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EXISTING HOMES

Weatherization Specification Manual

AIR SEALING AND MECHANICAL VENTILATION
MINIMUM VENTILATION LEVEL WORKSHEET

  • Controllable, continuous mechanical ventilation is the mainstay for good indoor air quality and maintaining acceptable moisture levels, ensuring appropriate ventilation levels for occupants or the building.
  • Weatherization Specifications Manual—Mechanical Ventilation

    • The ventilation specification addresses when and how mechanical ventilation shall be installed in the Existing Homes program. It requires calculating prescribed ventilation needs, accounting for:
    • Occupancy
    • Bedrooms
    • Volume
  • Typical homes and typical occupancy will provide approximately 0.35 ACHnat and require 15CFMnat per occupant.

How to Calculate Minimum Ventilation Level (MVL)

Step 1: MVL based on known occupancy; MVL = # of occupants x 15 CFMnat
Step 2: MVL based on number of bedrooms; MVL = (# of bedrooms + 1) x 15 CFMnat
Step 3: MVL based on volume; MVL = (0.35 x volume)/60

Identify the most restrictive target and compare with Ventilation Potential

How to Calculate Ventilation Potential

Ventilation Potential = CFM50/N>

Where N is a correlation factor: N = C x H x S

C = Climate factor, a function of annual temperature and wind.
C = 20 is an appropriate approximation for most areas in the Northwest.

H = Height-correction factor

S = Wind-shielding correction

Number of Stories 1 1.5 2 3
Correction Factor "H" 1 0.9 0.8 0.7
Wind Exposure Well Shielded Normal Exposed
Correction Factor "S" 1.2 1 0.9

Example

1,500 ft2
3 bedrooms
5 occupants
8-foot ceilings
One level
Normal wind exposure
Tested 2,100 CFM50

Compare Ventilation Potential to MVL to determine air sealing target. If Ventilation Potential is greater than MVL, determine target then proceed with additional air sealing.

Determine Target

Based on Volume
Volume = 1,500 ft2 x 8’ = 12,000 ft3
Target = (.35 x 12,000 ft3) ÷ 60 min = 70 CFM

Target CFM50 = ACH x N
Target CFM50 = (70 x 20)
Target CFM50 = 1,400 CFM50

Based on Occupants
MVL = 5 x 15 = 75 CFM
Target CFM50 = 75 CFM x 20
Target CFM50 = 1,500 CFM50

Target based upon occupants is most restrictive, so the lower limit of air sealing without adding mechanical ventilation is 1,500 CFM50.

Copyright © 2011 Energy Trust of Oregon Existing Homes