Semi Permeable Assemblies

Semi-permeable assemblies utilize Class II or Class III vapor retarders that allow controlled moisture transmission, enabling bidirectional drying while providing adequate vapor control. These assemblies are optimal for mixed climates where both heating and cooling seasons create vapor drive conditions.

Vapor Retarder Classifications

Class II Vapor Retarders

Class II vapor retarders have permeance values between 0.1 and 1.0 perm (5.7 to 57 ng/(Pa·s·m²)).

Common Materials:

Class II retarders provide moderate vapor control while permitting seasonal drying. They reduce but do not eliminate vapor transmission, allowing assemblies to recover from wetting events.

Class III Vapor Retarders

Class III vapor retarders have permeance values between 1.0 and 10 perms (57 to 574 ng/(Pa·s·m²)).

Common Materials:

Class III retarders provide minimal vapor control while allowing significant drying. They are suitable for climates where bidirectional drying is critical.

Kraft Paper Vapor Retarders

Kraft paper bonded to fiberglass insulation batts serves as an integrated vapor retarder with Class II performance.

Performance Characteristics:

• Permeance: 0.4-0.6 perm (typical)
• Thickness: 30-50 lb kraft paper
• Temperature stability: -40°F to 180°F
• Installation: Stapled to framing members
• Coverage: Overlapping flanges at studs

Advantages:

• Integrated with insulation (single installation step)
• Inexpensive compared to separate membranes
• Provides labeled insulation R-value
• Acceptable permeance for many climates
• Familiar to contractors

Limitations:

• Permeance not adjustable
• Tears during installation
• Gaps at electrical boxes and penetrations
• Compression at staples creates leakage paths
• No airtightness contribution

Installation Requirements:

Install kraft facing toward conditioned space (warm-in-winter side). Overlap flanges at framing members by minimum 1 inch. Seal tears with compatible tape. Cut neatly around outlets and repair facings.

Latex Paint as Vapor Retarder

Multiple coats of latex paint on gypsum board create a Class III vapor retarder suitable for many climates.

Permeance Data:

Variables Affecting Permeance:

• Paint formulation (acrylic, vinyl-acrylic, 100% acrylic)
• Film thickness per coat (wet mil thickness)
• Number of coats applied
• Surface texture and absorption
• Substrate type and porosity
• Temperature during application
• Curing time between coats

Climate Applicability:

Latex paint vapor retarders function adequately in:

• Climate Zones 3 and 4 (mixed climates)
• Heating-dominant zones with moderate cooling
• Assemblies with exterior insulation
• Walls with water-managed cladding systems

Not recommended for:

• Extreme cold climates (Zone 6+ without additional control)
• High indoor humidity environments (pools, spas)
• Assemblies without drying capacity
• Locations with persistent interior moisture sources

Material Selection Criteria

Climate-Based Selection

Cold-Dominant Climates (Zones 5-8):

Select Class II vapor retarders (0.1-1.0 perm) to limit outward vapor drive during heating season. Ensure adequate exterior permeance for inward drying during summer.

Mixed Climates (Zones 3-4):

Class III vapor retarders (1.0-10 perms) allow bidirectional drying. Latex paint systems provide sufficient control for typical residential construction.

Hot-Humid Climates (Zones 1-2):

Class III or higher permeance required. Avoid interior vapor barriers that prevent inward drying from exterior wetting and air conditioning condensation.

Assembly Configuration

Cavity Insulation Only:

Vapor retarder required on winter-warm side per code. Class II recommended for Zones 5+, Class III acceptable for Zones 3-4.

Exterior Continuous Insulation:

Continuous insulation reduces vapor drive and lowers dewpoint location. Allows higher permeance interior layer. Calculate minimum R-value ratio per code.

Reservoir Claddings:

Materials that absorb and store water (brick, stucco) require higher interior permeance to facilitate inward drying. Class III minimum.

Moisture Load Considerations

Standard Residential Occupancy:

Indoor humidity typically 30-40% RH in winter. Class III vapor retarders adequate for most locations.

High-Moisture Sources:

Indoor pools, spas, large aquariums, humidified buildings require Class II or lower permeance. Mechanical dehumidification often necessary.

Tight Construction:

High-performance air barriers reduce unintended moisture transport. May require lower vapor retarder permeance to control diffusion.

Smart Vapor Retarders

Variable permeability membranes adjust permeance based on ambient relative humidity.

Performance Range:

Mechanism:

Polyamide or nylon film with hygroscopic properties. Molecular structure opens at high RH, closes at low RH. Response time 24-72 hours to humidity changes.

Applications:

• Mixed climates with significant seasonal variation
• Renovations where existing vapor barrier uncertain
• High-performance assemblies requiring bidirectional drying
• Cold climates with risk of summer inward vapor drive

Installation:

Install on interior side of cavity insulation. Overlap seams 6 inches minimum. Seal penetrations with compatible tape. Provide continuous coverage.

Bidirectional Drying Principles

Semi-permeable assemblies must allow drying in both directions to manage moisture from multiple sources.

Inward Drying Requirements:

• Exterior cladding moisture from rain
• Solar-driven vapor drive from wet cladding
• Air conditioner condensation in cooling climates
• Humid exterior air infiltration

Outward Drying Requirements:

• Interior moisture generation (occupants, cooking, bathing)
• Humidification systems
• Construction moisture in new buildings
• Plumbing leaks and interior water exposure

Design Strategy:

Interior permeance > 1.0 perm allows inward drying. Exterior sheathing and cladding system must provide water management and adequate permeance (typically >5 perms for sheathing, >10 perms for weather-resistive barrier).

Code Requirements

IRC and IBC specify vapor retarder requirements by climate zone. Class I, II, or III acceptable depending on location and assembly configuration. Exception provided for assemblies with sufficient exterior insulation to control condensation risk.

Compliance Paths:

1. Install Class I, II, or III vapor retarder per climate zone
2. Provide minimum exterior insulation R-value ratio
3. Demonstrate through hygrothermal analysis
4. Use materials listed in exception tables

Semi-permeable assemblies offer balanced moisture control for variable climate conditions, enabling assemblies to self-regulate and recover from wetting events while maintaining adequate vapor control during peak vapor drive seasons.