building sciences Roof Sheathing Minimum R-50 Rigid Insulation in Two or More layers With Horizontal and Vertical Joints Staggered Plywood Roof Sheathing Roofing Membrane (Vapor Permeable Liquid Applied or Roofing Felt) Vented Space Shingles Roofing Paper Air Control Layer/ Vapor Control Layer Wood Decking Timber Rafter or Exposed Joist Figure 3: sloped cathedralized Vented compact Roof. The roof cladding is still ventilated. Keep your structure to the inside of the air control and vapor control layers. The intermediate cladding is a layer of plywood, which is vapor semipermeable and it is covered with a vapor permeable roofing membrane (aka “tar paper”) or one of those new fancy-dancy liquid-applied vapor open membranes (don’t forget to seal the joints in the plywood for more air control redundancy). Fully Adhered Roofing Membrane Coverboard and Hygric Buffer Rigid Insulation (Minimum Two Layers; Joints Offset) Screw Attachment Gypsum Sheathing (Paperless) Fully Adhered Air Control Layer/Vapor Control Layer Screw Attachment Metal Deck Figure 4: Flat Roof. Note the fully adhered air control layer and vapor control layer on the paperless gypsum sheathing. Note the multiple layers of rigid insulation with the joints staggered. Note the fully adhered roofing membrane on the fiberboard hygric buffer. Unfortunately, also note the long screws that connect the upper membrane to the structure. the thermal control layer. The dotted line represents a ventilated cladding. The roof is a vented attic. Classic, simple stuff. Any bad stuff from inside that gets past the bold line and gets into the shaded stuff can pass right on through and get to the outside through the dotted line and the big sloped hat with holes. You don’t need a computer simulation to understand this. You don’t even need a psychrometric chart. A few little tricks of the trade—think of cladding as a dehumidifier that sucks moisture out of the shaded stuff. That means the back of the cladding will get wet if there is a flaw in September 2011 www.info.hotims.com/37990-36 68 ASHRAE Journal