Last summer I received a call about renovating a small low-income rental unit in Point Reyes Station, Calif. Its owners, the local community land trust, wanted to replace the foundation, remodel the kitchen, and make some minor upgrades to the interior. They mentioned that the utility bills were extremely high, so they also wanted to add insulation.
The 80-year-old two-bedroom house didn’t look too bad from the outside, but when I inspected the interior I found all kinds of problems. The ceiling was insulated with R-13 fiberglass batts, and several batts installed between the floor joists hung down into the crawlspace. Some portions of the exterior walls had been insulated during an earlier remodel, but others contained no insulation at all.
There was also clearly a lot of air leakage. Fifty years of electrical and plumbing additions had left the house riddled with holes, and the siding was nailed directly to the studs with no sheathing or air barrier behind it. I told the owners that unless we air-sealed the building, adding insulation would do little to lower the heating bills.
Developing a Plan
Click the square at the upper-left corner of the illustration to enlarge.
The good news was that the house had a floor area of less than 850 square feet and 8-foot ceilings, so the amount of air-sealing needed was relatively small. We decided to try to make the house as tight as possible within the budget we had to work with, which wouldn’t allow us to replace doors and windows. We also needed to preserve the interior finishes, so the stud bays would have to be accessed from the exterior.
The primary insulation in the walls and floor would be dense-pack cellulose, with blown loose-fill in the attic. A new layer of OSB sheathing would be followed by an inch of rigid foam and housewrap, with an exterior finish of fiber-cement siding applied over a vented air space (see illustration).
Cool and breezy. To measure our ability to eliminate leaks, we did blower-door tests at the beginning and end of the job. The initial test showed there to be 1,330 cfm of leakage at 50 pascals, or a very porous 11.9 air changes per hour (1,330 cfm x 60 minutes / 840 sq.ft. x 8 ft. = 11.9 ACH). When we saw those numbers, we weren’t surprised that the previous tenants had complained about heating costs.