Among common insulation materials, which has the highest typical R-value per inch?

• A. Polyiso (6.5–7.0 per inch)
• B. Fiberglass (3.1–3.7 per inch)
• C. Mineral Wool (3.0–3.3 per inch)
• D. Cellulose (3.5–3.8 per inch)

Answer: (A)

R-value per inch measures how much a material resists heat flow for each inch of thickness—the higher the number, the better the insulation per inch. Polyiso is the strongest here because it’s a closed-cell plastic foam with very low thermal conductivity. Its structure traps air in dense, uniform cells, so heat has fewer paths to travel. That yields a typical R-value around 6.5 to 7.0 per inch.

Fiberglass relies on glass fibers and pockets of air, but it has higher heat transfer paths than foam, giving about 3.1 to 3.7 per inch. Mineral wool is dense and has good fire resistance, but its thermal conductivity is higher than polyiso, around 3.0 to 3.3 per inch. Cellulose is treated paper fibers that can perform around 3.5 to 3.8 per inch, depending on install density and compaction. So the foam-based polyiso provides the highest insulation per inch among these common materials.