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ACH50 house volume – floor to ceiling or floor to floor?

tundracycle | Posted in General Questions on

How do you calculate building volume for ACH50? I’ve searched and cannot find this.

In a three level home (LL+1st+2nd) with 9′ ceilings and 2′ floor trusses is the volume for ACH50 the sq ft of each level times 9 or times 11? Does it matter if the floor truss space is empty or filled with blown fiberglass?


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  1. this_page_left_blank | | #1

    There are a few different schools of thought on this, and different organisations have different takes on it. However, in all of them I think the basic concept is "floor to drywall to drywall". If those floor trusses are enclosed, they wouldn't count. Some standards exclude interior walls, stairs, etc.

    1. tundracycle | | #2

      Thanks Trevor. Exactly what I needed to know.

  2. burninate | | #3

    I don't know how official standards are defined, but the thing that makes the most sense to me is insulated and/or airsealed volume, to include flue spaces.

    Substantial non-walkable space is a relatively new thing in homebuilding; This conversation would be much less significant with 2x10 floor joists. It's only with the advent of the open plan first floor, and substantial growth in home size, that it started to become reasonable to use sizable floor trusses.

    Simultaneously, building science itself, airsealing, and blower door testing is only today starting to become a thing that your typical builder thinks about; Many inspectors still don't have much grasp or interest.

    I would imagine that the difference is usually assumed to be negligible in the grand scheme of things.

    1. tundracycle | | #5

      Depends on how you define negligible perhaps? In this particularly case using floor to ceiling results in an ACH50 of 1.9 while including the floor truss space gives 1.3.

      If a primary purpose of the ACH50 calc is to allow somewhat equal comparison of one structure to another then that difference seems more than negligible? In the real life scheme of things it may actually not matter a ton (or even 1/16 ton) but if we are going to use ACH50 as a standard then it s/b as standardized as reasonably possible?

      If a builder says that he will build for me a quality house and among the attributes is that it will be tight with an ACH50 of 1.0 or less and I agree to build with him only to find out that he includes the floor truss space and other 'non air flow' space and so the house is actually quite leakier at 1.5 then I will not have gotten what I expected.

      BTW, I think we agree on what it should be which is basically the 'total free airflow space' so floor to rock to rock + ducts. E.G., the space the air will change in.

  3. fourforhome | | #4

    It's just a calculation. Do both and specify the parameters.
    You can please everybody.

  4. tundracycle | | #6

    The guys who did our blower door tests said that according to ResNet they are supposed to use exterior measurement as volume for ACH50. So include all exterior walls, interior walls, all floor truss space, all space behind knee walls, etc. So effectively out to the most exterior extent of the insulation envelope.

    Does that sound correct? That is different than I have ever understood this calculation and gives a quite different meaning to Air Changes.


  5. user-723121 | | #7

    To me the volume of a home for this calculation is all of the area inside the primary air barrier were ever that happens to be. A cold climate house with a polyethylene warm side vapor barrier would use the interior wall dimensions. A Zip wall with a cold side air barrier would include the exterior walls in the calculation. Basement dimensions, between floors, vaulted ceilings should all be included in the volumetric calculation as they can all be subject to air leakage.

    1. tundracycle | | #8

      The measurement is called 'Air Changes' though. Air doesn't change in spaces that are largely sealed off and in particular if they are filled with insulation. Since this is called Air Changes / Hour then Trevor's comment above seems the closest; floor to rock wall to rock ceiling - the space where air does in fact change.

      As well, what I've been able to find states 'floor to ceiling for the conditioned square footage'.

      A bigger question though is that this is supposed to be a standard. If one guy uses one calculation and says my house is 1.9 ACH50 and another guy measures a different but perfectly identical house, gets the exact same CFM50 but says 1.2 ACH50 simply because he uses a different calculation for volume then that's a problem and the standard called 'ACH50' is pretty meaningless.

  6. joshdurston | | #9

    IMHO, it depends on what your purpose and goals are.
    If you're the general contractor trying to get a low number you can stack the deck in your favor by using the outside dimension, thus inflating the volume (and hence reducing the leakage rate).

    As a stakeholder in the property interested in high performance, you want the number to be harder to achieve so a smaller volume (interior surfaces) will make it tougher to get a "good" number and force your contractors to try harder.

    Personal option... I think volume based standards are flawed, it should be per sqft, or even absolute per dwelling. I kinda like CFM50. A residential dwelling should have an upper cap on leakage, forcing big home to be built better. Adding 1000sqft of area to house shouldn't entitle someone to leak more CFMs (which a volume ratio allows). A small house should be allowed to be leakier than a big home since it actually will be a lower absolute energy consumer.

    ACH50, is like measuring vehicle fuel efficiency as mpg/ton (instead of just MPG), which makes heavier vehicles looks better even though they are burning more fuel.

