Attics can be built as part of a home’s conditioned building envelope, or they can be separate from it, or unconditioned, An unconditioned attic, per the International Residential Code (IRC), chapter 8, on roof and ceiling construction, must be paired with a vented roof. Conditioned attics can have vented roofs too, and venting can help to prevent moisture buildup at the roof sheathing and elsewhere in the attic. It’s also one way to prevent ice dams. In all cases, the devil is in the details—enough vent opening space, continuous vent channels, the right level of insulation, continuous air-sealing, and other moisture management strategies. We’ve covered attics and roof venting a lot on GBA. For any number of deep dives into the topic, start here:

• All About Attic Venting – GreenBuildingAdvisor
• Creating a Conditioned Attic – GreenBuildingAdvisor
• All About Roof Venting – GreenBuildingAdvisor
• Is Venting an Unconditioned Attic Necessary? – GreenBuildingAdvisor
• Lstiburek’s Rules for Venting Roofs – GreenBuildingAdvisor
• Five Cathedral Ceilings That Work – GreenBuildingAdvisor

Yet the questions keep coming.

In a recent post in the Q&A Forums, GBA member “PatSauce” is requesting some info on unvented attics. Pat is designing a house in Los Angeles County, climate zone 3B, with traditional Craftsman-roof details that can’t accommodate soffit vents. The fire-zone location adds another vote for an unvented roof. Pat doesn’t want to use spray foam and would like to avoid the cost and complexity of exterior insulation installed over the roof deck. And like so many people working through these details, he is concerned with the potential for moisture accumulation, mold growth, and rot.

“According to the IRC requirements in section R806.5.1.3, both air-impermeable and air-permeable insulation can be used and rigid-foam insulation board can be used as the air-impermeable layer applied directly to the underside of the…