ASHRAE Journal - June 2011 - 70

building sciences
1.5 in. Concrete Slab Surfacing 0.5 in. Sand Layer 4 in. Hydrophobic Ash Layer Asphalt and Felt Waterproof Membrane Sloped Cast-in-Place Concrete Gravel Filter Surface Drain Concrete Slab Surfacing Sand Layer Hydrophobic Fly Ash Sand and Fly Ash Mixture Asphalt and Felt Membrane

Figure 1 (left): Soviet-Era Protected Membrane Roof (PMR). Pretty impressive technology for the time. Figure 2 (right): More Soviet Era Stuff. Great roof drain. Could have also been the design for a shower drain. For real, not kidding. any other option, and they don’t save energy compared to an otherwise well-insulated roof. They should not be a code or bylaw requirement, and you shouldn’t be encouraged to use them except for architectural reasons that are, in the truest sense of the word, “architectural.”7 What do we know about building them? A great deal. The earliest ones go back to the old Soviet Union in the 1940s (Figure 1 and Figure 2). Think of them as PMRs or IRMAs with dirt and grass ballast. We sure know about PMR and IRMA assemblies, and we sure know about green roofs. For the newbies, PMR stands for “protected membrane roof ” and IRMA stands for “inverted roof membrane assembly.” In these roofs the water control layer is (not always! fully adhered, alas) directly on the structural deck, and the thermal control layer and ballast are located above (Figure 3). One of the great features of these systems is that the water control layer is also the assembly air control layer and vapor control layer, aka, the “Perfect Roof ” (see ASHRAE Journal, May 2007 “The Perfect Wall”). The biggest benefit of the PMR is that the membrane is, well, protected. Protected from the things that destroy most membranes: heat, cold, ultraviolet radiation and mechanical damage from above. By keeping a “fully adhered” (yup, this “fully adhered” thing is a big deal) quality membrane below the insulation, it will never see temperatures much different than room temperature for its entire life, and will never be exposed to sunlight. It does not take a Ph.D. (“professional hairdresser”) to figure out this means that the membrane will last a very long time. In green roofs, mechanical damage happens easier because of folks working on the roof (aka “gardening”), and the costs of getting at a leak are much more expensive than other roofs, so protecting the membrane should be a no-brainer. Therefore, the PMR is tailor-made for green roofs. Even the Feds have figured this out; the General Services Administration requires PMRs for all vegetative roofs—and the brain trust at City Hall in my hometown.8 In taking apart PMR or IRMA (including terrace and green roofs) that are 10 and 20 years old and older, we have learned that the insulation layer can get saturated if it is not drained
Ballast (Rock, Pavers, Earth) Filter Fabric Extruded Polystyrene Insulation

Sloped Concrete Topping; Slope Minimum 2% to Drains

Concrete Structural Deck Drainage Gap, i.e., Drainage Mat or Grooved Insulation Fully Adhered Roof Membrane

Figure 3: PMR and IRMA. Protected membrane roofs and inverted roof membrane assemblies date back in North America to before I was born—a time when the Toronto Maple Leafs were winning Stanley Cups. Note that drainage below the extruded polystyrene (XPS) layer is necessary to keep it from becoming saturated over time. above and below. This approach was not common practice in the early days; it should be standard practice today. For drainage, all that is needed is a small gap, 1/8 in. (3 mm) or so, provided by drainage layer materials. A good slope of at least ¼ in. (6 mm) in 12 in. (305 mm) is important so that the water can run away. And the membrane should be protected even as it turns up at parapets, curbs, and penthouses to ensure that the membrane remains durable everywhere. There is one, and only one, insulation that can be used in this type of assembly: extruded polystyrene. Use a thick layer or a series of thick layers—as thick as you can get to reduce water uptake. Surface area results in water uptake. Less surface area, less uptake. For example, one 4 in. (102 mm) layer of rigid insulation has less uptake than two layers of 2 in. (50 mm) rigid insulation. All of the other insulation types get too wet and hold too much water.

7. Oh boy, an engineer commenting on architecture. Well, to paraphrase Georges Clemenceau “architecture is too important to be left to architects.” 8. Check out the GSA’s P100 Facilities Standards that establish design standards and criteria for new buildings. And guess what, the “Toronto Green Roof Construction Standard” is fabulous if you actually want to know how to build them. I shouldn’t be surprised. My old friend Hitesh Doshi from Ryerson University chaired the Technical Advisory Group. Well done, Hitesh. Just don’t forget about the drainage above and below the rigid insulation. Once again I mention it, and in a footnote, so I am really serious about this.

ASHRAE Journal

ashrae.org

June 2011

ASHRAE Journal - June 2011

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