Once all of the trades were roughed in and inspected and framing was inspected, we began the insulation in the exterior walls. We ran into a bit of a challenge with this home when it came to choosing and installing the insulation we would use. We prefer dense-packed cellulose over fiberglass insulation because it performs better due to the way it slows down air flow in the wall cavities. When you slow down the movement of air inside the walls, you greatly slow down heat loss by convection (loss of heat from air to a solid, like drywall and wall sheathing). Fiberglass is not a bad product, but it lets air flow through it fairly easily (think of a cheap furnace filter; made of fiberglass). So we chose dense-packed cellulose.
Note: we could have used a damp-spray cellulose material, but we have found in the past that the foam sheathing does not allow the insulation to sufficiently dry, leaving wet areas of insulation in the walls. Even with dehumidifiers running in a home for two weeks the insulation still has a moisture content that makes us nervous. With a tight, high-performance home, moisture in the walls is a very bad thing, and can lead to mold issues down the road. We also had the option of damp sprayed fiberglass, but again we were concerned about added moisture into the structure.
The trouble came when we discovered that the staggered stud walls that we had built, although excellent at heat loss prevention, made it significantly more challenging to insulate than a standard framed wall. Generally with cellulose, the material is blown into each stud cavity under pressure, causing the insulation to fill the cavity nice and tight. By building the staggered stud walls, the stud cavities are no longer enclosed, but rather open to the next cavity, and the next, and so on. It becomes impossible to create any pressure and pack the insulation in. See the photo below that illustrates our dilemma:
Help came from Mark Lee of Better World Builders here in Kalamazoo. His company does very high quality energy improvement work, and is acting as our insulation contractor on the ZERH project. Mark came up with a way of using a fabric material (made for this purpose), stapled to the sides of the studs, to create enclosed cavities that would accept the dense-packed insulation. His crew did a great job of installing the material, filling the insulation tightly around all of the framing, wires, electrical boxes, and other items in the walls. The photos below show the fabric in place in the walls:
With the 2 inches of dense foam on the exterior walls as sheathing and the dense-packed cellulose inside the walls, we end up with an R-value of about 32, or about 78% better than the 2012 energy code, which won’t be adopted here in Michigan until early next year. This should insure energy savings and a comfortable living space for the life of the home.