ASHRAE Journal - September 2011 - 73

is the single most important thing in this approach. Having said that, we also need some vapor control. In Climate Zones 2 through 4 use a liquid-applied vapor control layer yielding a Class III rating (something between 1 and 10 perms wet cup). In Climate Zone 5 continue to use the liquid, but go to Class II. If you are in Climate Zone 6 and higher, go to Class I and consider a foilbacked gypsum. Stay away from that sheet polyethylene junk because it is a nightmare to install. Stick with something that sticks to the gypsum board: either a coating on the inside, or a foil-facing stuck on the topside. In all of these assemblies, insulate the attic ceiling with lots of insulation. Start at R-30 in the South. Hit R-50 when you get to Climate Zone 5. And, go up from there for Climate Zones 7 and 8. We are not done yet with Figure 2. Ventilate this puppy. Use continuous soffit vents and lots of upper vents for the attic. When you get into high snow load areas, you can’t use ridge vents, which is why Pete invented gable vents and monitors and dormers and cupolas. What if I don’t want to do a classic vented attic? I can’t lie to you. It gets ugly. Let’s kick up the ugliness in stages. Let’s say you want a cathedralized roof assembly over your pool. OK, I can see that happening. Go with the approach in Figure 3, Page 68. 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 liquidapplied vapor open membranes (don’t forget to seal the joints in the plywood for more air control redundancy). Remember, that you absolutely have to over-roof one of these things with a vented roof. And, we have already beaten to death the roof-towall connection. They love these roofs in snow country, so don’t forget the cupolas to provide the upper roof vents that will not get blocked with all that snow. Next stage of ugliness. Flat roofs. Can’t we just say no? Please? Yeah,
5Controls membrane blisters from moisture vaporizing due to incident solar radiation. What are the odds that there might be moisture in this assembly? I mean, with this assembly being over a pool and all? Think of this as a roof assemblies’ version of concrete air entrainment.

right. OK, here we go. Check out Figure 4. 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.5 Unfortunately, also note the long screws that connect the upper membrane to the structure.

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September 2011

ASHRAE Journal

ASHRAE Journal - September 2011

Table of Contents for the Digital Edition of ASHRAE Journal - September 2011