Synopsis: Fiberglass insulation is the most popular residential insulating product sold today. Unfortunately nearly all of it is poorly installed, so it doesn’t perform as well as it could. Common installation problems include gaps or voids around pipes, ducts, and wires, and batts that are compressed or improperly sized for the cavity. Lee Kurtas, an insulation professional in Houston, describes how to install fiberglass insulation so that it fits right and provides good thermal performance. He describes the sequence of a whole-house insulating job, including air-sealing and where to start insulating, then discusses how to efficiently and effectively delegate the work, and where energy raters are likely to focus their inspections. A comprehensive sidebar describes the RESNET rating system and provides illustrations on what  RESNET Grades I, II, and III look like in an insulated wall without drywall. Photos: Patrick McCombe

When building science and home efficiency really took off in the mid-1990s, insulation contractors started hearing regularly about how the type of insulation used affects a building’s energy efficiency. Blower-door testing and thermal imaging of existing homes proved that fiberglass — as it’s typically installed — didn’t perform as well as other types of insulation, especially spray foam. As a result, builders and architects doing projects with energy-performance benchmarks started specifying spray foam as a way to ensure better airtightness and thermal resistance.

These builders and architects liked spray foam because it seals around penetrations such as pipes, ducts, and wires, and because it fills odd-shaped cavities quickly and easily. But spray foam does have a significant downside: cost. Insulating a typical new home with spray foam in my area costs two to three times what it costs to use fiberglass. Because of its low cost, fiberglass insulation is easily the most popular building insulation here…