Perm Ratings

Perm ratings quantify the rate at which water vapor transmits through materials under specified conditions. The permeance value serves as the fundamental metric for selecting vapor retarders and designing moisture-safe building envelope assemblies.

Permeance Definition and Units

Permeance measures the time rate of water vapor transmission through unit area of flat material induced by unit vapor pressure difference between two surfaces.

U.S. (I-P) Units:

• Perm (grain/h·ft²·in.Hg)
• 1 perm = 1 grain/(h·ft²·in.Hg)
• Standard test conditions: 73.4°F (23°C), 50% RH

SI (Metric) Units:

• ng/(Pa·s·m²)
• 1 perm = 57.4 ng/(Pa·s·m²)
• Conversion factor: 57.4 × (I-P perms) = SI perms

Permeability vs. Permeance:

• Permeability: intrinsic material property independent of thickness
• Permeance: specific to tested thickness
• Relationship: Permeance = Permeability / Thickness

ASTM Classification System

ASTM E1745 establishes the standard classification for vapor retarders based on permeance values.

Class Definitions

Classification Significance

Class I Materials:

• Provide maximum resistance to vapor diffusion
• Risk of moisture accumulation if installed on wrong side of assembly
• Limit drying potential in both directions
• Examples: polyethylene film, aluminum foil, sheet metal

Class II Materials:

• Moderate vapor diffusion resistance
• Allow some drying capacity
• Suitable for moderate climate zones
• Examples: kraft-faced insulation, asphalt-coated paper

Class III Materials:

• Significant vapor transmission allowed
• Good drying potential
• Appropriate for assemblies requiring bidirectional drying
• Examples: latex paint, unfaced insulation

Test Methods and Standards

ASTM E96 Water Method

The standard test method for water vapor transmission of materials.

Test Procedure:

• Material specimen sealed over dish containing water or desiccant
• Assembly placed in controlled environment
• Weight change measured over time
• Permeance calculated from transmission rate

Test Conditions:

• Procedure A (Desiccant Method): 90°F, 0% to 50% RH gradient
• Procedure B (Water Method): 73.4°F, 50% to 100% RH gradient
• Procedure C (Inverted Cup): Various temperature/humidity conditions

Calculation:

Permeance = (W × t) / (A × Δp)

Where:

• W = weight change (grains)
• t = time (hours)
• A = test area (ft²)
• Δp = vapor pressure difference (in.Hg)

ASTM E96 Dry Cup Method

Used for very low permeance materials.

Advantages:

• Better resolution for low-perm materials
• Reduced test duration for impermeable specimens
• More accurate for Class I materials

Limitations:

• Different results than wet cup method
• Not representative of all field conditions
• Temperature-dependent results

Material Perm Ratings

Class I Materials (≤0.1 Perm)

Class II Materials (0.1-1.0 Perm)

Class III Materials (1.0-10 Perm)

Permeable Materials (>10 Perm)

Variable Permeance Materials

Advanced vapor retarders with humidity-dependent permeability provide adaptive moisture control.

Operating Principle

Dry Conditions (Winter):

• Material exhibits low permeance (Class II)
• Reduces outward vapor drive
• Protects insulation from condensation

Humid Conditions (Summer):

• Material exhibits higher permeance (Class III)
• Allows inward drying
• Prevents moisture accumulation

Permeance Response Curve

Typical “smart” vapor retarder characteristics:

Common Products

• Nylon-based membranes
• Modified kraft facings
• Polymer-based “smart” sheets
• Coating systems

Factors Affecting Permeance

Temperature Dependence

Water vapor permeability increases exponentially with temperature.

Arrhenius Relationship:

μ(T) = μ₀ × exp[-E/(R×T)]

Where:

• μ(T) = permeability at temperature T
• μ₀ = reference permeability
• E = activation energy
• R = gas constant
• T = absolute temperature (K)

Practical Impact:

• Doubling of permeance possible over 20-40°F range
• Cold-side materials exhibit lower transmission
• Summer conditions increase vapor flow rates

Humidity Effects

Many materials exhibit non-linear permeance response to relative humidity.

Hygroscopic Materials:

• Wood products: permeance increases at high RH
• Gypsum board: moisture content affects transmission
• Paper facings: significant RH dependence

Non-Hygroscopic Materials:

• Plastics: minimal RH effect
• Metals: no RH dependence
• Foams: slight variation with humidity

Aging and Degradation

Long-term permeance stability varies by material type.

Stable Materials:

• Polyethylene films
• Metal foils
• Glass
• Extruded polystyrene

Degrading Materials:

• Asphalt coatings (oxidation)
• Some paints (weathering)
• Kraft facings (moisture cycling)
• Adhesives (UV exposure)

Design Considerations

Climate Zone Application

Cold Climates (Zones 6-8):

• Class I or II vapor retarder on interior (warm side)
• Permeable materials on exterior (cold side)
• Prevents winter condensation in insulation

Mixed Climates (Zones 4-5):

• Class II or III vapor retarder recommended
• Consider variable permeance materials
• Balance winter protection with summer drying

Hot-Humid Climates (Zones 1-3):

• No interior vapor retarder recommended
• Class III or permeable interior finish
• Vapor control on exterior if required

Assembly Perm Analysis

The “5:1 rule” for assembly design:

Winter-Dominated Climate:

• Exterior materials should have ≥5× the permeance of interior
• Ratio = Perm(exterior) / Perm(interior) ≥ 5
• Ensures any diffused moisture can escape outward

Summer-Dominated Climate:

• Interior materials should have ≥5× the permeance of exterior
• Ratio = Perm(interior) / Perm(exterior) ≥ 5
• Allows inward-driven moisture to dry toward interior

Continuous Insulation Impact

Exterior continuous insulation alters vapor pressure profiles.

