Materials Comparison
Material permeability comparison enables informed selection of building envelope components based on hygrothermal performance requirements. Systematic evaluation of vapor transmission characteristics across material categories reveals optimal combinations for climate-specific assembly design.
Comprehensive Permeability Table
Comparative permeance values for common building materials at standard conditions (73°F, 50% RH):
| Material Category | Material | Thickness | Permeance (perm) | Class |
|---|---|---|---|---|
| Vapor Barriers | Aluminum foil | Any | 0.00 | I |
| Polyethylene 6 mil | 6 mil | 0.06 | I | |
| Polyethylene 4 mil | 4 mil | 0.08 | I | |
| EPDM membrane | 45 mil | 0.05 | I | |
| Low Permeance | XPS rigid insulation | 1 inch | 0.6-1.2 | II |
| Closed-cell spray foam | 1 inch | 0.4-1.0 | II | |
| Oil-based paint (3 coats) | - | 0.3-1.5 | II | |
| Kraft-faced insulation | - | 0.2-0.5 | II | |
| Medium Permeance | OSB sheathing | 7/16" | 0.5-2.0 | II/III |
| Plywood | 3/8" | 0.7 | III | |
| Latex paint (2 coats) | - | 10-20 | III | |
| Building paper #15 felt | - | 5-10 | III | |
| High Permeance | Unpainted gypsum board | 1/2" | 40-50 | III |
| Housewrap | - | 10-60 | III | |
| Fiberglass batt (unfaced) | Any | >100 | Permeable | |
| Mineral wool | Any | >100 | Permeable |
Insulation Material Permeability
Thermal insulation materials span the complete permeability spectrum:
Highly Permeable Insulations (>20 perm)
Fibrous insulations:
- Fiberglass batts unfaced: >100 perm, completely vapor-open
- Mineral wool batts: >100 perm, fire-resistant permeable option
- Cellulose loose-fill: >100 perm, hygroscopic buffering capacity
- Cotton/denim batts: >100 perm, sustainable permeable insulation
Application: Cavity insulation in assemblies requiring vapor permeability, compatible with any vapor control strategy.
Semi-Permeable Insulations (1-20 perm)
Board insulations:
- Mineral wool rigid board: 20-30 perm, exterior continuous insulation
- EPS (expanded polystyrene) 1 inch: 2.0-5.7 perm depending on density
- Polyisocyanurate unfaced 1 inch: 3.0-5.0 perm
Application: Exterior insulation where moderate drying capacity required, thermal bridge reduction.
Low Permeance Insulations (<1 perm)
Closed-cell foams:
- XPS (extruded polystyrene) 1 inch: 0.6-1.2 perm
- Closed-cell spray polyurethane 1 inch: 0.4-1.0 perm
- Polyisocyanurate foil-faced 1 inch: 0.02-0.05 perm
Application: Applications requiring combined thermal and vapor control, below-grade insulation, roof systems.
