Mapping out the roof assembly for a new house in Climate Zone 6B, Steve Mackay has settled on long I-joists insulated with a mix of closed-cell spray polyurethane foam and blown-in fiberglass. He doesn’t plan on venting the roof, and he wants to be certain his design will be problem free.
Here’s what it looks like: 14-inch I-joists with 5 inches of closed-cell foam sprayed on the underside of the roof desk. The balance of the insulation will be fiberglass (Mackay wants to use the BIBS system) with no interior vapor barrier. In all, he gets R-67 in the roof cavity, with R34 coming from the foam, and R-33 from the fiberglass.
Mackay has read an article on the topic by GBA’s Martin Holladay, and he’s confident the assembly will work. With an inside design temperature of 68°F and an outside design temperature of 22°F (an average of the three coldest months of the year) and a relative humidity of 35%, the inside surface of the spray foam will be about 45°F. That’s well above the dew point of the air and safe from the threat of condensation.
“I’m very confident this will work well,” he writes in a Q&A post. “However, the ccSPF [closed-cell spray polyurethane foam] is expensive and the insulation contractor suggested using 2 inches of ccSPF under the roof deck and then using ocSPF [open-cell spray polyurethane foam] to get the R-value I’m looking for.”
The assembly would be cheaper than Mackay’s original plan. But does this change his calculations on where the first condensing surface is? And with the change, does the risk of condensation inside the assembly go up?
That’s where we start this Q&A Spotlight.
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