Last month, Coastal Connection visited a job site in Maine ski country to follow along on the roof and wall framing for a super-insulated, high-performance addition designed using Passive House methods. This week, we went back to the site and found Jesper Kruse and his carpenters working on the roof overhangs for the building.
Kruse was occupied with laying out the eave of the low-slope part of the addition's roof. Taking into account the planned strapping and sheathing (not yet applied to the roof), as well as the wall strapping that will later be required for vertical board wall siding, required a little bit of head-scratching on the contractor's part.
Meanwhile, the rest of the Maine Passive House carpentry crew (Todd, Steve, and Caleb) worked to install the last of the overhang framing on the addition's steep south-facing gable. The crew set up pump jacks, applied permeable weather-tight Pro Clima Mento Plus membrane lapping from the I-joist wall build-out up onto the I-joist roof rafters, and then attached an overhang outrigger to the wall over the membrane.
To follow along, view the slideshows: Layout for Roof Overhang (Eave) and Framing Roof Overhangs
The building science details for this roof system involve a method that is increasingly common for cathedral roofs in Passive House construction. The underside of the rafters will receive a smart vapor barrier fabric (Pro Clima Intello) designed to resist the passage of moisture into the insulated cavities from interior space, but still allow the cavities to dry to the inside if necessary. The upper side of the rafters will receive a water-tight, air-tight, vapor-open fabric (Pro Clima Mento Plus) intended to allow drying to the outside. Over the rafters and fabric, the crew will first apply cross-strapping to form a vented air space, then OSB sheathing, then asphalt felt and fiberglass-asphalt shingles. The design is intended to minimize the risk of vapor intrusion while maximizing the system's ability to dry out. We'll keep you updated as work continues.