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    • HERS RATER WRITTEN EXAM >
      • Section 1 Building Science Fundamentals >
        • 1a. Basic Terms & Definitions >
          • 1. Airflow in Buildings
          • 2. Equipment Efficiencies
          • 3. Power and Energy
          • 4. Effective Leakage Area
          • 5. Area Weighted R-Value
          • 6. Baseload / Seasonal Energy Use
          • 7. Driving Forces (Including Natural and Mechanical)
          • 8. Behavior of Radiation
          • 9. Thermal Resistance / Transmittance: R and U Values
          • 10. Latent / Sensible Heat
          • 11. Total Equivalent Length
          • 12. Dehumidification / Humidification
          • 13. Convert Pressure Units
          • 14. Thermal Bridges
          • 15. Pressure Boundary
          • 16. Stack Effect
          • 17. Exfiltration and Infiltration
          • 18. Natural / Mechanical Ventilation
          • 19. Net Free Area
          • 20. Input & Output Capacity
          • 21. Peak Electrical Demand
          • 22. Permeability and Perm Rating
          • 23. Standby Loss
          • 24. IAQ (indoor air quality): Moisture, CO, Dust
        • 1b. Principals of Energy, Air & Moisture Thermodynamics >
          • 1. Thermodynamics: Conduction, Convection, Radiation, ΔT
          • 2. Factors That Affect Insulation Performance
          • 3. House Pressurization/Depressurization by Various Forces
          • 4. Heat Gain / Loss
          • 5. Power and Energy
          • 6. Moisture Transport Mechanisms
          • 7. Identify Areas of Highest Relative Humidity
          • 8. Principles of Combustion
        • 1c. Combustion Safety >
          • 1. Combustion Analysis
          • 2. Carbon Monoxide (CO) Testing
          • 3. Combustion Appliance Venting, Draft, Combustion Air & Sizing
          • 4. Understand Combustion Safety Issues
      • Section 2 Buildings and Their Systems >
        • 2a. Building Components >
          • 1. Identify basic duct configurations and components
          • 2. Identify Basic Hydronic Distribution Configurations and Components
          • 3. Identify Basic Structural Components of Residential Construction
          • 4. Thermal Boundaries and Insulation Applications
          • 5. Basic Electrical Components and Safety Considerations
          • 6. Basic Fuel Delivery Systems and Safety Considerations
          • 7. Basic bulk water management components (drainage plumbing gutters sumps etc)
          • 8. Vapor barriers/retarders
          • 9. Radiant Barrier Principles and Installations
          • 10. Understand Fenestration Types and Efficiencies
          • 11. Understand Issues Involved With Basements, Crawlspaces, Slabs, Attics, Attached Garages, Interstitial Cavities, and Bypasses
          • 12. Understand Issues Involved With Ventilation Equipment
          • Understand Basic Heating / Cooling Equipment Components Controls and Operation
          • Understand Basic DHW Equipment Components Controls and Operation
          • Identify Common Mechanical Safety Controls
          • Identify Insulation Types and R-Values
          • Understand Various Mechanical Ventilation Equipment and Strategies: Spot, ERV, HRV
        • Conservation Strategies >
          • Appropriate Insulation Applications and Installation Based On Existing Conditions
          • Opportunity for ENERGY STAR Lighting and Appliances
          • Identify Duct Sealing Opportunities and Applications
          • Understand Importance of Air Leakage Control and Remediation Procedures
          • Blower Door-Guided Air Sealing Techniques
          • Water Conservation Devices and Strategies
          • Domestic Hot Water (DHW) Conservation Strategies
          • Heating & Cooling Efficiency Applications
          • Proper Use of Modeling to Determine Heating and Cooling Equipment Sizing and Appropriate Energy
          • Understand the Use of Utility History Analysis in Conservation Strategies
          • Appropriate Applications For Sealed Crawlspaces Basements and Attics
          • Identify / Understand High Density Cellulose
          • Appropriate Applications for Fenestration Upgrades Including Modification or Replacement
        • Comprehensive Building Assessment Process >
          • Determine Areas of Customer Complaints / Concerns in Interview
          • Understand / Recognize Need For Conducting Appropriate Diagnostic Procedures
          • Interaction Between Mechanical Systems, Envelope Systems and Occupant Behavior
        • Design Considerations >
          • Appropriate Insulation Applications Based On Existing Conditions
          • Understand Fire Codes as Necessary to Apply Home Performance in a Code-Approved Manner
          • Understand / Recognize Building Locations Where Opportunities for Retrofit Materials
          • Understand Climate Specific Concerns
          • Understand Indoor Environment Considerations for the Environmentally Sensitive
          • Understand Impact of Building Orientation, Landscape Drainage, and Grading
          • Opportunity Potential Renewable Energy Applications: Geothermal, Photovoltaic, Wind
          • Understand Impact of Shading on Heating / Cooling Loads
          • Awareness for Solar Gain Reduction / Solar Gain Opportunities
          • Understand Need for Modeling Various Options For Efficiency Upgrades
      • Measurement & Verification of Building Performance >
        • Measurement & Verification of Building Performance >
          • Air Leakage Test Results
          • Understand Building Shell / Envelope Leakage
          • Apply Fundamental Construction Mathematics and Unit Conversions
          • Calculate Building Tightness Levels (Minimum Ventilation Requirements)
          • Calculate Heating Degree Days and Cooling Degree Days
          • Identify Proper Appliance and Combustion Appliance Venting
          • Ventilation calculations and strategies
          • Proper methods for identifying / testing fuel leaks
          • Blower door setup, accurate measurement and interpretation of results
          • Combustion Appliance Zone (CAZ): depressurization, spillage, draft, Carbon Monoxide (ambient and flue)
          • Carbon Monoxide (CO) evaluation: ambient
          • Proper applications and use of temperature measuring devices
          • Pressure pan and room to room pressure diagnostics
          • Recognize contributing factors to comfort problems
          • Inspect for areas containing moisture or bulk water in undesirable locations
          • Understand and inspect for basic electric safety (e.g. frayed wires, open boxes, etc)
      • RESNET HERS RATER National Standards & Project Specifications >
        • Understand applicability content and intent of BPI National Standards – Do no harm, make buildings more healthy, comfortable, durable and energy efficient
        • Recognize need for a professional local/state/national codes evaluation
        • Be able to specify appropriate materials and processes needed for building performance projects
      • Analyzing Buildings Systems >
        • Recognize need for air sealing measures and their impact on other building systems
      • Conduct and Communications >
        • Conservation strategies
        • Conservation strategies
    • HERS RATER FIELD EXAM >
      • How To Put The House Under Worst Case & CAZ
      • What's What? Pa, CFM, CFM50, CAZ, Draft, Room Pressure
      • What To Know In The Attic
      • What To Know In The House
    • BLOWER DOOR TEST >
      • Manometer Setup
    • RESNET STANDARDS >
      • RESNET Standards Decoded
  • ESSENTIALS
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    • COMMON AUDITOR / CREW MISTAKES
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Building Science Fundamentals

