ASHRAE 160 Standard

ASHRAE Standard 160 provides design criteria for moisture control in buildings to prevent mold growth and material deterioration. The standard establishes surface relative humidity limits and temperature-RH combinations that predict conditions favorable for mold development.

Standard Scope and Application

ASHRAE 160 applies to:

• Residential buildings
• Commercial buildings
• Institutional facilities
• New construction and major renovations
• All climate zones

The standard addresses moisture control during both design and construction phases.

Design Criteria Philosophy

ASHRAE 160 establishes performance-based criteria:

Fundamental Principle: Building envelope assemblies must be designed to limit moisture accumulation such that mold growth does not occur under normal occupancy conditions.

Performance Metric: Surface relative humidity and temperature combinations at material interfaces.

Time-Dependent Assessment: 30-day running average method accounts for material moisture buffering capacity.

Surface RH Limits

The standard specifies maximum allowable surface relative humidity based on surface temperature:

Critical Threshold: Surface RH should not exceed 80% when surface temperature is above 41°F (5°C) for extended periods.

Temperature-RH Combinations

Mold growth potential increases with both temperature and humidity. Critical combinations:

Optimal Mold Growth: 77-86°F (25-30°C) with RH > 85%.

30-Day Running Average Method

The standard uses a 30-day running average calculation to account for material moisture buffering:

Calculation Method:

1. Calculate hourly surface temperature and RH for one year
2. Compute 30-day running average RH for each hour
3. Compare running average against criteria limits
4. Identify exceedance periods

Formula:

RH_30day(t) = (1/720) × Σ[RH(t-i)] for i = 0 to 719 hours

Where:

• RH_30day(t) = 30-day running average RH at time t
• RH(t-i) = hourly RH at time t minus i hours
• 720 hours = 30 days × 24 hours/day

Compliance Criterion: The 30-day running average RH must not exceed limits in the table above.

Climate Data Requirements

ASHRAE 160 analysis requires hourly climate data:

Essential Parameters:

• Outdoor dry-bulb temperature (hourly)
• Outdoor relative humidity (hourly)
• Solar radiation (direct and diffuse)
• Wind speed and direction
• Rainfall data
• Ground temperature

Data Sources:

Minimum Duration: One full year (8760 hours) of climate data required.

Material Properties for Analysis

Surface conditions depend on material hygrothermal properties:

Required Properties:

Material Classes:

• Class I (vapor impermeable): < 0.1 perm
• Class II (vapor semi-impermeable): 0.1-1.0 perm
• Class III (vapor semi-permeable): 1.0-10 perm
• Class IV (vapor permeable): > 10 perm

Analysis Methods

ASHRAE 160 compliance can be demonstrated through three approaches:

1. Simplified Prescriptive Method

Application: Simple assemblies in mild climates.

Requirements:

• Continuous air barrier
• Thermal insulation meeting code minimums
• Vapor retarder placement per climate zone
• Proper flashing and drainage

Limitations: Not applicable for complex assemblies or severe climates.

2. Intermediate Calculation Method

Application: Standard assemblies with known properties.

Process:

1. Calculate steady-state temperature distribution
2. Determine surface temperatures at critical interfaces
3. Calculate surface RH from indoor/outdoor conditions
4. Compare against criteria limits

Tools: Spreadsheet calculations, simple software.

3. Advanced Hygrothermal Simulation

Application: Complex assemblies, innovative designs, severe climates.

Process:

1. Model full assembly with all layers
2. Input hourly climate data
3. Run transient heat and moisture simulation
4. Calculate hourly surface conditions
5. Apply 30-day running average
6. Verify compliance

Software: WUFI, DELPHIN, MOISTURE-EXPERT, hygIRC.

Critical Surface Locations

Analysis must evaluate RH at moisture-sensitive interfaces:

Exterior Assemblies:

• Exterior sheathing inner surface
• Interior surface of exterior insulation
• First condensing surface inward from exterior
• Roof deck underside
• Rim joist interfaces

Interior Conditions:

• Behind vapor retarders
• Cold corners and thermal bridges
• Window and door perimeters
• Basement walls
• Slab edges

Indoor Humidity Assumptions

ASHRAE 160 specifies design indoor humidity levels:

Residential Buildings:

Additional Sources:

• 1.0 gal/day per occupant (moisture generation)
• Cooking, bathing, laundry activities
• Mechanical ventilation rate: 0.35 ACH or 15 CFM/person

Commercial Buildings: Maintain 40-60% RH per ASHRAE 55 with mechanical systems.

Compliance Verification

Demonstrating compliance with ASHRAE 160 requires:

Documentation Requirements

Design Phase:

• Climate data source and location
• Assembly drawings and specifications
• Material property data
• Calculation method and results
• Critical surface identification

Compliance Report Must Include:

Verification Procedures

Step 1: Identify Critical Surfaces

• Locate first condensing surface
• Identify thermal bridges
• Consider material compatibility

Step 2: Determine Surface Conditions

• Calculate hourly temperature
• Calculate hourly RH
• Apply 30-day running average

Step 3: Compare Against Criteria

• Check maximum monthly average
• Check 30-day running average
• Identify exceedances

Step 4: Evaluate Compliance

• Pass: All criteria met
• Fail: Any exceedance requires redesign

Failure Consequences and Redesign

When analysis shows non-compliance:

Common Solutions:

• Increase thermal resistance (reduce surface temperature depression)
• Relocate vapor control layer
• Add ventilation or drainage
• Modify interior humidity control
• Change materials to more vapor-open
• Add exterior insulation
• Improve air sealing

Redesign Verification: Rerun analysis with proposed modifications until compliance achieved.

Relationship to Building Codes

ASHRAE 160 referenced by:

• International Energy Conservation Code (IECC)
• International Residential Code (IRC)
• Local jurisdictions for moisture control verification
• Green building rating systems (LEED, NGBS)

Code Requirement: Some jurisdictions mandate ASHRAE 160 analysis for assemblies with insulation exceeding code minimums or in moisture-prone climates.

Limitations and Considerations

Standard Limitations:

• Addresses mold only, not structural moisture damage
• Assumes proper construction and installation
• Does not cover construction moisture
• Limited to normal occupancy patterns
• Does not address water intrusion events

Conservative Nature: The 80% RH limit at warm temperatures provides safety margin below actual mold germination thresholds (typically 85-95% RH).

Professional Judgment: Complex projects require experienced building science analysis beyond minimum standard compliance.

Implementation Strategy

Design Process Integration:

1. Preliminary assembly design based on thermal performance
2. ASHRAE 160 analysis at design development
3. Iteration if non-compliant
4. Final verification before construction documents
5. Construction quality assurance to match design assumptions
6. Post-occupancy verification if concerns arise

Quality Control: Site inspections to verify proper installation of air barriers, vapor retarders, insulation continuity, and flashing details critical to moisture performance.