Forum member “rfb” is building a new house to replace one that burned in a wildfire near the Northern California coast, Climate Zone 3. In this Q&A post, they describe the local weather as hot and dry in the summer, cool at night, and rainy in the winter with some occasional snow.

The house is to be equipped with rooftop solar panels and a battery storage system. An unvented attic seems to be a logical choice in this fire-prone area—and it is recommended by the 2021 International Wildland-Urban Interface Code (IWUIC) to reduce damage by wildfire—but most of the literature concerning unvented attics refers to locations either in the cold Northeast or the hot and humid Southeast. Rfb is unsure about them in a mild coastal climate.

The situation is complicated by space limitations, which the homeowner/builder says forces the HVAC air-handler and ductwork into the attic. Given this, they wonder whether the attic should be vented.

Rfb is also worried about the inability to detect roof leaks in a sealed space. Then there’s concern about insulation type—spray foam is efficient, but not fire-proof, while the more fire-resistant fiberglass batts would need a vapor diffusion vent, a detail that may not be advisable in this climate.

If mechanical equipment and ducts must stay in the attic, what kind of roof assembly will work in a mild region? This is the topic for today’s Spotlight.

Identifying strategies

Let’s start with the HVAC plan in general. Because high performance–home building pros discourage putting HVAC equipment and ducts in the attic—particularly in new construction, when the slate is clean for smart HVAC design—one reader suggests creating an integrated service cavity between the attic floor and the ceiling below to house the HVAC equipment; this would result in…