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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