    1. tundracycle | | #12

      It should not be a pick your standard. If it is then, as I said above, ACH50 is meaningless. As a property owner I also don't want my builder trying harder but spinning his wheels trying to meet an impossible target.

      I agree on ACH50/SqFt of surface area. That would be a much better standard.

      CFM50 by itself is not comparable. You are going to have different sized and shaped homes and you need a way to compare them somewhat accurately which is what CFM50/SqFt will do.

      If I'm a single person living in a 600 sq ft home, why should I be able to leak as much as 5 people living in a 2400 sq ft home? Or 3 people who live in a 4400 sq ft home that is also where they do business?

      Realistically the answer might be a hefty energy surcharge that encourages people to build more energy efficient so that they use less and pay less of the surcharge but that comes with a long list of other problems so let's leave it to another thread.

    2. this_page_left_blank | | #13

      I agree that leakage per area of living space makes the most sense. A house with 10 foot ceilings is not 25% more efficient (air leakage wise) than the same house with 8 foot ceilings.

  7. brad_rh | | #10

    I'm surprised no one has the answer to this. The recent energy codes (IECC ) specify a blower door test, and an ACH value, surely they spec how to measure the volume?

    I like Josh's idea of CFM/sqft or CFM/person.

    1. tundracycle | | #11

      Brad, I agree, it should not be this difficult and confusing to find out.

      I'm beginning to wonder if there are perhaps two standards; 1) Used in most countries, by PHI and by The Energy Conservatory which is floor to walls to ceiling (so not including 'dead space' such as floor truss space) and 2) A more lax standard developed by ResNET that results in a greater volume and so better (but false?) scores.

      I agree with CFM50/SqFt of surface area. Or perhaps CFM75. I think that would be much more accurate and meaningful. Unfortunately that is not what we have today though I do think that it should at least always be reported so that people can get use to it and so that it can become the standard.

  8. Expert Member
    RICHARD EVANS | | #14

    W Ramsay,

    If we are lucky- Michael Maines will see your question and offer a thorough answer for all of us.

    Personally, I've never liked the ACH metric as it favors larger buildings. PHIUS got it right when they opted for CFM per TFA (Treated Floor Area) over the rigid 0.6 ACH that PHI uses. TFA is a little complicated but you can read about it on the PHIUS website.

    I believe the standards for air infiltration come from RESNET. RESNET, like PHI, uses "Conditioned Floor Area" or CFA rather TFA.

    Here is their definition of CFA:

    "Conditioned Floor Area (CFA) – The floor area of the Conditioned Space Volume within a building, minus the floor area of attics, floor cavities, crawlspaces, and basements below air sealed and insulated floors. The following specific spaces are addressed to ensure consistent application of this definition:
     The floor area of a wall cavity that is Conditioned Space Volume shall be included.
     The floor area of a basement shall only be included if the party conducting evaluations has
    o Obtained an ACCA Manual J, S, and either B or D report and verified that both the
    heating and cooling equipment and distribution system are designed to offset the
    entire design load of the volume, or,
    o Verified through visual inspection that both the heating and cooling equipment and
    distribution system serve the volume and, in the judgement of the party conducting evaluations, are capable of maintaining the heating and cooling temperatures specified by the Thermostat section in Table 4.2.2(1) of ANSI/RESNET/ICC 301- 2014.
     The floor area of a garage shall be excluded, even when it is conditioned.
     The floor area of a thermally isolated sunroom shall be excluded.
     The floor area of an attic shall be excluded, even when it is Conditioned Space Volume.
     The floor area of a floor cavity shall be excluded, even when it is Conditioned Space
     The floor area of a crawlspace shall be excluded, even when it is Conditioned Space

    However, In your case, you are dealing with a multi-family building. This actually does complicate things quite a bit for the reasons you're described. (If you de-pressurize your unit and it is poorly air sealed, then you may be sucking air in from your neighbor's unit.)

    There are different ways of measuring air tightness in multi unit buildings without a common entrance. One way is called Compartmentalization (The other methods are more complex and likely wouldn't be relevant to you anyway.) Here is RESNET's requirements if you go this route as I think it may answer your question:

    "Compartmentalization metric (CFM50/ft2 of enclosure area) – For the purposes of
    defining the test boundary, the surface area of the dwelling unit enclosure, inclusive of all bounding walls, floor, and ceiling, shall be calculated as follows:
    i. The wall height shall extend from the finished floor to the bottom side of the floor decking above the target dwelling unit for non-top floor level dwelling units and to the exterior enclosure air barrier for the top floor.
    ii. The floor and ceiling area shall extend to the centerline of the adjacent walls."

    1. Expert Member
    2. Expert Member
      RICHARD EVANS | | #16

      Sorry... Long Week!

      I misread your question. Looks like you are not living in a multi-family unit after all. That makes things easier...

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