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Building Envelope Tightness (BET) Test:
Blower Door Testing
Blower door testing for DET Verification can only be done after walls and ceilings are installed. It’s best to wait until everything is done, or at least waiting for the floor coverings to be installed.
Blower door testing is used to determine the house’s air exchange rate with outdoor air. To compute it, the building volume needs to be calculated. There are building code definitions to decide what conditioned spaces are inside or outside the building envelope.
Topics in this section include:
Blower Door Test Procedure
The primary goal of building envelope tightness testing (aka blower door testing) is limiting the rate of conditioned air changes with outdoor air. This includes walls, floors, and ceilings around “conditioned” space which creates the air barrier around the building envelope.
Our procedure for blower door testing is as follows:
0. Figure out the volume of “conditioned” space: Volume
1. Prepare the house for testing
2. Depressurize house to predetermined state: 50 Pascals (Pa)
3. Determine rate of airflow needed to maintain pressure: CFM50
4. Calculate number of air changes per hour at 50 Pa: ACH50
ACH50 = (CFM50 * 60) / Volume
Success criterion for passing the blower door test is 7 air changes per hour at 50 Pascals: ACH50 ≤ 7. In most cases, blower door testing results in a natural air exchange rate per hour of 0.35: ACHnat ≤ 0.35. Said another way, all the air in the house is replaced in 3 or more hours.
Note: If blower door testing fails, it has to be repeated. If it passes, outdoor air ventilation may need to be added. See the discussion, “What If the Test Passes?”
The basis for computing air changes per hour for blower door testing is the volume of conditioned space in the house. Computing a house’s air volume can take an incredible amount of time to figure out when they have complex geometries. Usually, the roof creates the complexity. Some custom homes have wicked wall geometries too. Home InSight has developed methods to figure it out with intelligent estimates.
Normally, volume takes less than ½ hour to figure out after finding the floor area needed for the duct tests. On occasion, it may be necessary to spend more time figuring it out. These houses usually have a foamed roof line and tend to be high-performance homes, requiring energy ratings, such as a HERS Index.
The method we use will get the volume within 15% of its true value. For most houses, that’s good enough for blower door testing because either the house will be loose or tight. Rarely will it be on the test boundary.
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