When conducting a blower door test, one of the ways we express the findings is with air changes per hour at 50 Pascals (ACH50). As an example, a new home is blower door tested and found to be 2.75 ACH50, this means the entire volume of air inside the home is exchanging with the outside air 2.75 time per hour at the test pressure of 50 Pascals. When discussing blower door testing with builders and homeowners, it can be hard for them to relate to the ACH50 number other than they know the home needs to be less than a certain level of air tightness to meet current code requirements. Expressing the tightness as natural air leakage can sometimes be helpful. How can we calculate the natural air change rate? Well, there’s a formula:
ACH50/n-Factor = ACHnat
Easy enough formula, but what is n-Factor? The n-Factor (also called the LBL Factor) was developed a few decades ago by the Lawrence Berkeley Laboratory (LBL) as a way to calculate the natural air change rate by using the blower door test results. They came up with a map of the U.S. and Canada that uses wind data for the given location, how well shielded the home is, and how many stories the home has. The illustration and chart are used to determine the n-Factor for a given area.
Using the area I where live as an example, the arrowhead of Minnesota, the zone two portion of the table would be used. Next you would have to determine the shielding of the home. I haven’t seen a lot of information on what the parameters are for a well-shielded home compared to an exposed home, so I use the well shielded table when the home is surrounded by a dense woodland or tall buildings. The exposed…
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