Hi Folks,

We are planning a lean-to style addition for our home in climate zone 5b. It’s an adaptive re-use project—former mechanic’s garage into a home. Because it’s a former industrial building, we’ve got a near-flat roof (1:12) which we plan to match in the addition. Since it’s low-slope, we’ve decided on a warm, unvented roof. The roof and walls will be monopoly framed and wrapped in taped zip, with 2” of exterior foam applied to the walls and 4” of exterior foam applied to the roof. Eaves will be applied outboard of the sheathing. In the joist and walls cavities we will blow in cellulose to meet the appropriate ratio as explained here: https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing

We will put strapping over the exterior foam insulation to hang the siding and for fastening down the metal roof.

With this kind of assembly, what kind of air barrier is needed on the inside of the addition? The interior of the walls in part of the addition (my shop and studio) will be plywood-backed. Is a layer of plywood and painted drywall sufficient as an interior air barrier?

Thanks in advance.

Elizabeth