You didn’t want to use fiberglass insulation on this project. But you’re trying to avoid spray foam because of its outsized embodied emissions. You got a couple of bids for net-and-blow cellulose, excited by the carbon-storing capabilities of this recycled material. One bid was really high; it seemed like the contractor wasn’t interested in a small job like yours. The other was reasonable. But that contractor couldn’t get there for two weeks, which would mean losing your spot on the drywaller’s schedule. So here you are in aisle 18 of the big-box store, questioning your green credentials and hoping you can get your dolly through the parking lot without running into anyone you know.

An anecdotal truth

Fiberglass batts have a bad reputation among energy-conscious builders, with some justification. Compared to other insulation types, they are particularly susceptible to installation defects. A lack of proper training and slipshod work practices means that most batt insulation performs well below its rated R-value.

Despite their shortcomings, fiberglass batts remain the most popular insulation product in U.S. homes. They’re inexpensive, readily available, and don’t require special installation equipment. Compared to spray foam, foam board, and even mineral wool, they have lower emissions associated with their manufacture.

These considerations mean that sometimes, using fiberglass batts makes sense. Once that decision has been made, how do we get the best possible result? In this article, we’ll walk through best practices for product selection and installation. We’ll focus mainly on wall cavities but will also cover floors, attics, and cathedral ceilings.

Reasons fiberglass batts underperform

Standard-density fiberglass batts have a nominal R-value of about 3.8 per inch of thickness. Higher-density batts can provide about R-4.3 per inch. These numbers bracket the same general range as cellulose, mineral…