October 4th, 2010 by Bjorn Nelson
Last week our efforts in taping, sealing, and detailing an airtight building envelope paid off when we conducted our first airtightness test and received a very encouraging result. Our friends at Hammer and Hand, green builders who also do home performance testing, helped us out by conducting a blower door test of our building. Our result was 0.45 air changes/hour at 50 pascals!
A blower door test is a diagnostic tool that measures the airtightness of a building by depressurizing the house. This depressurization exaggerates the home’s air leaks, making the leaks easier to measure and locate. A calibrated fan is placed into a door opening, which is then temporarily sealed around the fan, and the fan is used to blow air out of the building to create a pressure differential. Pressure sensing instruments then measure the airflow (in cubic feet per minute) needed to create a 50-pascal pressure change. By factoring in the interior volume of the building, we can then determine the amount of the building’s air that gets displaced per hour at this pressure (air changes per hour at 50 pascals pressure – ACH50).
Blower door installed at our kitchen patio our test result of 153 cfm at 50 pascals
We conducted the blower door test at this early stage to determine if we are on the right track for meeting the Passive House Standard of 0.6 ACH50 and in order to have the opportunity to easily find and seal leaks with tape or mastic while the framing is still exposed. We were able to check around wall seams and window perimeters and could actually feel the air seeping through in some areas while the house was depressurized. Our official blower door test, which we will conduct at the end of construction, may differ somewhat, but last week’s test gives us a good sense of where we will be at. Between now and then, we will have to produce a few new penetrations through our outer walls for the ventilation system and our mini-split heat pump, we will seal and gasket them carefully, we will add tape to the outside of the window perimeter, and the interior drywall will provide an additional layer of airtightness as well.
looking for leaks
as a basis of comparison, here are some air changes per hour at 50 pascals:
0.45 – Shift House (preliminary)
0.60 – a requirement for Passive House Certification
1.50 – voluntary standard for Canadian R-2000 green building program
2.50 – routinely achieved in new homes in Minnesota
3.00 – Concordia House (LEED Platinum house built by ROOT)
3.90 – the median for new homes in Wisconsin (2002 study)
5.40 – energy star rated manufactured homes
7.00 – 2009 International Residential Code requirement
n/a – LEED for homes has no specific air-tightness restrictions
David Keefe, the manager of training services for Vermont Energy Investment Corporation, recently wrote in an article on blower-door testing, “Houses with less than 5 or 6 ach50 are considered tight, and those over 20 are quite leaky.” Many older U.S. homes are so leaky that a third to a half of the home’s heat loss comes from air leaks.