Insulation materials

R-value per inch matters most where space is tight. In an open attic you have unlimited depth, so the cheapest material per R-value usually wins - that is blown fiberglass or cellulose. In a 2x4 wall cavity or against a rim joist you have 3.5 inches and no more, so the high-R-per-inch material earns its price. That single distinction explains most of the right answers in this category.

Fiberglass is the default: cheap, widely installed, $0.30-$1.50 per sq ft in material. It performs poorly if compressed or installed with gaps, so quality of install matters more than brand. Cellulose is recycled paper treated with fire retardant, at $0.60-$2.30 per sq ft material; dense-packed it also slows air movement, which fiberglass does not. Mineral wool costs more ($1.40-$2.10 per sq ft material) and buys you fire resistance, water tolerance and sound damping - a real advantage in basements, garage ceilings and party walls.

Spray foam is the material to be most careful about. Closed cell delivers about R-7 per inch and doubles as an air and vapor barrier, which is genuinely valuable in rim joists, crawl spaces and cathedral roofs. But it is applied as a two-part chemical reaction on site, and if the mix ratio, temperature or thickness per pass is wrong, it can cure badly and leave a persistent odor. Isocyanates - a core spray foam ingredient - cause occupational asthma and irritation of the eyes, nose, throat and skin, per OSHA, and OSHA notes some isocyanate compounds are classified as potential human carcinogens. This is a hazard to the crew during application and the reason your household and pets must be out of the house for the manufacturer's stated re-occupancy period.

Bad spray foam is very expensive to undo. Removal runs about $1.50-$3.00 per sq ft, and up to $5.00 where it has to be scraped by hand. Fiberglass installed badly can be pulled out in an afternoon. That asymmetry should shape how much you vet a foam contractor versus a batt contractor.