How do spray foam and blown-in actually compare?
Spray foam does two jobs at once - it insulates and air-seals - and closed-cell delivers roughly R-6 to R-7 per inch, the highest of any common option. Blown-in fiberglass or cellulose insulates only, at lower R per inch (R-2.2 to R-3.8), but costs far less per square foot: $1 to $4 versus $2 to $5 for foam (HomeAdvisor, 2026).
Where does foam earn its higher price?
Foam makes the most sense where space is tight or air-sealing is the goal. Open-cell foam is lighter, around R-3.5 to R-3.7 per inch, and lets water vapor move through more easily; closed-cell is denser and provides a greater R-value where space is limited. For a straightforward attic-floor top-up, blown-in usually hits the target R-value for less money - foam earns its premium mainly at rooflines, rim joists, and tight spots that blown-in cannot fill evenly.
Sources
2 cited- Spray foam vs blown-in cost and R-value: HomeAdvisor, Attic Insulation Cost, 2026 (checked 2026-08-19).
- Open- and closed-cell spray foam R-value per inch, and where blown-in vs foam fits best: U.S. Department of Energy, Insulation Fact Sheet, DOE/CE-0180, 2008 (checked 2026-08-19).