What is a Blower Door?

What is a Blower Door?

Every home has air leaks, but the cumulative total leakage can vary widely based on whether air sealing efforts have been made. To measure the amount of leakage in a home a tool called a blower door is used.

Air leaks can be simple and inexpensive to seal if you can just find them, and that is where the blower door comes in. A blower door is a piece of diagnostic equipment that is used to measure the air leakage in a house, and to see how the house rates on a scale of “leaky” to “tight”. If a house is already tight, air sealing is obviously not required. But if it is too leaky, a blower door test can tell you how serious the leakage is and aid in locating the leaks. Once the leaks are sealed, the blower door can be used to calculate the actual amount of reduction in airflow.

How They Work

A blower door consists of a frame and flexible panel that fit in a doorway, a variable-speed fan, a pressure gauge to measure the pressure differences inside and outside the home, and an airflow manometer and hoses for measuring airflow. The fan pulls air out of the house, lowering the air pressure inside. The higher outside air pressure then flows in through all unsealed cracks and openings.


A blower door consists of a frame and flexible panel that fit in a doorway, a variable-speed fan, a pressure gauge to measure the pressure differences inside and outside the home, and an airflow manometer and hoses for measuring airflow


There are two types of blower doors: calibrated and uncalibrated. A typical Energy Audit would involve the use of a calibrated door. A calibrated blower door has several gauges that measure the amount of air pulled out of the house by the fan. Uncalibrated blower doors can only locate leaks in homes. They provide no method for determining the overall tightness of a building. The calibrated blower door’s data allow the auditor to quantify the amount of air leakage and the effectiveness of any air-sealing job.

These are some reasons for establishing the proper building tightness:

  • A reduction in excess energy consumption resulting from air leakage
  • Avoiding condensation problems caused by excessive moisture
  • Eliminate uncomfortable drafts caused by air leaking in from the outdoors
  • Determine the amount of mechanical ventilation required for achieving acceptable indoor air quality

The blower door results can be expressed in a few different metrics. The most common one is air changes per hour (ACH), or how many times a house’s air completely replaced in a given hour. Since we take our blower door measurement at 50 Pascal most codes and standards reference the air changes at that elevated pressure (ACH50), but we can also calculate the air changes under natural conditions (ACHn).

For example, a code-built new home with decent air sealing might have 7 air changes per hour at 50 Pascal (ACH50), meaning if we kept the blower door running for an hour it would pump in enough air to completely replace the home’s air 7 times. This would translate to about 0.35 natural air changes per hour (ACHn), or about one complete air replacement every 3 hours.


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