Spray Foam vs Fiberglass

Fiberglass batts: R-3.1 to R-3.8 per inch, $0.40–$1.10 per sq ft installed. Open-cell spray foam: R-3.5 to R-3.9 per inch, $1.20–$2.20. Closed-cell: R-6 to R-7 per inch, $1.75–$3.50 per sq ft at 2 inches.

Closed-cell foam is the only one of the three that is also an air barrier, a vapor retarder and a structural stiffener in one pass.

Rim joists, cathedral ceilings, unvented roof decks, crawl space walls, van and shed conversions, and any cavity too irregular to fill cleanly with batts. In these places foam solves air and vapor problems that fiberglass simply cannot.

Open attic floors and standard stud walls in new construction, where you can install to Grade I and air seal separately for a fraction of the cost. Adding a proper air-sealing pass plus deep fiberglass usually beats spending the same money on thinner foam.

Whole-house spray foam typically adds $4,000–$12,000 over fiberglass and returns 10–25% on heating and cooling. That is often a 12–25 year payback on energy alone — worth it if you value comfort, quiet and moisture control, less so as a pure investment.

Foam must be installed at the right temperature and ratio; bad installs off-gas or shrink and are expensive to fix. Get a written product data sheet, confirm the installer's certification, and stay out of the house for the manufacturer's recommended re-entry period.