What Is R-Value? Spray Foam Thermal Performance Explained
R-value measures how well insulation resists heat flow — higher is better.
Spray foam R-values
- Open-cell: ~R-3.5 per inch
- Closed-cell: ~R-6 to R-7 per inch
Closed-cell gives more insulation in less thickness — useful in 2×4 walls or tight rafter bays.
DOE recommendations by climate zone
The US Department of Energy recommends attic insulation of about R-30 to R-60 depending on your climate zone (higher in the north). Your contractor calculates the thickness needed to hit the target.
Air sealing matters too
R-value isn’t the whole story. Spray foam also seals air leaks, which fiberglass doesn’t. A material that stops drafts can outperform a higher-R material that lets air through.