Drying To Exterior

Overview

Drying to the exterior represents a moisture management strategy where wall assemblies are designed to permit outward vapor diffusion, allowing moisture that enters the assembly to escape through the exterior cladding and sheathing. This approach requires careful coordination of material permeance values and understanding of seasonal vapor drive directions.

Fundamental Principles

Vapor Drive Direction

The direction of vapor flow through assemblies is governed by vapor pressure differentials:

Winter Conditions:

Interior vapor pressure: 800-1200 Pa (heated, humidified spaces)
Exterior vapor pressure: 200-400 Pa (cold, dry air)
Net vapor drive: Outward (interior to exterior)
Dominant drying direction: Limited during heating season

Summer Conditions:

Exterior vapor pressure: 1400-2000 Pa (warm, humid air)
Interior vapor pressure: 600-800 Pa (air-conditioned spaces)
Net vapor drive: Inward (exterior to interior)
Critical consideration: Assembly must handle bidirectional drying

Shoulder Seasons:

Vapor drive varies with diurnal temperature cycles
Solar-driven moisture can create outward pulses
Maximum drying potential occurs during these periods

Exterior Sheathing Permeance Requirements

Permeance Thresholds

High Permeance (>10 perms):

Fiber-faced gypsum sheathing: 15-60 perms
Asphalt-impregnated fiberboard: 20-50 perms
Unfaced rigid mineral wool: 30+ perms
Application: Maximum drying potential for moisture-sensitive assemblies

Moderate Permeance (5-10 perms):

Plywood (1/2"): 0.5-1.5 perms (insufficient for drying)
OSB (7/16"): 0.5-1.0 perms (insufficient)
Coated fiberboard: 5-10 perms
Application: Requires careful detailing and climate analysis

Low Permeance (<1 perm):

Exterior foam insulation (>1" XPS/polyiso): 0.4-1.0 perms
Foil-faced products: <0.1 perms
Application: Creates drying barrier, requires interior drying path

Material Selection by Climate Zone

Cold Climates (Climate Zones 6-8):

Minimum sheathing permeance: 10 perms
Preferred materials: Fiber-faced gypsum, coated fiberboard
Avoid: Impermeable foam without interior drying strategy
Rationale: Outward vapor drive dominant, maximize drying capacity

Mixed Climates (Climate Zones 4-5):

Target sheathing permeance: 5-15 perms
Balance inward/outward drying potential
Consider: Membrane systems with variable permeance
Rationale: Bidirectional vapor drive requires flexibility

Cold-Humid Climates (Marine 4, 5):

Minimum sheathing permeance: 15 perms
Enhanced drainage plane ventilation
Avoid double vapor barriers at all costs
Rationale: High exterior moisture loads require maximum drying

Winter Drying Potential

Drying Mechanisms

Vapor Diffusion:

Primary mechanism when temperature gradient exists
Rate determined by: ΔP × permeance / thickness
Effectiveness: Limited during peak winter (low outdoor vapor pressure)
Enhancement: Air space ventilation between sheathing and cladding

Air Leakage Drying:

Exfiltration carries significant moisture loads
Rate: 50-100 times greater than diffusion for equivalent air/moisture flow
Control requirement: Must be managed, not relied upon
Risk: Condensation at cool surfaces during exfiltration

Ventilation Drying:

Rainscreen air spaces provide convective drying
Minimum gap: 3/8" for effective drainage and drying
Optimal gap: 3/4"-1" for maximum ventilation
Enhancement: Top and bottom vents create stack effect

Drying Capacity Calculations

Winter drying potential (grains/hr·ft²):

Diffusion Only:

Q = (P_i - P_o) × M / R_total
Where: P = vapor pressure (Pa), M = permeance (perms), R = resistance (1/perm)
Typical winter rate: 0.01-0.05 grains/hr·ft²
Limitation: Insufficient for bulk water events

Enhanced with Ventilation:

Add convective term: h_c × ΔW × A
Where: h_c = convection coefficient, ΔW = humidity ratio difference
Typical enhanced rate: 0.1-0.5 grains/hr·ft²
Application: Rainscreen systems significantly improve drying

Assembly Design Requirements

Layering Strategy (Exterior to Interior)

Exterior Cladding:
- Permeable or ventilated
- Examples: Wood siding, fiber cement, brick veneer with weeps
Drainage Plane:
- Minimum 3/8" air space
- Continuous from bottom to top
- Top and bottom ventilation openings
Weather Resistive Barrier (WRB):
- Permeance >10 perms preferred
- Examples: Spun-bonded polyolefin (30-60 perms)
- Avoid: Vapor impermeable membranes
Exterior Sheathing:
- Permeance >10 perms for cold climates
- Structural capacity as required
- Examples: Fiber-faced gypsum, coated fiberboard
Insulation:
- If exterior: Limit thickness to control sheathing temperature
- If cavity: Ensure sheathing stays above dewpoint
- Hybrid systems: Balance thermal and vapor control
Interior Vapor Control:
- Class III (1-10 perms) for bidirectional drying
- Examples: Vapor retarder paint, kraft facing
- Avoid: Polyethylene in mixed/marine climates

Critical Details

Ventilation Opening Design:

Net free area: Minimum 1:100 (opening:wall area)
Bottom openings: Bug screen reduces area 50%
Top openings: Must prevent rain entry while allowing airflow

WRB Integration:

Seal all penetrations
Maintain continuity at corners and transitions
Integrate with fenestration flashing
Tape all seams with compatible tape

Foam Sheathing Considerations:

