Posted by: lcowie | November 16, 2010

Improving Indoor Air Quality

Do you have enough airflow in your home? The way to determine this is to consider the volume of your home and the number of occupants in the home. This is where Building Airflow Standard (BAS) comes into play.

The healthy air turnover is 35% of volume per hour. If your home is 1700 square feet, with 8 ft ceilings, you will multiply the numbers together to get your volume, which is 13,600 cubic feet. You multiply this number by 0.35 and divide by 60 to get the cubic feet per minute leaving your home. In this case, there are 79.3 cfm of air leaving your house each minute.

To calculate healthy airflow among occupants, you take the number of people living in your home and multiply that number by 15 (cubic feet per minute). If you were conducting an energy audit and didn’t know how many occupants were in the home, you’d look at the number of rooms. Keep in mind that the master bedroom generally holds two people, so take the number of rooms and add 1 (for that master bedroom). If there are 4 people living in your home, take 4 times 15, and you get 60.

To determine the healthy amount of airflow specifically for your home, take the higher of these two numbers – 79 vs. 60. Multiply the 79 by .70 because healthy airflow extends to within 70% of the number you have chosen. After multiplying by 0.70, you get 55.

These numbers may not make sense until you run the blower door test. The blower door will give you the cubic feet per minute (at 50 Pascals) of air leaving your home. You will want to compare the blower door reading to your calculation. The blower door, however, is going to give you a four-digit number. Right now, we have a two-digit number. There are more calculations to be had.

You will want to divide your blower door reading by the N-factor of your home. The N-factor takes into account the climate zone of your home, its shielding, and how many stories there are. BPI provides a table with this information, so you can refer to that. If your home is in climate zone 3, with normal shielding and 2 stories, your N-factor is 17.2. Divide the blower door reading (at 50 Pascals) by 17.2 and you will be left with a comparable two-digit number.

Is the two-digit number above 79, between 79 and 55, or below 55? If it is above 79, that means you have enough airflow moving through your house. If it is between 79 and 55, you may want to add some mechanical ventilation to get 79 cubic feet per minute leaving your home – you can add the difference. If the number is below 55, you have to install mechanical ventilation that will give you at least 79 cubic feet per minute leaving your home.

These numbers all come from house volume and number of occupants. These numbers will change by house.

Leave a comment for more information about the Building Airflow Standard (BAS).

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Responses

  1. What do you think about HEPA air purifiers, do they make a difference?


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