By Randy Williams

What is a blower door test?

If you’re wondering about this air-tightness test, there may be no better person to explain than our guest contributor Randy Williams. Randy makes his living as a contractor and energy efficiency specialist. In this piece, he’ll walk you through the fundamentals behind blower door testing. 
We’ve known for a few decades that improving the air tightness of a home and installing a properly designed mechanical ventilation system can greatly enhance the performance of a home. Having the ability to verify the airtightness of the structure is key. We perform verification by using a tool called a blower door. I’ll explain what you need to know about performing the test, interpreting the results, and how it relates to code. 

What is a blower door test?

A blower door test is a test to confirm the continuity and integrity of a structure’s air barrier.

This leads to another question: What is an air barrier? An air barrier is the part or parts of a structure that creates the separation between inside and out. An air barrier is typically several different building products assembled in a way that keeps the outside air outside and the inside air in. A properly designed air barrier needs to be defined. Is it the wall and roof sheathing? Maybe it’s the closed cell spray foam installed inside the wall cavity, or it could be the interior finished drywall. It’s also going to include windows and doors. It can be all these products and more.

Most importantly, the air barrier needs to be continuous. No holes, no cracks, no breaks … continuous. Think of the children’s song, “the foot bone is connected to the leg bone, the leg bone is connected to the knee bone ... ” Every part of the air barrier needs to be connected and continuous.

Why do we care? Why is blower door testing important?

Uncontrolled air moving into (infiltration) and out of (exfiltration) a building causes all sorts of issues. The operational costs of the home (heating and cooling) are a big part. According to the Department of Energy, up to 40%* of the cost to heat and cool a home is due to air leaks. Air leaks also affect comfort, indoor air quality, and the durability and longevity of the structure. Energy isn’t cheap. We use insulation to slow the movement of heat into or out of a structure. We have found that air sealing is just as important, if not more important, than having high levels of insulation. Insulation performs best if air is not moving through it. 

How does a blower door work?

How can we test the continuity and integrity of an air barrier, sometimes referred to as the “tightness” of a building? The most common way is with a pressurization test, called a blower door test. By using a large fan placed in a door or window, we can positively or negatively pressurize a structure to a specific pressure (50 Pascals is common in residential construction). The fan is set into an expandable frame that fits most doors and some larger windows. A panel is fitted around the expandable frame with a hole to accommodate the fan. The fan is controlled by a tool called a dual-channel manometer. The manometer monitors inside and outside pressures and calculates the flow rate of air moving through the fan based on the fan speed and size of the opening in the fan housing — the fan opening is adjustable. When using the newest equipment available, a blower door test can be controlled by a computer, tablet, phone, or by the manometer itself. 
In residential, we aim for 50 Pascals of pressure differential between inside and outside when performing blower door testing.  

What is a Pascal?

A Pascal is a unit of pressure. One Pascal is one Newton per square meter, one kilogram per square meter, or one joule per cubic meter. That doesn’t help much. How about 0.2 inches of water column is equal to 50 Pascals? Place a straw in a cup of liquid and suck up ¼ inch and you’ve achieved a pressure more than 50 Pascals. An even more relatable description: 50 Pascals, produced during a blower door test, is the equivalent of about a 20 mile per hour wind blowing on all sides of the home at the same time. 
The flow rate of air moving through the fan is noted as cubic feet per minute (CFM). A cubic foot of air is roughly the size of a basketball. During a blower door test, it’s fairly common for the flow rate to be 500 CFM or more — 500 basketballs moving through the fan every minute. 500 CFM might be bad, or it might be good. It is all dependent on the size and volume of the structure. 

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