Very Low Permeance Materials

Very low permeance materials (Class I vapor retarders per IRC/IBC) exhibit permeance values below 0.1 perm (5.75 × 10⁻¹² kg/Pa·s·m²), effectively blocking water vapor transmission through building envelope assemblies. These materials function as vapor barriers in climates requiring strict moisture control.

Material Classifications

Metallic barriers provide absolute vapor impermeability:

• Aluminum foil (0.00 perm): Laminated facings on rigid insulation, reflective barriers
• Sheet metal (0.00 perm): HVAC ductwork, mechanical equipment enclosures
• Copper sheet (0.00 perm): Roof underlayment in specialized applications
• Stainless steel (0.00 perm): Food processing facilities, clean rooms

Polymer membranes combine low permeance with mechanical durability:

• Polyethylene 6 mil (0.06 perm): Standard crawlspace vapor barrier, slab-on-grade underslab
• Polyethylene 4 mil (0.08 perm): Wall cavity installation in extreme cold climates
• Cross-laminated polyethylene (0.02-0.05 perm): Premium crawlspace and underslab applications
• Reinforced polyethylene (0.03 perm): Heavy-duty foundation waterproofing

Elastomeric membranes provide seamless application:

• Rubber membrane roofing EPDM (0.01-0.05 perm): Single-ply roofing systems
• Bituminous coatings (0.02-0.08 perm): Foundation dampproofing, below-grade walls
• Spray-applied rubber (0.03-0.06 perm): Air and vapor barrier systems
• Peel-and-stick membranes (0.02-0.05 perm): Window and door rough opening flashing

Performance Characteristics

Water vapor transmission rate calculations for 1000 ft² assembly at 0.50 inches Hg vapor pressure difference:

Application Requirements

Climate zone specifications: Building codes mandate Class I vapor retarders in specific configurations:

• Cold climates (Zones 6-8): Interior vapor barriers in heating-dominated assemblies
• Marine climates (Zone 4 Marine): Exterior vapor barriers preventing inward vapor drive
• Hot-humid climates (Zones 1-2): Exterior impermeable claddings require interior permeability

Installation specifications:

• Continuous coverage across framing members without gaps or tears
• Sealed penetrations for electrical boxes, plumbing, mechanical services
• Overlapped seams minimum 6 inches with compatible tape or adhesive
• Mechanical fastening with compatible cap fasteners at 12-18 inches o.c.

Condensation Risk Management

Very low permeance materials prevent vapor diffusion but concentrate risk at discontinuities. Critical design considerations:

Cold side condensation: Vapor barriers placed on warm side of insulation prevent condensation on cold surfaces. Errors in placement cause:

• Trapped moisture at sheathing interface
• Ice formation within wall cavities
• Structural decay and mold growth

Double vapor barrier prohibition: Placing impermeable layers on both sides of assembly:

• Eliminates drying potential in either direction
• Concentrates construction moisture indefinitely
• Violates building code requirements

Air leakage concentration: Gaps in vapor barrier create high vapor flux zones:

• Local moisture accumulation exceeds diffusion-only calculations
• Requires sealed air barrier separate from vapor retarder

Special Applications

Underslab vapor barriers: 10 mil polyethylene or equivalent under concrete slabs:

• Prevents soil gas infiltration (radon, methane)
• Blocks ground moisture transmission
• Requires granular capillary break layer above
• Mandatory for conditioned spaces per IRC

Crawlspace ground covers: 6 mil minimum polyethylene:

• Reduces crawlspace relative humidity
• Protects wood framing from moisture damage
• Requires sealed seams and perimeter attachment
• Minimum 6-inch overlap at seams

Cold storage facilities: Continuous exterior vapor barriers:

• Prevents warm-side vapor drive into insulation
• Maintains insulation R-value at design conditions
• Requires robust air sealing at all penetrations

Compatibility Considerations

Material selection must account for:

• Substrate compatibility: Adhesive systems matched to substrate (concrete, wood, metal)
• UV resistance: Exposed applications require stabilized polymers
• Temperature range: Low-temperature flexibility for cold climate installation
• Chemical resistance: Resistance to soil chemicals, cleaning agents
• Puncture resistance: Mechanical protection during construction

Testing and Verification

ASTM E96 (wet cup method) determines permeance below 0.1 perm. Quality assurance requires:

• Third-party certification to stated permeance values
• Batch testing for large projects
• Field verification of continuous coverage
• Thermal imaging to identify installation defects

Very low permeance materials provide essential moisture control when properly specified, located, and installed according to climate-specific hygrothermal principles. Misapplication creates worse performance than omitting vapor control entirely.