Blower Door Test

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Measuring the Tightness of the House

All structures leak air through cracks, around pipes, under doors, and even through walls that aren't well sealed. This is an energy issue because cold air leaking in during the winter has to be heated, hot air leaking in during the summer has to be cooled. The "tightness" of a structure is specified by its air exchange rate, which is the number of times per hour that all the air in the volume of the structure is exchanged with outside hour. A typical house has an air exchange rate of 1 to 2, meaning that all the air in the house is exchanged with outdoor air once or twice an hour (every 30 to 60 minutes). Hard to believe, but true. Newer construction with better insulation and better weatherstripping should have an exchange rate a bit less than one. The EPA recommends a minimum exchange rate of 0.35 to avoid the build-up of indoor air pollution from cooking, cleaning products, and outgassing of chemicals from paints and carpets. Particleboard and plywood emit formaldehyde; carpet emits the yummy sounding 4-phenylcyclohexane. A typical house has enough air exchange to handle all this, but at the price of higher energy use.

The tightness of a house can be measured with a blower door test. We had a test done about 8 months after we moved in. As the photo shows, a special door is attached tightly to one of the exterior doors – in our case, to the door leading into the garage. The fan blows air out of the house, lowering the pressure inside the house. The fan speed is adjusted so the the house has a negative pressure of 50 pascals (equivalent to 0.0075 pounds per square inch). The fan is calibrated, so the operator can convert the fan speed to an air exhaust rate in cubic feet per minute. This is the rate at which air is being pulled into the house through all the various leaks. Some simple calculations using the exhaust rate and the measured house interior volume give the air exchange rate.
As the fan was running you could easily feel the air flow wherever there were air leaks. One of the larger sources of leaks turned out to be recessed can lights in the upstairs that protrude up into the attic space. There was also some leakage around pipes coming through the garage wall to the tub in the master bathroom. A surprising source of leaks – at least to us, this seems to be well known to experts – is around electric outlets and electric switch plates. The inside of an interior wall is not tightly sealed at the top where it reaches the attic, so cold air seeps down into the wall space and then leaks in through electric outlets and switches that extend into the wall. Inexpensive foam pads can be – and should be – placed behind the cover plates on switches and outlets to provide a reasonable amount of sealing. You can also get plastic plugs to plug all outlets that aren't in use.

The good news was that my efforts to stuff loose straw into all the gaps in the bale walls paid off; there was no noticeable leaking through the walls or around window and door frames.

Despite the inevitable leaks, our air exchange rate ended up at 0.3 exchanges per hour. This is a very tight house. In fact, tighter than EPA's recommended minimum of 0.35. However, we don't find this a reason for concern. We designed the house to make minimal use of any toxic or hazardous materials. We have no carpet, very little particleboard, and all the interior paints are low VOC (volatile organic compounds) paints.

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