Patrick’s Barn: Blower-Door Testing
One of my most recent projects as a Fine Homebuilding associate editor was to develop a magazine article and companion video with building-science expert Larry Armanda. Our mission was to show how to run a blower-door test and interpret the results. I’ve been curious about the performance of the new barn my wife and I just built, so I decided to try a blower-door test to see if the video or magazine article had overlooked or glossed over any of the steps.
I think we did a good job with our content because I found doing the test quite easy. Of course, it helped that I was able to learn from Larry, who has hundreds of blower-door tests under his belt. The barn performed pretty well, too. It’s worth explaining that the barn has two sections. The smaller lean-to section where I keep our lawn tractor is separated from the rest of the barn by a steel fire door. It’s the same kind of weatherstripped door that separates an attached garage from the rest of a house. This lean-to section has my homemade carriage doors, which admittedly need better weatherstripping.
When I opened the fire door connecting the two spaces, the blower door registed about 740 cfm, which is equivalent to just under 5 ACH at 50 Pa. With the fire door closed, the number dropped to about 500 cfm, which is 3.2 ACH at 50 Pa. I think both numbers are quite respectable, given that we didn’t use spray-foam insulation, taped exterior sheathing, or exotic air-sealing materials.
We also have a woodstove with a 6-in. flue connected to the outside. With a little more work, such as sealing the conduit that supplies the electrical subpanel, I think I can get the final number for the conditioned space below 3 ACH at 50 Pa. I think I can also do much better in the semiconditioned, lean-to section by installing better weatherstripping on the carriage doors. Anybody know the best weatherstripping for homemade doors?
Look for Larry’s blower-door article in the February/March 2014 edition of Fine Homebuilding magazine.
You can read more about my barn here.
Once you've turned off all combustion appliances and ventilation fans, closed and locked all windows and exterior doors, and opened all interior doors, assemble and size the blower door's knock-down aluminum frame.
After sizing the frame, remove it and cover it with the fabric panel that closes off the opening and surrounds the fan. The blower door's design is rugged and elegantly simple.
Surprisingly, the most difficult part for my first blower-door test was sizing the frame. The secret is making it a little loose before adding the fabric cover and properly positioning the cam-shaped locks so they exert the right amount of pressure when tightened.
Once you get the door set up, the test takes minutes. I was totally geeked out by the very precise controller that adjusts the fan speed and by the two-channel manometer that compares inside and outside air pressure. How much fan pressure it takes to register a 50-Pa pressure difference between inside and outside is the foundation of blower-door testing.
When this fire door separating the lean-to section from the rest of the barn is closed, the barn's airtightness is about 3.2 ACH at 50 Pa. When it's open, it's about 5 ACH at 50 Pa. The difference is the leaky weatherstripping around my homemade carriage doors, which are visible in the background.
A blower-door test is one of the most accurate ways to register the airtightness of a building. Assembling the door and running the test is surprisingly easy. The door's powerful fan depressurizes the building while its digital manometer measures indoor and outdoor air pressure.