What is a blower door?

A blower door is a tool that an energy auditor uses to measure the air leakage into and out of your home.  Originally developed in the 1970’s the units have evolved with technological advancements.  Today they are much more precise, lighter weight and more affordable.  The reduced set up time allowed by their more compact design had led to blower door testing to become a key element in the energy audit process.

How it works

When air pressure and air flow are controlled and measured, they can provide data about how airtight a building is.  There are three variables involved:  pressure, flow and holes or leaks.  A change in one of these factors will produce a change in at least one other factor.  Since the goal of a blower door test is to locate air leaks in the building envelope, data regarding flow and pressure provide information about holes.

During the blower door test, the interior air pressure is reduced in relationship to the outside air pressure.  This air pressure differential is roughly equal to the pressure created by a 20mph wind hitting the building.  Since the blower door forces air through cracks and holes, the locations of the leaks can be indentified.  The drafts entering the building can often be felt with the hand.  Smoke and infrared imaging can also be used to locate smaller, more subtle leaks.  It is often assumed, especially by the homeowner that poorly sealed windows and doors are often the major culprits of air leaks.  In reality, leaks in other areas are usually much more significant.  The difference in air pressure between the interior and exterior is greater both at the bottom level and up high, so leaks in the basement or crawl spaces, as well as in the attics are the most important to locate.