An energy audit is a diagnostic visit that measures the efficiency of a building. By using state-of-the-art technology to discover and pinpoint areas of inefficiency, an energy audit is the best starting point when considering building performance upgrades. But how does an energy auditor interpret blower door test results? Here are the key metrics:
Airtightness and the building enclosure
Energy audits use blower doors, which are installed in an exterior door of a home or building. All windows and doors (and fireplaces or other operable dampers) are shut. The blower door fan either blows air in (pressurizes) or blows air out (depressurizes) the building. This creates either a positive or negative pressure in the building’s interior. In an effort to stabilize the pressure in the building, air will start to move in or out of the building. Blower door test results can determine the airtightness of the building, and can be presented in a number of different formats.
CFM50 is the airflow (measured in cubic feet per minute) from the blower door fan needed to create a change in building pressure of 50 pascals. The most common measure of building airtightness, CFM50 indicates the total air leakage in the building enclosure.
An old and uninsulated, two-story, wood framed house would likely produce a CFM50 test result between 4,000 and 8,000, which is very leaky. A new modern house built to a strict airtightness standard would likely produce a test result between 600 and 1,000 CFM50, which is very tight (in this instance, mechanical ventilation would probably be needed to keep the home’s air quality good).
Percent reduction in CFM50
In high-performance weatherization programs for leaky houses, homeowners can expect to see reductions in CFM50 as drastic as 40-50%.
Air Changes per Hour at 50 pascals (ACH50)
ACH50 compares the CFM50 to the conditioned interior volume of the building. This measurement (ACH50) tells us how many times per hour the entire volume of air in the building is replaced when the building envelope is subjected to pressure equaling 50 pascals. Many airtightness test standards for new houses have specified a maximum allowable ACH50 leakage rate of 3 ACH50.