R-Value Isn't the Only Number That Matters
Fiberglass batt insulation achieves R-3.1–3.7 per inch (blown fiberglass: R-2.2–2.7/inch). Spray polyurethane foam comes in two types: open-cell at R-3.7/inch, and closed-cell at R-6.0–7.0/inch. Closed-cell spray foam achieves more than twice the R-value per inch of fiberglass batts. For applications where thickness is constrained — cathedral ceilings, under-slab applications, or walls where you don't want to lose interior space — closed-cell foam is the only practical way to achieve adequate R-values in tight spaces. But for accessible attics with unlimited depth, fiberglass or blown cellulose at $0.03–$0.05/sq ft installed vs. spray foam at $1.00–$1.50/sq ft is almost always the better economic choice.
Air Sealing: Where Spray Foam Wins Decisively
Fiberglass batts insulate but don't seal air leaks. Air infiltration accounts for 25–40% of home heating and cooling energy loss (Department of Energy). Spray foam adheres to surfaces and expands into gaps, cracks, and penetrations, creating both insulation and air barrier in a single application. Studies of spray foam retrofits consistently show 20–40% reductions in air leakage (measured in air changes per hour at 50 Pascals). This air sealing benefit often exceeds the incremental R-value benefit — particularly in older homes where air infiltration is the dominant energy loss mechanism rather than conductive heat transfer through walls.
Moisture Management: Open vs Closed Cell Behavior
Open-cell spray foam is vapor-permeable (allows moisture to pass through). Closed-cell is a vapor barrier (blocks moisture). In humid climates, applying closed-cell foam to the interior of exterior walls can trap moisture between the foam and the exterior sheathing — creating mold conditions if the assembly isn't designed correctly. Building scientists recommend closed-cell foam for below-grade (basement walls, under-slab) and cold-climate roof assemblies where it belongs on the cold side; open-cell or fiberglass for above-grade wall cavities in most mixed-humidity climates. The correct foam application depends on climate zone and assembly location.
Payback: Where to Prioritize Your Insulation Budget
Attic insulation has the highest return of any insulation application — heat rises, attic bypasses are common, and marginal R-value additions in accessible attics are cheap. Blown fiberglass or cellulose at R-49–60 in a previously R-11 attic typically pays back in 3–7 years. Spray foam for rim joists (where foundation meets house frame) is one of the highest-value spray foam applications: a 100-linear-foot rim joist sealed with 2" closed-cell foam costs $300–$500 and can save 5–10% on heating costs. Wall cavity spray foam is the most expensive and often lowest-return application — the upgrade from R-11 existing insulation to R-20 spray foam rarely pays back in less than 15 years on energy savings alone.