Building Science-Natural Air Leakage

This blog first appeared on the Green Building Advisor website.

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 Continue reading “Building Science-Natural Air Leakage”

Building Science-Tight Homes and Negative Pressures, When Should We Be Worried?

This post originally appeared on the Green Building Advisor Website.

A couple years ago I was asked to perform a blower door test on a new home.  The home was small with a footprint of only 1130 square feet.  When the test was completed, the test report indicated an air leakage rate of 91 CFM at the test pressure of 50 Pascals, .33 ACH50.  Completely unexpected for this code-built house.  (I actually measured the volume of the home again and ran the test two more times before I was convinced I had an accurate test.)  The contractor contacted me some time later and indicated the homeowners were noticing air coming through the exterior light switches when the dryer was operating.  The dryer was producing close to the same negative pressure as my blower door every time it was being used, a negative pressure of about 50 Pascals.  Is this a problem?

Continue reading “Building Science-Tight Homes and Negative Pressures, When Should We Be Worried?”