In areas where we did have access from inside, we insulated and air-sealed the rim-joist sill first with Sashco Big Stretch to seal all connections to the sill plate and rim joist. Prior to spray-foaming, we added 2-inch blocks of Rockwool Comfortboard over the rim joist. This vapor-open buffer zone allows moisture to dissipate into neighboring materials and dry into the building
On this bedroom wall, which extends to the kitchen below, we used CertainTeed’s MemBrain, a material that allows water vapor to pass through when humidity levels are high while stopping airflow. The membrane worked well to reduce the amount of cooking odors that permeated into the bedroom.
Air-sealing is not relegated to one phase of construction. Throughout the entire build, we continually seal anywhere air can penetrate or convect, as seen in this interior plumbing wall after the shell was sealed. Reducing air movement in walls is also important for improving fire safety, deadening sound (sound travels through air), and reducing unwanted moisture movement. In this case, these walls will be tiled on both sides.
Access hatches from the exterior into a crawlspace and from the interior into a knee-wall attic area (right) are often leaky. For each one, we took care to build insulated access hatches with Zip System sheathing and foam-rubber gaskets.
There are three separate attics with three separate access points. This one was the smallest. The air-barrier here consists of 1 1/2-inch XPS foam that we cut into the rafter bays to create a 1-inch ventilation space below the roof sheathing. We do not rely on the closed-cell foam as a singular air barrier.
We used the trick of drawing a line around the structure on plan sections to see where the discontinuities of the control layers were (see “Air Barrier Basics,” by Steve Bazcek, Jan/19). In this area, we had to remove an existing slab to create continuity from slab to foundation to sill. The vapor-barrier is sealed to the foundation where a liquid-applied barrier continues to the sill plate. The sill plate is gasketed and caulk-sealed to the foundation.