High R-Value Exterior Insulation:

• Raises sheathing temperature
• Reduces condensation potential
• May eliminate need for interior vapor retarder

Critical R-Value Ratio:

R(exterior) / R(cavity) > Critical Ratio

Critical ratio varies by climate (0.2 to 0.5 typical).

Code Requirements

International Building Code (IBC)

IBC Section 1404 addresses vapor retarders.

Requirements:

• Class I or II vapor retarder required in Climate Zones 5 and higher
• Exceptions for specific assemblies
• Compliance with IECC Chapter 4

Exceptions:

• Basement walls
• Below-grade portions of walls
• Construction where moisture/weathering deterioration is prevented
• Walls of groups R-2, R-3, R-4 in Climate Zones 1, 2, 3

International Residential Code (IRC)

IRC Section R702.7 specifies vapor retarder installation.

Class I or II Required:

• Climate Zones 5, 6, 7, 8, Marine 4
• Installed on interior side of frame wall

Class III Permitted:

• With specific exterior insulation ratios
• With vented cladding systems
• In Climate Zones 1-4 except Marine 4

ASHRAE Standards

ASHRAE 90.1 Section 5.5.15:

• Vapor retarder requirement based on climate
• Exception for demonstrated moisture-safe assembly

ASHRAE 160:

• Moisture analysis methodology
• Performance-based compliance option
• Hygrothermal modeling requirements

Testing and Verification

Quality Control Testing

Manufacturing QC:

• ASTM E96 at regular intervals
• Batch testing for consistency
• Specification compliance verification

Field Verification:

• Visual inspection for damage
• Continuity testing
• Seam integrity confirmation

Installation Quality

Critical Details:

• Continuous vapor retarder plane
• Sealed penetrations (electrical boxes, pipes)
• Sealed joints and seams
• Overlap requirements (6 inches minimum)

Common Defects:

• Tears or punctures
• Unsealed penetrations
• Gaps at transitions
• Inadequate overlap

Measurement Uncertainty

Test Variability

ASTM E96 typical coefficient of variation:

Specification Tolerances

Recommended specification approach:

• Class I: specify maximum perm (e.g., “0.1 perm maximum”)
• Class II: specify range (e.g., “0.1 to 1.0 perm”)
• Class III: specify minimum perm (e.g., “1.0 perm minimum”)
• Include test method reference (ASTM E96, Method A or B)

Multilayer Assemblies

Series Resistance Calculation

For layered assemblies, total vapor resistance equals sum of individual resistances.

Resistance (Reperm):

Reperm = 1 / Permeance

Total Assembly Resistance:

Reperm,total = Reperm,1 + Reperm,2 + … + Reperm,n

Assembly Permeance:

Perm,assembly = 1 / Reperm,total

Example Calculation

Wall assembly:

• Interior paint: 5 perm
• Gypsum board: 4 perm
• Kraft-faced insulation: 0.4 perm
• OSB sheathing: 0.7 perm
• Housewrap: 50 perm

Reperm values:

• Paint: 1/5 = 0.20
• Gypsum: 1/4 = 0.25
• Kraft: 1/0.4 = 2.50
• OSB: 1/0.7 = 1.43
• Housewrap: 1/50 = 0.02

Total: 4.40

Assembly permeance: 1/4.40 = 0.23 perm (Class II)

Controlling Layer: Kraft facing dominates resistance.

Advanced Considerations

Air Leakage vs. Vapor Diffusion

Vapor transport via air leakage typically exceeds diffusion by 10:1 to 100:1 ratio.

Relative Importance:

1. Air barrier continuity (primary concern)
2. Vapor retarder placement (secondary concern)
3. Material perm rating (tertiary concern)

Combined Strategy:

• Seal air leakage paths first
• Design vapor control second
• Select appropriate perm ratings third

Moisture Storage Capacity

Materials with high moisture storage can buffer vapor transmission.

Hygric Buffer:

• Wood products: 50-150 kg/m³ moisture capacity
• Gypsum: 20-40 kg/m³
• Concrete: 80-120 kg/m³

This storage delays moisture accumulation and can prevent condensation.

Capillary vs. Diffusion Transport

At high moisture contents, capillary liquid transport exceeds vapor diffusion.

Transition Point:

• Occurs at 95-98% RH
• Capillary flow 100-1000× faster than diffusion
• Important for rain penetration, not typical diffusion scenarios

References and Resources

ASTM Standards:

• ASTM E96: Standard Test Methods for Water Vapor Transmission
• ASTM E1745: Standard Specification for Plastic Water Vapor Retarders
• ASTM E2178: Standard Test Method for Air Permeance of Building Materials

Building Codes:

• International Building Code (IBC)
• International Residential Code (IRC)
• International Energy Conservation Code (IECC)

ASHRAE Resources:

• ASHRAE Handbook—Fundamentals, Chapter 26: Heat, Air, and Moisture Control
• ASHRAE Standard 160: Criteria for Moisture-Control Design Analysis
• ASHRAE Standard 90.1: Energy Standard for Buildings

Industry Guidelines:

• Building Science Corporation: Moisture Control Strategies
• Oak Ridge National Laboratory: Moisture Control Guidance
• Canada Mortgage and Housing Corporation (CMHC): Vapor Barriers