tinyurl.com/34wcqwz. Figure 12a (top): Steel Stud Parapet “Old Timer.” Wood blocking and a cant anchored to the structural deck restrain membrane shrinkage at parapet. Notice the continuity of the control layers. Figure 12b (bottom): Steel Stud Parapet “New Pups.” Large backer rod supporting a bunch of extra membrane that lets things move when they have to move. The “Zen” approach to membrane movement. Use a more dimensionally stable membrane and then let things move when they have to. Again, notice the continuity of the control layers. ashrae.org February 2011 56 ASHRAE Journal