ASHRAE Journal - August 2009 - 64

Photo 1 (left): World’s largest Canadian beer cooler is located in Westford, Mass. Barn retrofit with 8 in. (203 mm) of rigid foam on the exterior walls and 10 in. (254 mm) of rigid foam in one layer on the roof. Notice that the rigid foam on the roof is in one layer. Because it is in one layer, it leads to convection. Photo 2 (right): Professional embarrassment. Convective cells evident in frost melt pattern on shingles. Complex 3-D Airflow Networks By Joseph W. Lstiburek, Ph.D., P.Eng., Fellow ASHRAE L ittle things can be big things. You build structures that seem like they will work, and then the real world intrudes and reminds you that you are not as smart as you think. You realize that the fine points turn out to be vital. It’s simple to build a compact unvented cathedral ceiling. Take a flat unvented compact roof and slope it (Figure 1). No big deal, right? Not quite. Sloping roofs can result in convection cells, and worse. It turns out to be important to have multiple layers of rigid insulation with the joints offset horizontally and vertically (staggered) and to have an air barrier directly on the structural deck (Figure 2). Ask me how I know?* Check out what this crazy person did: 8 in. (200 mm) of rigid foam on the exterior walls and 10 in. (254 mm) of rigid foam in one layer on the roof (Photo 1).† The key thing to note is that the rigid foam on the roof is in one layer. Because it is in one layer, it leads to convection (Photo 2).‡ The convection loop in this example is not catastrophic because it is not coupled to the interior space. Note the air barrier sheet membrane under the rigid insulation. If the air barrier had not been present, the convection cells would have been coupled to the interior, and there could have been moisture transport * † from the interior into the roof assembly. With moisture transport from the interior, the roof likely would have been doomed as it is located in a cold climate. As it is, the convection only lead to professional embarrassment and an energy penalty. It turned out that the professional embarrassment was much worse than the energy penalty. Let’s add a bit more complexity, and enjoy the outcome. Structural insulated panels (SIPs) are an elegant enclosure system: a foam core with exterior and interior linings of oriented strand board (OSB). The panels slip together to make walls and roofs and sometimes even floors and foundations. Easy, fast, no thermal bridges, it can’t get better than that. Let’s go to Juneau, Alaska, and check a bunch of SIP roof assemblies (Photo 3). The first sign that something was amiss were mushrooms growing out of the shingles. I call that the Wolfgang Puck effect (Photo 4). Looking a little deeper (pun intended), we find moisture damage along the panel joints and the ridge line (Photo 5). The pattern of damage is distinctive and can be found in many cold climate SIP roof failures. It is called “ridge rot” (Figure 3) and can easily be avoided by dealing with panel joints. It is a classic case of three-dimensional airflow networks and convection cells. I have good judgment, and good judgment comes from experience, and experience comes from bad judgment. I have lots of experience. This is the world’s largest Canadian beer cooler under construction. It happens to be in Westford, Mass., and for years was my office before it became a bar. OK, that’s not quite true. It has a bar inside one part of it. The rest of it is where building scientists hang out when they are not causing trouble. Note to the Standard 90.1 committee: see all that foam on the outside, which is something that you folks are saying can’t be done because it is not structurally possible, practical or safe? Well, there it is. The cladding is 15 years old and does not appear to be falling off the building. ‡ When I first saw this after the first frost and I was standing on the sidewalk looking at my roof mumbling to myself that I am never, ever going to tell people about just how stupid I am, this old guy walking his dog stops and looks up at the roof and then says to me: “You know that if you had put up that rigid insulation in two layers with the joints offset horizontally and vertically, you would not have gotten those three-dimensional airflow networks.” I looked at him dumbfounded and asked, rather humbled, who he was. He said: “I’m your neighbor, and I used to be a roofer.” Old guys know stuff. They can be a pain, but that’s because they know stuff. 64 ASHRAE Journal ashrae.org August 2009

ASHRAE Journal - August 2009

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