Thermal Insulation R-Values
Thermal Insulation & Acoustics · 8 specs
Direct Answer
Thermal insulation materials are standardized under ASTM C518 and the International Energy Conservation Code (IECC) by their thermal resistance (R-value per inch of thickness, hr·ft²·°F/Btu / RSI in m²·K/W), ranging from fiberglass batts (R-3.1/in) to polyisocyanurate and closed-cell spray foam (R-6.5 to R-7.0/in).
Source: ASTM C518 / ASTM C177 / IECC (International Energy Conservation Code) / ASHRAE 90.1 · Last reviewed Aug 28, 2026
Specs
| Governing Standards | ASTM C518 / ASTM C177 / IECC Chapter 4 / ASHRAE 90.1 |
| R-Value Definition | R = Thickness (inches) ÷ Thermal Conductivity (k-factor) in (hr·ft²·°F)/Btu |
| Fiberglass Batts (Standard) | R-3.1 to R-3.4 per inch | Density: 0.5–1.0 lb/cu ft | Permeable to air/vapor (Vapor Perm > 50) |
| Mineral Wool / Rockwool | R-3.8 to R-4.2 per inch | Non-combustible up to 1,177°C (2,150°F) | Excellent acoustic dampening |
| Expanded Polystyrene (EPS) | R-3.6 to R-4.0 per inch | White rigid beadboard | Vapor permeable |
| Extruded Polystyrene (XPS) | R-5.0 per inch | Blue/Pink closed-cell board | Moisture resistant, continuous foundation insulation |
| Polyisocyanurate (Polyiso) | R-6.0 to R-6.8 per inch | Foil-faced rigid foam | Superior continuous exterior sheathing |
| Closed-Cell Spray Foam (ccSPF) | R-6.5 to R-7.0 per inch | 2.0 lb density | Class II vapor barrier (< 0.8 Perm) + structural racking strength |
Thermal Insulation Material R-Value Comparison Matrix
| Insulation Material Type | R-Value per Inch (ASTM C518) | RSI Value per 25mm (Metric) | Vapor Permeance (Perms) | Water Absorption | Fire / Ignition Behavior |
|---|---|---|---|---|---|
| Fiberglass Batts | R-3.1 to R-3.4 / in | 0.55 to 0.60 RSI | > 50 Perms (Open) | High (Loses R when wet) | Non-combustible fibers |
| Mineral Wool (Rockwool) | R-3.8 to R-4.2 / in | 0.67 to 0.74 RSI | > 30 Perms (Open) | Hydrophobic (Repels water) | Fireproof (2,150°F / 1,177°C) |
| Dense-Pack Cellulose | R-3.6 to R-3.8 / in | 0.63 to 0.67 RSI | Vapor permeable | Absorbs moisture | Borate fire-retardant treated |
| EPS Rigid Foam (Beadboard) | R-3.6 to R-4.0 / in | 0.63 to 0.70 RSI | 2.0 to 5.0 Perms | Moderate (2% to 4%) | Requires thermal barrier (drywall) |
| XPS Rigid Foam (Extruded) | R-5.0 / in | 0.88 RSI | 0.8 to 1.5 Perms | Very Low (< 0.7%) | Requires thermal barrier |
| Polyiso (Foil-Faced) | R-6.0 to R-6.8 / in | 1.05 to 1.19 RSI | < 0.05 Perms (Vapor Barrier) | Low (< 1.0%) | Excellent thermal sheathing |
| Open-Cell Spray Foam (0.5#) | R-3.6 to R-3.8 / in | 0.63 to 0.67 RSI | 10 to 20 Perms | High (Absorbs water) | Excellent acoustic air-seal |
| Closed-Cell Spray Foam (2#) | R-6.5 to R-7.0 / in | 1.14 to 1.23 RSI | < 0.8 Perms (Vapor Retarder) | Zero (< 0.5%) | Air barrier + Vapor barrier + Structure |
Continuous Insulation (ci) vs Thermal Bridging in Studs
Wood studs (R ≈ 1.2 / in) and steel studs (R ≈ 0.001 / in) act as thermal bridges that bypass cavity insulation. A 2×6 wall with R-21 batt insulation has an effective overall assembly R-value of only R-15 due to 25% framing area framing loss. Adding 1.5 inches of exterior XPS continuous insulation (R-7.5 ci) breaks the thermal bridge completely.