HOW TO CONVERT CFM(50pa) TO CFM(natural)
1) The example table below, customized for Vermont, illustrates how to determine the
conversion factor, based on adjustments for wind shielding and building height. Use the map
on the reverse, and the ratios from the Custom Table Factors below the map, to create a
custom table for your geographic region.
2) Divide the CFM(50pa) blower door measurement by the conversion factor.
For example: Suppose the house is 2 stories above grade sited on an exposed hilltop. Also
suppose that the blower door measurement is 2480 CFM(50pa). The conversion then yields
200 CFM(natural) since 2480 divided by 12.4 = 200.
HOW TO CALCULATE THE BUILDING AIRFLOW STANDARD
(based on ASHRAE 62.2 – 2003)
1) Multiply the total heated square feet of the building by 1% (i.e., 0.01); then
2) Multiply the total number of occupants (or number of bedrooms + 1) by 7.5; then
3) Add the two answers to yield the minimum CFM ventilation requirements of the building.
For example: Suppose this same example house also has 2000 square feet of heated floor
area and has 4 bedrooms and 3 occupants. 1% of the heated floor area = 20, and 3 (number
of occupants) times 7.5 = 22.5. Based on the number of occupants, the minimum ventilation
requirement is then 42.5 CFM (20 + 22.5). Based on potential occupancy, the answer is
57.5 CFM (20 + 37.5), since 4 bedrooms + 1 = 5, and 5 times 7.5 = 37.5.
By either calculation, the air leakage rate of this example building is far greater than the
Building Airflow Standard (BAS) since 200 (the existing average CFM) is greater than 42.5
(required ventilation rate with current occupancy) or 57.5 (potential ventilation rate
required). If, after air sealing, the blower door measurement is 620 CFM(50pa), then
supplemental mechanical ventilation should be installed since 620 divided by 12.4 = 50
CFM. While the current occupancy does not require it (50>42.5), potential occupancy does
(50<57.5). Best practice in such event then means installing high performance low-sone
ventilation that is/can be suitably controlled to ensure adequate ventilation whenever natural
ventilation is inadequate.