Blower doors are spoken of in reverential tones in energy circles. Or at least they were a few years back. Now you can’t throw a manometer without hitting a contractor setting up a blower door. Which is a very, very good thing. With the incorporation of air flow standards into the various housing codes, blower doors are becoming essential. In fact, I tell customers that a simple shorthand for whether your insulator groks building science is whether they own/use/understand blower doors.
In the public, something I’ve found it pleasantly surprising was the breadth of folks who’d at least heard of blower doors. I’ve had a fair history over the years with construction and landscaping and had never heard of blower doors prior to my energy auditing days. So when a customer chirps up that they’ve heard of blower doors, it’s pretty cool even if they haven’t yet seen one in action. And once you do see it in action, their usefulness for building diagnostics is pretty inescapable.
What Is A Blower Door (And CFM50, ACH50, CFM/ft2, etc.)
A blower doors is at the conceptual heart of the push in modern residential construction toward tightening the building envelope. The blower door depressurizes residential buildings (or if you have a really, really large one then commercial buildings) revealing the tightness of and flaws in the building enclosure.
What Is A Blower Door – The Nuts And Bolts (and Manometers)
A blower door has three components, the fan, manometer and sheathing structure. The fan is very large and adjustable, able to move enough air to depressurize most small residential structures. Manometers are finely tuned pressure gauges measuring minute variations in the pressure between the exterior and the interior of the fan. Manometers themselves are very useful for testing flue gas pressures, zone diagnostics (testing pressures between different sections of the house) and combustion zone testing. Why, I ought to write a post just about manometers (*Mental Note*)
The mounting frame is an adjustable aluminum frame with a vinyl fabric sheathing. When setting up, the frame is loose fit into the door frame then tightened with expanding gaskets. The fan is fit into the lower half and attached to the manometer to measure air flow.
With an Energy Conservatory blower door (the guy pictured on the left), there is a green air hose which goes to the exterior of the building and a red air hose attaching the fan to the manometer. Why am I mentioning this? Because the green hose needs to be 6+ feet or so away from the fan to avoid the fan’s exterior exhaust. And if you forget the green hose prior to installing the frame and fan, you’ll need to take the fan out and crawl through the bottom of the blower door to do so.
This is known as the energy auditor crawl of shame…and we’ve all done it.
And What Are Blower Doors Used For?
Blower doors are primarily used for building tightness but have meandered into other parts of home performance. A brief aside about measuring building tightness: The blower door test as most auditors understand it is called the ‘single point blower door test': it’s a CFM (cubic feet per minute) measurement taken once at 50 pascals of negative pressure.
The issue is that this can’t be called an accurate measure of a building’s year-round air leakage any more than a single day’s worth of weather will reflect an area’s annual climate. There are a jillion different measures for the data produced by blower doors, all of which need to be taken with a few trainloads of salt.
All of which is to say that blower doors can give a sense of the magnitude of air leakage, identify more severe air leakage pathways in the building and reveal possibly unknown air leak issues in the building enclosure.
Blower doors are now often used by insulators during weatherization work. After performing air sealing and weatherization, a running blower door can help refine the work, making any flaws apparent. The blower door can help make the insulation and weatherization job more thorough, creating a tighter and warmer house.
Blower doors are also used for confirming air tightness standards for building efficiency programs. LEEDS, HERS, PassivHaus and new energy code standards all include building air tightness measures.
Air transported heat loss is a huge component of your heating bill. For decades, the tightness of the building envelope has been a secondary concern. But with the blower door as a prime tool, that’s changing and changing rapidly.