Low Permeance Materials
Low permeance materials (Class II vapor retarders) exhibit permeance values between 0.1 and 1.0 perm (5.75 × 10⁻¹² to 5.75 × 10⁻¹¹ kg/Pa·s·m²), providing moderate resistance to water vapor diffusion while permitting limited moisture transmission and drying potential.
Material Categories
Painted surfaces depend on coating thickness and formulation:
- Oil-based paint 3 coats (0.3-1.5 perm): Traditional interior wall finish
- Alkyd enamel 2 coats (0.5-1.2 perm): Bathroom and kitchen applications
- Vapor barrier primer + latex (0.4-0.8 perm): Compromise between vapor control and breathability
- Epoxy coatings 2 coats (0.2-0.6 perm): Industrial and commercial applications
Kraft-faced insulation:
- Asphalt-impregnated kraft paper (0.2-0.5 perm): Fiberglass batt facing, traditional wall insulation
- Foil-scrim-kraft (FSK) facing (0.02-0.2 perm): Rigid board insulation, ductwork liner
- Kraft laminated polyethylene (0.1-0.3 perm): Enhanced moisture protection
Foam plastic insulations:
- Extruded polystyrene 1 inch (0.6-1.2 perm): Exterior continuous insulation, below-grade applications
- Closed-cell spray polyurethane foam 1 inch (0.4-1.0 perm): Cavity insulation, air sealing
- Polyisocyanurate foil-faced 1 inch (0.05-0.1 perm): Roof insulation, wall sheathing
Specialty membranes:
- Vinyl wallpaper (0.3-0.8 perm): Interior finish with vapor retarding effect
- Asphalt-saturated felt (15 lb) (0.4-1.0 perm): Wall sheathing wrap, roof underlayment
- Grade D building paper (0.2-0.6 perm): Traditional water-resistive barrier
Permeance Characteristics
Material permeance varies with environmental conditions and installation details:
| Material | Nominal Permeance | Temperature Effect | Humidity Effect |
|---|---|---|---|
| Oil paint (3 coats) | 0.3-1.5 perm | Decreases at low temp | Increases at high RH |
| XPS 1 inch | 0.6-1.2 perm | Minimal variation | Minimal variation |
| Kraft paper (asphalt) | 0.2-0.5 perm | Minimal variation | Increases when wet |
| ccSPF 1 inch | 0.4-1.0 perm | Minimal variation | Minimal variation |
Application Strategies
Cold climate installations (Zones 5-8):
- Interior vapor retarder reduces winter condensation risk
- Permits some inward drying during summer cooling season
- Balances moisture control with drying potential
- Typical assembly: exterior permeable sheathing, low-perm interior finish
Mixed-humid climates (Zones 3-4):
- Low permeance suitable for both heating and cooling seasons
- Painted gypsum board provides adequate vapor control
- Avoids strict vapor barrier that prevents seasonal drying
- Flexibility accommodates bidirectional vapor drive
Assembly drying considerations:
- Class II retarders allow trapped construction moisture to escape
- Drying time 2-5× longer than highly permeable assemblies
- Sufficient for most residential and light commercial construction
- Inadequate for assemblies with high initial moisture content
Thickness Dependence
Foam plastic permeance decreases approximately linearly with thickness:
Extruded polystyrene:
- 1 inch: 1.0 perm
- 2 inches: 0.5 perm
- 3 inches: 0.33 perm
- 4 inches: 0.25 perm (transitions to Class I)
Closed-cell spray foam:
- 1 inch: 1.0 perm
- 2 inches: 0.5 perm
- 3 inches: 0.33 perm
- 3.5 inches: 0.29 perm (Class I threshold)
Performance Comparisons
Vapor transmission through 1000 ft² assembly at standard conditions (0.50 inches Hg vapor pressure difference):
| Material Configuration | Permeance | Daily Moisture Flow | Annual Accumulation |
|---|---|---|---|
| Oil paint, 3 coats | 0.5 perm | 0.25 lb/day | 91 lb/year |
| XPS 2 inches | 0.5 perm | 0.25 lb/day | 91 lb/year |
| Kraft-faced insulation | 0.3 perm | 0.15 lb/day | 55 lb/year |
| ccSPF 2 inches | 0.5 perm | 0.25 lb/day | 91 lb/year |
Installation Requirements
Painted vapor retarders:
- Apply minimum specified number of coats for rated permeance
- Ensure continuous coverage without holidays or thin spots
- Maintain wet film thickness per manufacturer specifications
- Allow proper cure time between coats
Faced insulation products:
- Install facing toward conditioned space (warm in winter)
- Seal all seams with compatible tape
- Staple flanges to framing members on interior face
- Avoid compression that creates gaps and air leakage paths
Rigid foam insulation:
- Stagger joints to avoid continuous vapor pathways
- Seal joints with compatible tape or foam adhesive
- Consider cumulative permeance of multiple layers
- Account for fastener penetrations reducing effective vapor resistance
Design Integration
Class II vapor retarders integrate into hygrothermal design through:
Climate-specific placement: Interior application in heating climates, exterior in cooling climates based on dominant vapor drive direction.
Drying pathway preservation: One side of assembly maintains permeance >5 perm to enable seasonal moisture removal.
Construction moisture management: Initial moisture content below 12% by mass in wood-based materials ensures drying occurs before mold risk develops.
Condensation plane verification: Dewpoint analysis confirms condensation potential remains at intended drainage plane, not within insulated cavity.
Low permeance materials provide practical vapor control for conventional construction, balancing moisture protection with the flexibility to accommodate imperfect installation and variable environmental conditions.