ASHRAE Journal - June 2010 - 52

Photo 6 (top): Amazing. Rube Goldberg would be proud. I won’t say in which state this technology was attempted, because I live there. Figure 4 (bottom): Conditioned Attic. Putting the ducts inside by moving the thermal boundary and pressure boundary to the roof deck. This can be done by insulating on the top of the deck or applying spray foam on the underside of the deck.

Figure 5: Compact Roof. Roof deck insulated on the topside. Note the air barrier membrane. If you don’t do this, the roof is doomed.You have been warned. A fully adhered membrane is recommended. Further, note that there is not a vented space under the roof cladding to compensate for the thermal resistance of the snow layer. This roof should be used where the ground snow load is less than 50 lb/ft2 (244 kg/m2) and should have a thermal resistance greater than R-50. With ground snow loads greater than 50 lb/ft2 (244 kg/m2), compact roofs should have a ventilation space under the roof cladding such as in Figure 8.

facturer’s installation instructions. For ice dam control, more ventilation is better. Photo 6 is not the answer. A minimum 2 in. (51 mm) air gap is necessary between the underside of the roof deck and the top of the insulation layer in ice dam regions—not the typical code specified 1 in. (25 mm). An ice dam region is anywhere the ground snow load exceeds 30 lb/ft2 (146 kg/m2). Where does this come from? Old guys. More about them later. So, what do we do about ductwork and heating systems? Put them inside. Or, move the inside so that they become inside (Figure 4). Note that this approach of moving the thermal barrier to the roof deck is not easy. You are now creating an unvented roof, which is OK if you get it right. The easiest way to do it right is insulate on the top of the roof deck (Figure 5). It can also be done with spray foam from underneath, but that is a topic for another day. So how do we handle ice dams with compact roof assemblies like that described in Figure 5? Don’t we need ventilation? Sometimes yes. Sometimes no. To figure this out we need to study the old masters once again,‡,§ and appreciate some basic physics. In theory, you can install enough insulation in a roof assembly so that the roof deck pretty much stays cold (close to the temperature of the outside) and sometimes even colder due to something called night sky radiation. So lots of insulation and an airtight roof and we are done, right? Well, not quite. The problem is the snow. It’s on the top of the roof deck, remember?

Figure 6: Snow Insulation. A plot of thermal resistance of snow versus density courtesy of the Cold Regions Research and Engineering Laboratory (CRREL) of the U.S. Army Corps of Engineers.

You will be shocked to know that snow has an R-value and a pretty good one at that.** It’s about R-1 per inch, depending on density (Figure 6). This is a pretty big deal. Think of snow on a roof as an insulating blanket that elevates the temperature of the roof deck.

‡ Wayne Tobiasson is another one of my unsung building science heroes. He has retired, finally, from the Cold Regions Research and Engineering Laboratory

(CRREL) of the U.S. Army Corps of Engineers. Wayne knows ice dams. A nice paper to get you started if you want to know more is “Guidelines for Ventilating Attics and Cathedral Ceilings to Avoid Icings at Their Eaves” by W. Tobiasson, et al., in Thermal Performance of the Exterior of Buildings VIII/Roof Design—Practice http:// tinyurl.com/ventilatingattics. § There are these two other guys who also know stuff. Not so old, but masters nevertheless. Check out “Venting of Attics & Cathedral Ceilings” by Bill Rose and Anton TenWolde. Published in the ASHRAE Journal, no less, October 2002. Not a column, a real paper, reviewed, the whole nine yards. This one is a gem just for the references cited. The words are real good, too. ** Canadians know this because we all live in igloos when we are not winning hockey gold medals.

ASHRAE Journal

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June 2010

ASHRAE Journal - June 2010

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