BUILDING SCIENCES Figure 4: The “Detroit Wall.” In this assembly, the water control layer and air control layer function of the fully adhered membrane or liquid applied layer over the exterior gypsum board in Figure 3 is replaced with a taped foil-faced polyisocyanurate or extruded polystyrene. The rock wool and cladding attachment stays the same. Same note as before about wood furring being used. So why do I want an air gap between the exterior cladding and the continuous rigid insulation? I can see needing it to control ice dams on roofs, but for walls? Ah, we have to deal with rain and hydrostatic pressure. And, yes, we were also here before in “Hockey Pucks & Hydrostatic Pressure” (ASHRAE Journal, January 2012). It also makes it easy to attach the cladding over the rigid insulation. This attachment thing can get complicated when we are retrofitting masonry walls on the exterior. But have no fear, there is a solution to this as well. Figure 5 uses wood 2 × 4s on the flat screwed into the masonry to provide a structural transition for 66 ASHRAE Journal Figure 5: CMU Wall. Wood 2 × 4s on the flat are screwed into the masonry to provide a structural transition for the cladding attachment. A liquid applied water and air control layer is applied directly to the masonry prior to the installation of the 2 × 4s. Rigid insulation is installed between the 2 × 4 furring and then a continuous layer of rock wool is installed. The exterior fiber cement cladding is installed as before. the cladding attachment. A liquid-applied water and air control layer is applied directly to the masonry prior to the installation www.info.hotims.com/44632-31 May 2013