22. Understand Permeability and Perm Rating

Our homes are meant to do two things with moisture.

  1. Prevent it from getting inside in the first place.
  2. Help it get out if and when it gets in.

Permeance and permeability tell us how well a material lets water vapor move (aka diffuse) through it (from wet to dry).  
Permeance depends on the thickness of the material and is what is referred to as the "perm rating."  This is like an R-value.

A vapor barrier is only a vapor barrier if it has a very low (less than 0.1) perm rating.  The lower the perm rating, the better and the more moisture it will block out. 

Class I: ≤ 0.1 perm (called impermeable)

Class II: 0.1 to 1.0 perm (called semi-impermeable)

Class III: 1.0 perm to 10 perms (called semi-permeable)

Permeability is just the rate at which water vapor will pass through a material, it does not depend on the thickness of a material.  However, if we divide permeability / thickness = perms.

Every material is permeable but how much water vapor diffuses through it depends on the size of the water vapor and what is in it (oils, dirt etc), the temperature of the home and outside, and the thickness of the material.

Next Section

  1. 1a. Basic terms and definitions
    1. Understand airflow in buildings / ducts: CFM, CFM50, CFM25, ACHn, ACH50, FPM
    2. Understand equipment efficiencies: AFUE, SSE, SEER, EER, HSPF
    3. Understand power and energy: watts, BTU/hr, ton of refrigeration  watt-hours, BTU, therm, decatherm
    4. Understand effective leakage area
    5. Understand area weighted R-Value
    6. Understand baseload / seasonal energy use
    7. Understand driving forces (including natural and mechanical: Pressure, temperature, moisture differential
    8. Understand behavior of radiation: emissivity, reflectivity, absorbtivity
    9. Understand thermal resistance / transmittance: R and U Values; including conversions
    10. Understand latent / Sensible heat: evaporation, condensation / specific heat, heat capacity
    11. Understand total equivalent length
    12. Understand basics of dehumidification / Humidification as well as measurement equipment
    13. Understand and convert Pressure units: Inches of Water Column (iwc), Pascal (Pa)
    14. Understand, identify thermal bridges
    15. Understand pressure boundary 
    16. Understand/define stack effect 
    17. Understand and define exfiltration and infiltration 
    18. Natural / mechanical ventilation 
    19. Understand net free area 
    20. Understand input / output capacity 
    21. Understand peak electrical demand 
    22. Understand permeability and perm rating 
    23. Understand standby loss 
    24. IAQ (indoor air quality): moisture, CO, dust
1b. Principals of energy, air & moisture
  1. Thermodynamics: conduction, convection, radiation, ΔT including air movement due to temperature gradients
  2. Factors that affect insulation performance: density, installation, moisture
  3. House pressurization/depressurization by various forces
  4. Heat gain / loss: internal, solar, heat transmission, air leakage 
  5. Power and energy: BTU content of fuels, capacity of combustion appliances and electrical appliances 
  6. Moisture transport mechanisms: bulk water, air leakage, diffusion, capillary action 
  7. Identify areas of highest relative humidity 
  8. Principles of combustion: combustion analysis, CO 
1c. Combustion science
  1. Combustion analysis: oxygen, flue-gas temperature, carbon monoxide 
  2. Carbon Monoxide (CO) testing of combustion appliances 
  3. Basics of: Combustion appliance venting, draft, and combustion air including identification of proper sizing/vent tables 
  4. Understand combustion safety issues: Combustion air, draft, worst case / baseline depressurization, spillage, backdrafting, unvented combustion appliances 
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