XPS >1": Permeance drops to 0.4-0.6 perms (vapor barrier)
Strategy: Use interior Class III vapor retarder for inward drying
Alternative: Limit foam thickness, use permeable foam types
Monitor: Sheathing temperature must stay >45°F to prevent condensation

Failure Modes

Inadequate Drying Capacity

Symptom: Chronic moisture accumulation at sheathing interface

Causes:

Impermeable exterior layers (foam >2", vinyl wallpaper effect)
Double vapor barriers (interior poly + exterior foam)
Insufficient ventilation gap
Blocked drainage path

Consequence:

Mold growth on sheathing
Wood rot in structural members
Reduced R-value of wet insulation
Fastener corrosion

Prevention:

Calculate total assembly permeance
Ensure drying path exists in at least one direction
Verify through hygrothermal modeling

Summer Inward Vapor Drive Issues

Symptom: Interior surface condensation during cooling season

Mechanism:

Solar heating drives moisture from wet cladding
Vapor moves inward through permeable sheathing
Condenses on cool interior vapor barrier or AC ductwork

Risk Factors:

Absorptive cladding (brick, stucco) after rain
Dark colors (increased solar absorption)
High cooling loads (low interior surface temperatures)
Impermeable interior layer (Class I vapor barrier)

Mitigation:

Use rainscreen to reduce cladding wetting
Interior Class III vapor retarder (allows inward drying)
Avoid interior polyethylene in Climate Zones 1-5
Enhanced interior ventilation during humid periods

Condensation at Impermeable Layers

Critical Location: First condensing surface encountered by vapor flow

Winter Scenario:

Outward vapor flow
Condensation at exterior impermeable layer
Examples: Cold side of exterior foam, vinyl wallpaper on exterior sheathing

Assessment:

Calculate dewpoint temperature through assembly
Compare to actual temperature profile
Verify surface temperature >dewpoint at all layers

Design Fix:

Remove impermeable layer, or
Increase thermal resistance exterior to layer (warm it above dewpoint), or
Reduce interior humidity (source control, ventilation)

Air Barrier Discontinuity

Problem: Air leakage deposits moisture at random locations

Common Failures:

Electrical penetrations
Plumbing chases
Rim joist connections
Window rough openings

Impact:

Localized wetting far exceeds diffusion loads
Ice dam formation (attic applications)
Condensation at sheathing

Solution:

Continuous air barrier system
Seal all penetrations with appropriate materials
Test building tightness: Target <3 ACH50 for cold climates

Climate-Specific Requirements

Climate Zone 6 (Cold)

Design Parameters:

Heating degree days: 5400-7200°F-days
Winter design temp: -10 to 0°F
Interior RH target: 30-40% at -10°F outdoor

Assembly Requirements:

Exterior sheathing: >10 perms
Rainscreen gap: Minimum 3/8"
Interior vapor retarder: Class II or III (0.1-10 perms)
Insulation level: R-20 to R-28 wall

Critical Consideration: Outward vapor drive dominant, prioritize exterior drying

Climate Zone 7 (Very Cold)

Design Parameters:

Heating degree days: 7200-9000°F-days
Winter design temp: -20 to -10°F
Interior RH target: 25-30% at -20°F outdoor

Assembly Requirements:

Exterior sheathing: >15 perms preferred
Mandatory rainscreen with ventilation
Interior vapor retarder: Class II (0.1-1.0 perms)
Insulation level: R-28 to R-35 wall

Critical Consideration: Extreme cold requires careful condensation control, but low absolute humidity reduces risk

Climate Zone 8 (Subarctic)

Design Parameters:

Heating degree days: >9000°F-days
Winter design temp: <-20°F
Interior RH target: 20-25% maximum

Assembly Requirements:

Exterior sheathing: >15 perms
Enhanced ventilation drying (1" air gap)
Interior vapor barrier: Class I acceptable (poly sheeting)
Insulation level: R-35+ wall

Critical Consideration: Extreme winter severity justifies interior vapor barrier, summer inward drive negligible

Marine Climate 4 (Cold-Humid)

Design Parameters:

High exterior moisture loads year-round
Moderate heating season
High cooling season humidity

Assembly Requirements:

Exterior sheathing: >20 perms (maximum drying)
Mandatory rainscreen: 3/4" minimum gap
Interior vapor retarder: Class III only (bidirectional drying critical)
Enhanced drainage plane design

Critical Consideration: Bidirectional drying essential, never use interior polyethylene

Design Verification

Hygrothermal Modeling

Tools:

WUFI (Fraunhofer IBP)
MOISTURE-EXPERT (Oak Ridge National Laboratory)
DELPHIN (TU Dresden)

Validation Criteria:

Sheathing moisture content: <20% year-round
No sustained condensation periods (>30 days)
Net drying during annual cycle
Surface RH <80% to prevent mold

Performance Monitoring

Sensor Locations:

Exterior sheathing outboard face
Mid-cavity (insulation)
Interior sheathing face (if present)

Measured Parameters:

Temperature: ±0.5°F accuracy
Relative humidity: ±2% accuracy
Calculated: Moisture content, dewpoint

Acceptance:

First winter: Verify no condensation
Annual cycle: Confirm net drying
Long-term: Stable or decreasing moisture levels

Summary

Drying-to-exterior assemblies provide effective moisture management in cold climates where outward vapor drive dominates. Success requires exterior materials with adequate permeance (>10 perms), ventilated drainage planes, and avoidance of double vapor barriers. Material selection must account for climate-specific vapor drive patterns, with particular attention to summer inward drive in mixed climates. Proper detailing of air barriers and integration of all weather-resistive elements ensures long-term durability and prevents common failure modes associated with trapped moisture.