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Alphabet soup: ACH, CFM, ELA, MLR, SF

user-1135248 | Posted in Energy Efficiency and Durability on

[You’ll find a duplicate of this as a comment on an old blog thread
about “how much ventilation”, but obviously that’s not the way to
get it actually seen and commented on.]

Before my retrofit I did a quick-n-dirty volume and surface area
calculation on my relatively simple Cape, and have just redone it
a bit more accurately. I’m now reading up on blower-door testing
because I expect that eventually the place will get one, and the
confusion over which numbers are which, better, meaningful, BS,
etc is *profoundly* irritating. Anybody who comes along talking
about CFM leakage per square foot of *living area* needs to be
shown the door and by that I mean as a prompt exit assisted by my
foot, not the door to mount the blower frame in … then you’ve got
the problem of Equivalent Leakage Area over the entire envelope vs.
MLR which only considers above-grade envelope surface, and for the
latter I would probably want to add in the below-grade area of my
basement bulkhead door to make things more realistically accurate.

By breaking the house into sections — attic, living floors, basement —
I know all my volumes and areas, and derived an easy reference point
that 1 ACH for the whole enclosed space, just shy of 15,000 cubic feet,
would be 248 CFM. That’s regardless of pressure, of course, and that
volume includes interior walls, closets, the volume of the remaining
part of the chimney inside, etc. But what is the “geek with the blower
door” going to say about what any CFM reading actually means in terms
of CFM/sf? How many “square feet” should be considered, and where are
they located?? How does anyone know how much “living” I’d do in my
unfinished basement before deciding whether to include that in any
such figures?

And then in a heavily-insulated house one has to consider where the
measurements were taken — inside or out? I started with my interior
dimensions because that’s effectively the volume where air will be
changed, measuring heights floor-surface to floor-surface to include
all of those thicknesses. But if I did the same measurement outside
now, with the exterior bulked up 5 or 6 inches more with polyiso and
furring and siding, everything would be different.

So really, “CFM50/sf” has to be *thoroughly* qualified and explained
in any test results if it’s going to mean jack. And it likely won’t
account for *where* leakage sites actually exist, i.e. would they
contribute to stack effect more or less. At least “ACH50” makes more
sense because it keeps everything in terms of volume, as long as you
know how that volume was determined.

The poor sod who eventually shows up here to test the place is probably
going to get a real earful about this, as he’s given an envelope with
*no* open penetrations to test. I mean, why should I leave my HRV
vents open if we’re testing the *envelope*?? I know how many CFM
that system gives me already.


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  1. davidmeiland | | #1

    Do you have a question?

  2. user-1135248 | | #2

    Well, yes, even if it might have been a bit vague. Anything
    in the lead post with a question mark indicates something I'd
    like to understand better, but to sum up: What useful data
    should I look for from a blower-door test, and what does the
    community consider misleading or fluff that they'd like to
    see testers not even bother to mention anymore?


  3. davidmeiland | | #3

    Well, I guess the only number I would really pay attention to is ACH50. If you can get that around 3 on a limited retrofit, you're doing great. CFM/sf is related to ACH50 by way of ceiling height, so I guess you could look at that... in general a result around .5 CFM50/sf is good, especially when a lot of houses test in at >1 CFM/sf.

    The "leakage area" stuff doesn't do much for me, maybe it should, I dunno.

    Of course during a test we pay attention to where the leaks are and how easy they are to reduce, and in the end, you do what you reasonably can. If you are doing work on a house, you really want your own blower door, or a good facsimile thereof, so you can do guided air sealing. Having a guy show up at the end to give you a single number is less than ideal.

  4. user-1135248 | | #4

    See, you've offered an unqualified "sf", so right there I don't know
    if you're talking about envelope, partial envelope, or living floor
    area. But I'll let it go, as it seems to be a really common
    specification problem as observed in my original premise.

    I would tend to agree that ACH50 is probably the best measure of how
    quickly my air would get replaced in an uncontrolled fashion. How I
    choose to dispose my floor space inside is totally irrelevant. But
    still, if I had a hole in the attic and a hole in the basement the
    effect of infiltration/stack would be much more profound than if the
    same two holes were both on the first floor -- hinting at the several
    discussions I've found on the impossibility of determining "natural
    ACH" just from blower-door tests.

    In case the point has been completely missed, my question really boils
    down to "why is this whole topic such a mess and how do we fix it".

    Once colder weather arrives I'll definitely be sticking an exhaust
    fan in one upstairs window and running around givin' the place the
    back of my hand, as it were.


  5. davidmeiland | | #5

    "Unqualified sf".... maybe, but almost everyone either uses square footage of floor area including the exterior walls... i.e. footprint of building... or square footage of floor coverings, inside the drywall. I use the former for a couple of reasons, but you could use the latter and they're really quite close, not enough to quibble over in my opinion. Example: a rectangular house with a 25x40 footprint has 1000 square feet including the walls, or ~936 if measured inside the drywall, a difference of 6.4%. I can't get excited about it. If you think your drywall is an excellent air barrier, you could use the 936 and get a slightly higher ACH50 from a given CFM50, raising the bar on yourself a bit compared to the guy who goes with 1000. Or, maybe try both and look at the difference.

    Now that you've actually boiled down your question, I don't see how the whole topic is a mess at all. You are correct that blower door testing creates an artificial condition and that natural leakage may not be easily predicted, for reasons like the one you use as an example. But, is it close enough for you, and if not, why not? What really matters is where the leaks are and where you can tighten economically, and how the house performs in actual use. Airflow standards give you something to shoot for, but IMO they are rules of thumb and no more... they may result in over- or under-ventilation of a specific house and that varies quite a but by occupancy, season, and other factors.

    There is no "set and forget"" with this stuff. You need to follow up on the work you do, when and where possible, and see what actual conditions develop over time. That is what I try to do. You need to develop and use your skills and experience to be a good building performance tech, and you need to let go of any idea that you can predict outcomes with perfect accuracy using the tools and standards

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