Which type of attic insulation achieves R-38 value with minimum thickness?

Closed-cell spray polyurethane foam achieves an R-38 thermal resistance rating with the minimum required profile, needing only 5.5 to 6.0 inches of applied depth compared to 10 to 14 inches for fibrous insulation materials.

Low-pitch roof assemblies, shallow truss heels, and unvented conditioned attics routinely pinch clearance down to less than 8 inches along exterior wall top plates. Under the 2021 International Energy Conservation Code (IECC Section R402), achieving prescriptive ceiling insulation values in constrained roof cavities requires maximum thermal density per inch to prevent perimeter energy loss and roof ice dams.

If you only do one thing: Specify medium-density, 2.0-pound closed-cell spray polyurethane foam applied at a finished depth of 5.75 inches to achieve R-38 while integrating an air and vapor barrier.

• Closed-cell spray foam (ccSPF): Yields R-6.5 to R-7.0 per inch, requiring 5.5 to 5.85 inches for R-38; its 2.0-pound per cubic foot nominal density functions as an air barrier and a Class II vapor retarder (under 1.0 perm) at depths above 1.5 inches.
• Polyisocyanurate rigid board: Delivers R-6.0 to R-6.5 per inch at nominal test conditions, reaching R-38 at 6.0 to 6.5 inches of thickness; it requires taped, staggered multi-layer installations to eliminate joint thermal bypass on flat attic platforms.
• Dense-pack cellulose: Delivers R-3.6 to R-3.8 per inch and requires 10.0 to 10.5 inches of finished depth; cellulose provides effective air-retarding density (3.5 pounds per cubic foot) but demands raised-heel energy trusses to fit over wall plates.
• Open-cell spray foam (ocSPF): Provides R-3.6 to R-3.8 per inch at 0.5-pound density, requiring 10.0 to 10.6 inches for R-38; ocSPF remains vapor-permeable and requires an applied vapor-retarder coating in International Residential Code (IRC) Climate Zones 4 through 8.
• Blown-in fiberglass: Averages R-2.5 to R-3.2 per inch, requiring 12.0 to 15.2 inches of uncompressed settlement depth to meet R-38, which necessitates full-height cardboard or foam wind baffles at every eave soffit vent.

• Watch out for: Exceeding the manufacturer's maximum single-pass thickness (typically 2.0 inches) during closed-cell foam application, which causes exothermic heat accumulation, core scorching, and shrinkage gaps.
• Watch out for: Code-mandated ignition barriers under IRC Section R316.5.3, which require 1.5-inch mineral wool, intumescent coating, or 1/2-inch drywall covering foam plastics in attics accessed for mechanical maintenance.
• Watch out for: Temperature de-rating in polyisocyanurate boards, which lose thermal efficiency (dropping toward R-5.0 per inch) when mean core temperatures fall below 25°F (-4°C).

Measure existing heel clearance at the roof eave boundary: if vertical space is under 7 inches, install closed-cell spray foam to achieve R-38 without blocking attic ventilation pathways.