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...

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