Step 22c – Test the Air Sealing

Dense pack cellulose insulation is not airtight, so it is important to ensure my air barrier is solid before I install it. I used a relatively simple device called a blower door to check for any weaknesses in the air barrier. The blower door consists of a large fan, a metal support to secure the fan inside a door frame, two small air hoses, and a manometer.

The preparation for a blower door test begins by sealing up any openings like ducts and conduits. I used some scrap packing Styrofoam and tape to cover all of the ERV ducts and the electrical conduit. Next, I assembled the door frame and fan assembly, and connected the power to the fan. One of the air hoses runs out a small hole in the blower door mount to the exterior where it is placed inside a bag to protect it from wind. The other end is plugged into the manometer. The second hose runs between the blower door fan and the manometer.

Once the preparation is done, the first step in running the test is called establishing a baseline pressure measurement. The manometer measures the pressure from the hose outside to get an idea of the outdoor air pressure. This will be subtracted out from the measured indoor pressure, so a blower door test run in Colorado can be compared to a blower door test in New Orleans, even though they have very different air pressures. Next, the fan is turned on, which begins forcing air out of the house. The standard blower door test creates a pressure difference of 50 Pascals, which is a pretty powerful negative pressure in the house. The manometer begins to measure the amount of air that is being moved out of the house. If the fan doesn’t need to work very hard to maintain the 50 Pascal pressure difference, the air barrier is working well. If the fan has to run faster to maintain the pressure difference, that means air is leaking into the house from the outside. This additional air must be pushed out by the fan to maintain the pressure difference, which is why the fan runs faster.

The manometer measures the amount of air flowing out of the house and through the fan in cubic feet per minute (CFM). This number is displayed on the manometer and will fluctuate as wind from outside alters the pressures acting on the house. The manometer can alternatively display the units of the air rushing out of the house as air changes per hour (ACH). This is a measure of how many times all of the air inside the house is drawn outside by the fan per hour. A higher CFM will remove more air from the house resulting in a higher ACH. It is preferred to measure the ACH so that results from a larger house and a smaller house can be compared to each other.

My first blower door test gave me an average ACH of 1.8 or so, which was an excellent number considering anything under 3 is considered to be pretty airtight. I was disappointed, however, as I was hoping to attain the Passivhaus standard of 0.6 ACH. I kept the fan running while I had some friends walk around the house with special smoke pencils. Watching the smoke curl up the walls, you could easily spot the leaks wherever the path of the smoke changed. We spotted quite a few miniscule leaks all around the ceiling perimeter, so I got back in the attic and worked on sealing them up.

After crawling up in the attic and doing a few more rounds with a can of spray foam and sealing up a few other small holes we found, the ACH dropped to 1.4, which was still not as low as I wanted, but I had to keep in mind that insulation and drywall will lower air infiltration as well. I was satisfied with the results and ready to move on to the next step in insulating.