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A blower door is a machine used to measure the airtightness of houses. The test will analyze the pressure differential from outside to inside, with a fan creating a pressure of 50 pascals inside the building.

50 pascals is used to imitate the pressure of a strong wind.

It can also be used to:

• measure airflow between building zones
• test ductwork airtightness
• help physically locate air leakage sites in the building envelope.
  

Blower door tests are commonly done on new construction to ensure building standards are met and they are also commonly done as part of an energy assessment, ie. in Ontario we have the Energuide rating.

Homeowners will often book an assessment before starting a renovation so they can take advantage of any government incentives for improvements made to their home. The Energy Advisor will perform the blower door test and do a complete walkthrough to locate problem spots around the house.

With any renovation based on increasing the airtightness and insulation levels, the work must include a plan for balancing the HVAC and, potentially, to install an HRV to ensure consistent fresh air flow.

This is why it's equally important to get the test done after the renovation as it is before.