In The Field With Neil: Depressurize or Pressurize Airtightness Test

Question: Should the blower door test be performed in a pressurized or depressurized mode?


The blower door test follows ASTM standard E779 (Standard Test Method for Determining Air Leakage Rate by Fan Pressurization), which states that this test method consists of mechanical pressurization or de-pressurization of a building and measurements of the resulting airflow rates at given indoor-outdoor static pressure differences.  From the relationship between the airflow rates and pressure differences, the air leakage characteristics of a building envelope are determined.  It is intended to quantify the air tightness of a building envelope and does not measure air change rate or air leakage.

So the answer is… either method is acceptable.

In general, we tend to depressurize buildings as it prevents a jet of air from being blown into the house during the test process.  However, under certain conditions it is necessary to conduct a blower door test by pressurizing the building.  For example, pressurization testing may be used to avoid the possibility of pulling known pollutants into the building during the test procedure (e.g. mold from wall cavities or crawlspaces). The pressurization test also requires an additional outside reference hose connected to the meter.

Remember it is fan sensor with reference to fan sensor location.  Therefore if we are using a DG-700 meter, the fan sensor would be installed on the B-side input tap and the outside reference connected to the B-side reference tap.

Tip from Neil:
When reporting your results – we assume that a depressurization
test was done.  If you tested otherwise, be sure to document it.

In comparing the results of a pressurization test to that of a depressurization test, in general the depressurization test will yield a slightly tighter structure.  The reason is quite simple.  A depressurization test tends to pull dampers closed (i.e. bath fans, kitchen fans and dryer vents), whereas the pressurization test will tend to force them open.

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