Sheathing Material Performance
Structural sheathing significantly influences assembly vapor performance:
| Sheathing Type | Permeance | Drying Capacity | Structural Rating | Cost Factor |
|---|---|---|---|---|
| Plywood 3/8" | 0.7 perm | Limited | Excellent | Moderate |
| OSB 7/16" | 0.5-2.0 perm | Variable | Excellent | Low |
| OSB coated | 3-6 perm | Moderate | Excellent | Moderate |
| Gypsum exterior | 20-30 perm | High | Good | Moderate |
| Fiberboard | 20-50 perm | Excellent | Fair | Low |
| Mineral wool board | 20-30 perm | Excellent | None (requires bracing) | High |
| Foam plastic | 0.5-5.0 perm | Limited | None (requires bracing) | Moderate-High |
Selection criteria:
- Cold climates: OSB/plywood acceptable, provides adequate outward drying with interior vapor control
- Mixed climates: Higher permeance sheathing (coated OSB, gypsum) improves bidirectional drying
- Hot-humid climates: Permeable sheathing essential when exterior cladding has low permeance
Interior Finish Comparison
Interior surface treatments determine inward drying capacity and summer vapor control:
| Finish System | Dry Permeance | Wet Permeance | Smart Retarder | Climate Suitability |
|---|---|---|---|---|
| Unpainted gypsum 1/2" | 50 perm | 50 perm | No | Hot-humid only |
| Latex 1 coat on gypsum | 20-35 perm | 20-35 perm | No | Mixed, hot-humid |
| Latex 2 coats on gypsum | 10-20 perm | 10-20 perm | Slight | Mixed climates |
| Latex 3 coats on gypsum | 5-10 perm | 5-10 perm | Moderate | Cold, mixed |
| Oil paint 3 coats | 0.3-1.5 perm | 0.3-1.5 perm | No | Cold climates |
| Vinyl wallpaper | 0.3-0.8 perm | 0.3-0.8 perm | No | Cold climates |
| Smart vapor retarder | 0.8 perm | 10-20 perm | Yes | All climates |
Weather-Resistive Barrier Performance
WRB materials must balance water resistance with vapor permeability:
| WRB Type | Permeance | Water Resistance | Durability | UV Resistance |
|---|---|---|---|---|
| Housewrap (spunbonded) | 10-60 perm | Excellent | 6-12 months | Limited |
| Housewrap (woven) | 20-50 perm | Good | 3-6 months | Poor |
| Building paper asphalt-free | 10-30 perm | Good | 3-6 months | Moderate |
| Liquid-applied membrane | 15-40 perm | Excellent | Permanent | Excellent |
| Peel-and-stick | 5-20 perm | Excellent | Permanent | Good (if protected) |
| Felt paper #15 | 5-10 perm | Fair | 6-12 months | Good |
Selection priority:
- Water resistance (primary function)
- Adequate permeance for climate (>5 perm minimum in most climates)
- UV resistance matching construction schedule
- Compatibility with cladding attachment method
Climate-Specific Assembly Comparison
Optimal material combinations vary by climate zone:
Cold Climate Assembly (Zone 6)
- Interior: Gypsum 1/2" + latex paint 3 coats (5-10 perm)
- Cavity: Fiberglass batt unfaced (>100 perm)
- Sheathing: OSB 7/16" (0.7 perm)
- WRB: Housewrap (30 perm)
- Cladding: Vinyl siding (permeable)
- Vapor drive: Outward in winter, moderate inward in summer
- Performance: Interior moderate control prevents condensation, exterior permeability enables outward drying
Mixed Climate Assembly (Zone 4A)
- Interior: Gypsum 1/2" + latex paint 2 coats (10-20 perm)
- Cavity: Fiberglass batt unfaced (>100 perm)
- Sheathing: OSB coated (3-6 perm)
- WRB: Housewrap (40 perm)
- Cladding: Fiber cement (8-12 perm)
- Vapor drive: Bidirectional seasonal variation
- Performance: Moderate permeance both sides enables drying in either direction
Hot-Humid Assembly (Zone 2A)
- Interior: Gypsum 1/2" + latex paint 1 coat (20-35 perm)
- Cavity: Fiberglass batt unfaced (>100 perm)
- Sheathing: Gypsum exterior (20-30 perm)
- WRB: Liquid-applied membrane (25 perm)
- Exterior insulation: Mineral wool board (25 perm)
- Cladding: Stucco over WRB (permeable)
- Vapor drive: Inward during cooling season
- Performance: High interior permeance enables inward drying from air-conditioned space
Design Decision Framework
Material selection follows systematic hygrothermal analysis:
Step 1: Identify dominant vapor drive direction based on climate and HVAC operation.
Step 2: Place vapor control (low permeance) on source side of vapor drive.
Step 3: Ensure opposite side provides adequate drying capacity (typically >5 perm minimum).
Step 4: Verify no intermediate condensing planes using dewpoint analysis.
Step 5: Confirm adequate drainage for bulk water management independent of permeability.
Step 6: Validate against building code vapor retarder requirements for climate zone.
Material permeability comparison reveals that successful assemblies balance vapor control with drying capacity, adapted to climate-specific boundary conditions and operational patterns.