It's the Hole - Understanding What a Blower Door Is for
There's been a lively discussion going on in the RESNET/BPI group on LinkedIn. I posted my article about how infiltration occurs at the surface, not in the volume, and it's generated some good comments. In that article, I said we need to stop talking about infiltration rates in terms of air changes per hour because there are too many problems with it.
Reading and responding to the LinkedIn comments has refined my thinking on the issue, so let's see if I can convince the holdouts.
Infiltration* is caused by two things:
- a pressure difference
- a pathway
If the pressure outside a building is the same as the pressure inside, you can open all the windows, and little air will move through them. Likewise, you can have a huge pressure difference across the building envelope, but if there are no holes, there's no air leakage. So, if you eliminate one or the other of those two conditions, you eliminate the infiltration.
Now, the Blower Door is really good at telling us about the pathway, that is, the hole in the house. The more air flow a Blower Door measures, the bigger the hole in the house. (When I say 'hole,' singular, I'm referring to the sum of all the leakage sites.)
What the Blower Door cannot do, however, is tell us anything about the pressure differences that the home experiences. When the Blower Door is operating, it puts the house under a controlled pressure difference. When the Blower Door is not running, the pressure in the house varies season to season and moment to moment. The driving forces for those changes are wind, the stack effect (warm air rising in the house), and mechanical systems.
Calculating the ACHnat based on a single point test attempts to factor in the effects of wind and stack effect, but at great cost to the precision. You go from ~3% uncertainty to 20%, 30%, possibly even 50% or more. I don't think it's helpful to tell a homeowner, "Your home leaks at the rate of 1 air change per hour...but it could be as low as 0.5 or as high as 1.5, and if you've got problems with the mechanical systems, it could even be 2 or more air changes per hour."
Pressure differences created by mechanical systems can eclipse those created by wind and the stack effect. The disconnected supply duct, the panned return, the 1200 cfm commercial range hood with no makeup air - all these things can dwarf the effects we're trying to capture in ACHnat.
I believe that the pathways for infiltration are really all we need to be concerned with when we're talking about Blower Door results. If a house has a hole, we want to know how big it is and what we can do to make it smaller. That's what the Blower Door is good at. We can still help homeowners without extrapolating to the point of meaninglessness if we just focus on the hole.
My point about normalizing the raw number, cfm50, to the surface area rather than the volume makes sense when you see it this way. If you've got a hole in the building envelope, you compare it to the size of the building envelope, not the volume.
Yes, pressure differences are important, but let's understand the limitations of the Blower Door. It's great at telling us about the pathways for air leakage through the building envelope. It can't tell us anything about the normal pressure differences a building will experience.
*When I talk about infiltration, I'm using that word to describe both the air that enters a building through the leaks and the air that leaves the building through leaks. Technically, those are infiltration and exfiltration, respectively, but I'm going to allow you to do a little of the work and understand that I'm talking about all of the leakage.