DuPont THERMAX Polyiso
DuPont THERMAX is a foil-faced polyisocyanurate (polyiso) rigid foam board for above-grade walls and roof assemblies. R-6.5 per inch at 75°F — the highest R-value per inch of any common rigid foam. Unlike most polyiso, THERMAX maintains R-value in cold temperatures (R-7.0/inch at 0°F). Foil facing (both sides) functions as an air barrier, Class I vapor retarder (0.03 perms), and thermal barrier — UL classified for exposed installation in some applications without additional drywall. Not for below-grade or under-slab use. Available at Home Depot (special order) and commercial building supply.
Material
Polyisocyanurate (polyiso)
Faced
Yes
Facing
Foil (both sides, 1 mil reflective aluminum)
R Value Per Inch
6.5
R Value Note
R-6.5 at 75°F mean temp; R-7.0 at 0°F — does not lose R-value in cold (unlike standard polyiso). Use R-5.5 for conservative design in Climate Zones 6+ per ASHRAE.
Board Size
4 ft × 8 ft
Model Comparison
2 models
| Specification | 1" | 2" |
|---|---|---|
| Model | THERMAX-1 | THERMAX-2 |
| r_value | 6.5 | 13 |
Shared Specifications
All models| material | Polyisocyanurate (polyiso) |
| faced | Yes |
| facing | Foil (both sides, 1 mil reflective aluminum) |
| r_value_per_inch | 6.5 |
| r_value_note | R-6.5 at 75°F mean temp; R-7.0 at 0°F — does not lose R-value in cold (unlike standard polyiso). Use R-5.5 for conservative design in Climate Zones 6+ per ASHRAE. |
| board_size | 4 ft × 8 ft |
| coverage_sqft_per_board | 32 |
| compressive_strength_psi | 25 |
| water_absorption_pct_max | 0.1 |
| vapor_retarder_class | I (foil-faced, 0.03 perms) |
| thermal_barrier_required | No |
| thermal_barrier_note | UL classified for exposed installation — foil facing qualifies as 15-min thermal barrier per NFPA 286 (walls up to 3", FM 4880 approved) |
| Application | Exterior continuous insulation, wall assemblies, roofing — above-grade only (not for below-grade or under-slab) |
| fire_rating | flame spread: ≤25 · smoke developed: ≤450 · standard: UL 723 / ASTM E84 · ul classified: true |
| standards | ASTM C1289 Type II Class 1, UL 1256, NFPA 286 |
| fm_approval | FM 4880 |
| warranty_years | 20 yr |