In this recent Q&A post, reader Carsonb says that the rafter bays of his new timber-framed home have recently been sprayed with 3-1/2 in. of closed-cell spray foam insulation as part of a flash-and-batt strategy. The unvented (hot) roof assembly forms a vaulted ceiling, and he has observed that while the bays have been insulated, many framing intersections and sistered rafters show uninsulated gaps. He asks, “Is this a major concern for condensation issues? Neither the insulator, builder, or the energy rater that looked over the install seemed concerned. But from my understanding, cold air can make it through the cracks located between sistered framing members, causing condensation on the underside of the OSB.” He also noticed a small section of a bay that the insulators neglected to foam. His question is two-fold: Should he be concerned about the potential for mold and rot? If so, what should he use to seal the gaps?

This is the topic for today’s Q&A Spotlight.

Avoid condensation

Michael Maines thinks that warm, moist indoor air should be prevented from reaching the cold sheathing, where it can condense. “While wood is pretty resilient and can absorb and release moisture without problems for a long time, eventually if the moisture builds up enough, you’ll get undesirable microbial action (a.k.a. mold and rot).” He adds that moisture issues may take up to 10 years or more to show up, or may not show up at all, even if the science is sound.

Akos raises another issue: the possibility of ice dams caused by small air leaks in spray-foamed roofs, citing two related posts concerning ice dams with new construction and air-sealing conjoined trusses. He admits that the leaks may be hard to spot, unless you use smoke sticks during…