R-value hierarchy across common materials
Fiberglass batts, the cheapest and most common retrofit, deliver around 3.2 R-value per inch. Mineral wool and stone wool run slightly higher at 3.3-3.5, with better moisture and fire performance. Rigid foam (polyisocyanurate and XPS) jump to 5.8-6.5 per inch, explaining their premium cost and frequent use in high-performance builds where wall depth is constrained. Spray polyurethane foam reaches 7 per inch but introduces application complexity and off-gassing concerns. Cellulose adds another option at 3.5-3.7 per inch, popular in green building for recycled content.
Choosing by performance requirements and cost tradeoffs
A builder targeting modest insulation might select fiberglass throughout for cost certainty and ease. Passive House and net-zero projects favor foam for superior per-inch density and air-sealing potential. In retrofit walls where depth is limited, foam's compactness often forces the choice, even at 2-3x the material cost. Regional building codes and moisture climate (cold vs hot-humid) add further constraints, as vapor barriers and drainage planes interact with material hygroscopicity and settling patterns over decades.