Wednesday, October 19, 2016

Blower Door Test

One of our goals during construction of the house was to make it air tight.  Of course, no house is ever completely air tight.  Some houses leak a little and other houses leak a lot.  The standard way of measuring air tightness of houses is a blower door test.  A fan is installed into the front door opening to blow air out.  An equal amount of air comes in through whatever leakage paths are in the house.  The technician adjusts the fan speed while measuring the pressure difference between the inside of the house and the outside of the house.  The technician determines the rate of airflow required to establish a 50 Pascal pressure difference.  That air flow rate is compared to the volume of the house to calculate the number of air changes per hour, called ACH50.

A low ACH50 is good.  We had a blower door test performed on our previous house, which was built in the early 1990s.  The ACH50 was about 10.  That house had many of the symptoms of poor air sealing, like rooms that were uncomfortably cold whenever it was windy in the winter.  Modern new constructions houses usually have an ACH50 of 3.0 - 4.0. To be certified under the very rigorous Passive House standard, a house needs an ACH50 of less than 0.6.  Builders of Passive Houses go to great lengths to get to that level.  When I calculated the heating and cooling load for the house, I used 1.5, although I hoped for better.

The floorplan of our house is not ideal for air sealing.  The fact only some of the house has a second story and the ceiling in the hangar is midway up the second story meant that we had more wall/ceiling intersections to seal.  We did a number of things to improve the air tightness.  The worst areas in most houses are the ceilings and the rim joist.  To reduce air leakage through the ceilings, we:
  • located the attic accesses either in the garage or in the gable ends to avoid leakage around attic access doors
  • avoided recessed can lights
  • used spray foam around the intersection of the walls and ceilings
To reduce air leakage around the rim joists, we:
  • applied a peel and stick membrane on the exterior extending from the foundation concrete to the wall sheathing 
  • used spray foam on the interior
To reduce air leakage through the walls, we:
  • taped the joints of the OSB sheathing
  • caulked around the framing on the interior side
  • selected high quality casement windows
So, did these things work?  Yes, they did.  Our blower door test result of 0.82 ACH50 proves it.

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