What’s a Blower Door Good For?
Hint: It’s NOT 'natural' air changes per hour
In last week's blog, I suggested that talking about infiltration rates in terms of air changes per hour isn’t an accurate way to portray air leakage. The problem is that you’re dividing by volume but the leaks happen at the surface. I don’t think ACHACH stands for Air Changes per Hour. This is a metric of house air tightness. ACH is often expressed as ACH50, which is the air changes per hour when the house is depressurized to -50 pascals during a blower door test. The term ACHn or NACH refers to "natural" air changes per hour, meaning the rate of air leakage without blower door pressurization or depressurization. While many in the building science community detest this term and its use (because there is no such thing as "normal" or "natural" air leakage; that changes all the time with weather and other conditions), ACHn or NACH is used by many in the residential HVAC industry for their system sizing calculations.50 is going away anytime soon, and I use it myself because everyone else does, even though it’s biased toward larger houses.
The quantity that drives me bonkers, though, is ACHnat, which stands for Air Changes per Hour under “natural” conditions. Every HERSIndex or scoring system for energy efficiency established by the Residential Energy Services Network (RESNET) that compares a given home to a Home Energy Rating System (HERS) Reference Home based on the 2006 International Energy Conservation Code. A home matching the reference home has a HERS Index of 100. The lower a home’s HERS Index, the more energy efficient it is. A typical existing home has a HERS Index of 130; a net zero energy home has a HERS Index of 0. Older versions of the HERS index were based on a scale that was largely just the opposite in structure--a HERS rating of 100 represented a net zero energy home, while the reference home had a score of 80. There are issues that complicate converting old to new or new to old scores, but the basic formula is: New HERS index = (100 - Old HERS score) * 5. rater and BPI class teaches people how to calculate this thing and what it means: A house with 1 ACHnat exchanges all it air with outside air once each hour. The only problem is that it isn’t true. Let’s start from the beginning.
Air leakage and blower door basics
Air leakage happens when you have both of the following:
- a pressure difference, and
- 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 enclosure, 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 air leakage.
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.) It doesn’t tell us anything about the distribution of the holes or their individual sizes and shapes, although we can walk around with the blower door running and locate the larger leaks. Joe Lstiburek wrote a great article about this last year: Just Right and Airtight.
The problem with extrapolation
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 main 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 effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season., 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 stack effect. The disconnected supply duct, the leaky 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.
Blower doors help us estimate how big the holes are
I believe that the pathways for air leakage 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, if it’s small enough to meet the code or program requirements, and what we can do to make it smaller. That's what the blower door is good at. We can still help builders and homeowners without extrapolating to the point of meaninglessness if we just focus on the hole.
My point about normalizing the (almost) 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 enclosure, you compare it to the size of the building enclosure, 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 enclosure. It can't tell us anything about the “natural” pressure differences a building will experience.
If we have this tool — the blower door — that gives us numbers with a precision of +/-3%, why would anyone want to manipulate the results into something with uncertainties of 20%, 30%, possibly even 50% or more?!
It's time to quit using ACHnat to talk about infiltration.
An alternative way to explain air leakage to homeowners
Defenders of the ACHnat make the seemingly valid point that it's conceptually easier to explain to homeowners. When you extrapolate from a house under test pressure to a house under “natural” pressure differences, the ACHnat tells you how quickly the air in your house is exchanging with outside air.
If, for example, a house comes in at 0.5 ACHnat, the energy auditor will tell the homeowner that all the air in their home is exchanged with outside air every two hours. Yes, it's an easy concept for homeowners to grasp, but, as I’ve shown above, it’s not true.
There’s another option here that has far less uncertainty. Blower door results can be turned into leakage areas. The two main ones are Effective Leakage AreaELA. Calculation in square inches equal to the total area of all air leaks in a building envelope. Defined by Lawrence Berkeley National Laboratory, the ELA is the area of a nozzle-shaped hole that would leak the same amount of air as the building does when pressurized to 4 pascals. Estimating leakage in this way enables one to visualize the cumulative impact many tiny holes can have on the airtightness of a house. (ELA) and Equivalent Leakage Area (EqLA). Both attempt to quantify the size of the hole based on the cfm50.
Another relation that I use also turns cfm50 into hole size: Each 1000 cfm50 is approximately equal to 1 square foot of hole. Don’t believe it? Set up your blower door and try it. Run your test as normal and find your result. Then go to the other side of the house and add a 1 square foot hole. You can do this most precisely with saws, but homeowners and builders probably won’t appreciate your leaving a permanent hole. I usually open a window and measure it to get one square foot. Run the test again, and you’ll find that it’s approximately 1,000 cfm50 higher.
So if you’ve done a blower door test and the result is 6,240 cfm50, the house has a hole that’s about 6 square feet. Point to that 2030 window in the kitchen and tell them that they’ve got a window that size completely open to the outside, 24/7/365. I’ve found that to be just as effective as telling them how many times the air in their house exchanges... and I feel a lot better about my truthfulness when I do so.
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