ASHRAE Journal - March 2011 - 68

building sciences

Figure 4: Insulating the Bottom of Decks Wrong. The water management is awesome. Note the continuity of the water control layer of the wall, connecting to the deck, connecting to the wall again. Also note the drainage gap in the wall behind the brick, the drainage gap on the deck under the tile, and the drainage gap behind the stucco. Note the base of the brick veneer is elevated and drains over the top of the tile. So what is the problem? Ah, the insulation. It is not on the top of the deck; it is underneath the deck. And it’s the wrong kind; it is air permeable. If it is underneath the deck it cannot be air permeable.

Photo 10 (top): Condensation and Decay are Bad. Air permeable insulation under deck can lead to condensation and decay. You would never construct an unvented roof this way. So why would you ever construct a deck this way? Photo 11 (bottom): Air Impermeable Insulation. Spray polyurethane foam is air impermeable. The low density stuff works in the South. The high density stuff works everywhere. Check out Figure 5. The only thing that changes is the type of insulation used under the deck. With spray polyurethane foam the problem of condensation goes away. But, which type of spray foam? The open cell or the closed cell? The closed cell works pretty much everywhere. The open cell only in the South. So what is the definition of the South? Go to the International Residential Code (IRC) for their definition of South. South means Climate Zones 4 and lower. It has nothing to do with the Civil War. At least I don’t think it does. So where are we with decks and balconies at this point? We drain and drain again, provide drainage gaps, slope to drains and add insulation on the top. If we can’t add insulation on the top, we insulate from underneath. When we insulate from underneath, we follow the International Residential Code (IRC) provisions for unvented roofs. Open cell, closed cell, vapor retarders, air impermeable. It is all there. After all, a deck is pretty much an unvented roof that you walk on. I already said that, didn’t I? Joseph W. Lstiburek, Ph.D., P .Eng., is a principal of Building Science Corporation in Somerville, Mass. Visit www.buildingscience.com.

Figure 5: Insulating the Bottom of Decks Right. The only thing that changes from Figure 4 is the type of insulation used under the deck. It is an air impermeable insulation—spray polyurethane foam. With spray polyurethane foam the problem of condensation goes away. But which type of spray foam? The open cell or the closed cell? The closed cell works pretty much everywhere. The open cell only works in the South. good. Condensation is bad. Decay is bad. You cannot use fiberglass or cellulose insulation here. You have to go the air impermeable insulation route. You have to go to spray foam (Photo 11).

**OK, this deck drains over the outside edge. But look at the nice geometry of the drip edge. Of course, with this detail we need “good mud.”

ASHRAE Journal

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March 2011

ASHRAE Journal - March 2011

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