Side-by-Side Comparison
Thermal Insulation & Acoustics
Thermal Insulation R-Values vs. Roof Pitch & Slope Standards
Comparing 2 standards across 14 specifications. 14 key differences identified.
Comparing:
Thermal Insulation R-Values
Roof Pitch & Slope Standards
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).
Roof pitch is standardized under the International Building Code (IBC) and IRC into rise-over-run slope ratios (e.g. 4:12 indicates 4 inches of vertical rise per 12 inches of horizontal run), defining pitch angles from 0° to 60° and determining mandatory weatherproofing coverings (membrane vs asphalt shingles vs metal).
Specification Comparison Table
14 Parameters| Parameter / Spec | ||
|---|---|---|
|
Governing Standards
|
ASTM C518 / ASTM C177 / IECC Chapter 4 / ASHRAE 90.1 | IBC Chapter 15 (Roof Assemblies) / IRC Section R905 / OSHA Roofing Fall Protection |
|
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 | — |
|
Notation Formula
|
— | Rise in inches per 12 inches of horizontal run (e.g. 4:12 or 4/12) |
|
Flat / Low-Slope Roof
|
— | 1/4:12 to 2:12 (1.19° to 9.46°) | Requires continuous welded membrane (TPO, EPDM, PVC, Built-Up BUR) |
|
Conventional Slope (Shingle Minimum)
|
— | 4:12 to 8:12 (18.43° to 33.69°) | Standard 3-Tab and Architectural asphalt fiberglass shingles with single underlayment |
|
Low-Slope Shingle Exception
|
— | 2:12 to 4:12 (9.46° to 18.43°) | Asphalt shingles permitted ONLY with double layer underlayment or self-adhering ice/water shield |
|
Steep Slope Roof
|
— | 9:12 to 12:12 (36.87° to 45.00°) | Excellent snow shed, Mansard/A-frame roofs |
|
Walkability / OSHA Threshold
|
— | ≤ 6:12 (26.56°): Walkable by foot | > 6:12: Mandatory OSHA roof brackets and fall arrest harness |