Medium Permeance Materials

Medium permeance materials (Class III vapor retarders at lower range) exhibit permeance values between 1.0 and 10 perm (5.75 × 10⁻¹¹ to 5.75 × 10⁻¹⁰ kg/Pa·s·m²), providing moderate vapor resistance while maintaining sufficient permeability to enable seasonal moisture drying in most climate conditions.

Material Categories

Wood structural panels:

• Plywood 3/8 inch (0.7 perm): Borderline low/medium permeance
• Plywood 1/2 inch (0.5 perm): Low permeance classification
• Oriented strand board 7/16 inch (0.5-2.0 perm): Variable by manufacturing process
• OSB with permeable coating (2-5 perm): Enhanced drying capacity
• Plywood exterior grade (0.4-1.5 perm): Depends on adhesive and veneer thickness

Painted gypsum board:

• Gypsum 1/2 inch unpainted baseline (50 perm)
• Latex paint 1 coat (20-35 perm): Minimal vapor retardance
• Latex paint 2 coats (10-20 perm): Moderate vapor control
• Latex paint 3 coats (5-10 perm): Significant vapor retardance
• Acrylic primer + latex topcoat (8-15 perm): Typical interior finish system

Building papers and felts:

• No. 15 felt asphalt-saturated (5-10 perm): Traditional roof underlayment
• No. 30 felt (2-5 perm): Heavy-duty applications, lower permeance
• Grade D building paper (3-8 perm): Wall sheathing weather barrier
• Synthetic underlayment (5-15 perm): Modern roofing underlayment

Modified sheathing products:

• Coated OSB water-resistive (3-6 perm): Integrated weather barrier
• Gypsum sheathing exterior (20-30 perm): High end of medium range
• Fiber-cement board (8-12 perm): Durable exterior substrate
• Structural fiberboard (10-20 perm): Permeable sheathing option

Permeance Variation Factors

Material permeance in this range varies significantly with environmental conditions:

Hygrothermal Performance

Medium permeance materials balance vapor control with drying capacity:

Moisture flux calculations for 1000 ft² assembly at 0.50 inches Hg vapor pressure difference:

Climate Compatibility

Cold climates (Zones 6-8):

• Exterior medium permeance acceptable with interior vapor control
• OSB/plywood sheathing provides adequate outward drying
• Interior latex-painted gypsum functions as “smart” vapor retarder
• Higher permeance during summer humidity enables inward drying

Mixed climates (Zones 3-5):

• Medium permeance optimal for both sides of assembly
• Accommodates bidirectional vapor drive
• Painted gypsum interior + coated OSB exterior typical
• Sufficient drying in both heating and cooling seasons

Hot-humid climates (Zones 1-2):

• Interior medium-high permeance essential for inward drying
• Latex paint on gypsum provides adequate vapor openness
• Exterior impermeable cladding acceptable with permeable interior
• Air conditioning creates inward vapor drive requiring interior permeability

Smart Vapor Retarder Behavior

Latex paint on gypsum exhibits “smart” vapor retarder characteristics:

• Permeance increases at high relative humidity
• Winter conditions (low interior RH): 15-20 perm, provides vapor control
• Summer conditions (high interior RH): 5-10 perm, allows inward drying
• Responds to seasonal vapor drive direction automatically

OSB and plywood show similar hygroscopic response:

• Dry winter conditions: Lower permeance, prevents condensation
• Wet summer conditions: Higher permeance, enables drying
• Self-regulating behavior improves assembly resilience

Assembly Design Integration

Typical wall assembly (cold climate):

• Exterior: Vinyl siding (high perm) over housewrap (high perm)
• Sheathing: OSB 7/16 inch (0.7-2.0 perm, medium-low)
• Cavity: Fiberglass insulation (>100 perm)
• Interior: Gypsum 1/2 inch with latex paint (10-15 perm, medium)
• Vapor drive: Primarily outward in winter, bidirectional drying capability

Roof assembly (mixed climate):

• Roofing: Asphalt shingles (0.0 perm)
• Underlayment: Synthetic (10 perm, medium-high)
• Sheathing: Plywood 1/2 inch (0.5 perm, low)
• Cavity: Ventilated air space or permeable insulation
• Interior: Gypsum with latex paint (10 perm, medium)

Construction Moisture Management

Medium permeance materials handle typical construction moisture:

Initial moisture content in new construction:

• Lumber framing: 15-19% moisture content (MC)
• Gypsum board: 0.5-1.0% MC by mass
• Concrete: 50-75% of mix water evaporates into assembly

Drying timeline with medium permeance envelope:

• First heating season: 60-70% of construction moisture removal
• First year complete: 85-95% moisture removal
• Two years: Equilibrium moisture content achieved
• Faster than low-perm assemblies, slower than high-perm

Installation Requirements

Medium permeance materials require standard installation practices:

OSB/plywood sheathing:

• Maintain 1/8 inch expansion gaps at panel edges
• Install per manufacturer nailing schedule
• Integrate with weather-resistive barrier per code
• Protect from prolonged wetting during construction

Painted finishes:

• Apply specified number of coats for design permeance
• Follow manufacturer wet film thickness recommendations
• Allow adequate cure time before enclosing assemblies
• Maintain continuous coverage without holidays

Building papers and felts:

• Horizontal lap minimum 2 inches, vertical lap 6 inches
• Seal around penetrations with compatible materials
• Install shingle-fashion to shed water
• Protect from UV exposure per manufacturer specifications

Medium permeance materials provide practical vapor control for mainstream construction, offering sufficient drying capacity to manage normal moisture loads while preventing excessive vapor transmission during peak heating or cooling conditions.