Last year, I designed and built my high-performance home in Shirley, Massachusetts. The design goal was a net-zero energy (NZE) house that would be comfortable, easy to build with low-skilled labor, and very affordable (around $100 per square foot, including the PV system that would offset 100% of the home’s energy use).
The NZE construction method that made the most sense was a simple wood-framed home with lots of insulation. I did several heat loss calculations, and compared different heating systems’ costs of installation, maintenance, and fuel. In the end, I decided to forgo purchasing a fossil-fuel-burning heating system. I estimated that I saved more than $20,000 by making this choice. Saving this money allowed me to spend more on insulation—and afford a solar-electric system. I chose dense-packed cellulose to insulate the walls (to R-42) and the roof to an average of R-100. There is just over 16,000 pounds of cellulose in the house.
One of the main benefits of cellulose is its ability to manage moisture. Many moisture-related problems are caused by air leaks, which bring moisture in with the air. Since dense-packed cellulose does not support convective air movement, the moisture-laden air cannot reach the condensing surface of the exterior plywood. With the help of National Fiber’s technical manager, we tested the wall assembly this past winter with a moisture meter. The testing revealed dry framing and dry exterior sheathing.
Most projects I’ve been a part of have considered the air-sealing separately from other construction. With my home, I integrated the air-sealing details into the building process. In the crawlspace, I installed a 16-mil vapor barrier against the ground and up the foundation walls. It’s reinforced and the seams are double-taped. The vapor barrier is mechanically fastened to the top edge of the block wall with plastic clips tapped into holes in the concrete.
Slight adjustments to the framing can result in better energy performance. For example, a common detail in homes is to have a central girder under the floor joists that is set into a pocket in the foundation wall. However, this leaves the girder subject to thermal bridging. My home has this central girder, but the connection between the girder and foundation was eliminated. Instead, an additional pier in the crawlspace handles the load. This allowed me to run a continuous layer of insulation up the foundation wall, and tie seamlessly into the rim-joist insulation. All this work is code-approved and requires no extra engineering—simply some forethought.
Exterior sheathing is 1/2-inch plywood covered with Grace Ice & Water Shield. Designed for use as roofing underlayment, this self-adhering product seals its own overlaps, and every puncture from nails and screws. I wrapped it around the house starting at the bottom, working up the walls and over the roof. Directly above each window and door, I left some paper on the backside so that I could lap window flashing under the higher course. This shinglelike lapping is the proper way, and the opposite of reverse-flashing, which will direct water into the structure.