When it comes to a building’s energy efficiency and long-term durability, air leakage through an assembly is undesirable. Air infiltration and exfiltration can account for significant energy losses; and moisture-laden air can lead to the rotting of lumber over time, compromising durability.
For the vented roof illustrated here, there is both intentionally designed air movement and unwanted air movement within the same assembly. The lower vent—located in the soffit—communicates with the vent located at the ridge (stack effect is what vents the attic). This system takes any moisture-laden air in the attic that is a result of diffusion or leakage from the house below and vents it out through the ridge. The attic air is then replaced by cooler, drier air entering through the soffit. While the sloped portion of the roof is vented and requires air movement for its success, the floor should be as airtight as possible for the conditioned house below to retain its heated or cooled air. So, how to achieve this . . .
Siga Majrex is a smart vapor retarder that allows for diffusion while maintaining an air barrier. It is staple-fastened to the underside of the roof truss (or ceiling joist). The critical transition in this assembly is the vapor retarder along the ceiling plane to the exterior air barrier, which, in this case, is Zip sheathing. To maintain continuity of the air barrier, two beads of sealant are applied at the exterior face of the top plate; and a 16-in. flap of the Majrex is centered and draped over the top plate. Then the sheathing is installed. The sheathing compresses the beads of sealant, resulting in an airtight connection between the interior vapor retarder/air barrier and the exterior sheathing/air barrier.
On the interior, the flap is rotated upward and stapled…
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