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Community and Q&A

Value of each 1/10 point ACH50?

tundracycle | Posted in General Questions on

Our new house in Minnesota unfortunately does not have exterior insulation so we’ll have a lot of thermal bridging. Construction is standard 2×6 w/ 2.25″ closed cell + blown cellulose to fill it out. In this case, how much is each 0.1 ACH50? ┬áIOW how important is it to get from say 1.2 to 1.0? Or to 0.9?

Our insulation contractor had said he’d get us to 1.0 but is having difficulty getting there. One option he has proposed is bringing in a company that effectively aeroseals the entire house and they say they can get us to 0.8 or better depending on windows & doors. He’s saying that he’ll split the cost with us if we want to do that. I’m still waiting to hear from them on what chemicals they use and what issues there might be with their method.


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  1. Aedi | | #1

    It depends on a few factors and on your own preferences. In general, the primary reason to get below ~1.5 ach50 is bragging rights, and it sounds like the added expense of you getting to 1.0 ach50 on your project will not be offset by long term energy savings. This is especially true if you do not have an HRV or ERV, since that air sealing will just be countered by higher ventilation rates and confer zero energy savings.

    Does your contract mandate the insulation contractor get you to 1.0 ach50?

  2. tundracycle | | #2

    We'll have two Braun ERV250 ERV's. House is 7300 sq ft.

    Aerosealing is:

    I think that there is more than just energy savings and lower energy use. Potentially there's a comfort element if leaks allow in enough cold air near where someone is sitting.

    There are two IAQ bits; 1) Any air coming through wall cavities or other leaks will pull in whatever is in those cavities so off-gassing from spray foam, etc and 2) bring in outdoor humidity creating potential for mold grown near leaks (and which then gets pulled in to the house with off-gassing stuff). I've no idea how much of an issue these are. At one time we thought CO2 levels of 2500 were OK, then we said 1000 and now it's 500-600.

  3. Jon_R | | #3

    > potential for mold grown near leaks

    In terms of wall moisture, there is well known benefit to slightly positive pressure in Summer and slightly negative in Winter. The former also helps with air quality. No data, but I'd guess that pressure adjustment is more important than going from 1.5 ACH to 1.0 ACH.

    You can calculate the estimated seasonal energy loss for a given [email protected] (but I'm feeling lazy).

  4. Aedi | | #4

    I'm familiar with aerobarrier, and am of the opinion that it is too expensive to make sense for new construction, at least from the quotes I've seen posted on here.

    Leaks large enough to be felt by the occupant are low hanging fruit, and your insulation contractor should have already taken care of them. Using a thermal camera during a blower door test (at negative pressure) can help verify, and would be substantially cheaper than aerobarrier.

    Your other concerns should be easily controlled by keeping your home ventilation at the ASHRAE recommended rates. They generally aren't issues even in homes with twice as much infiltration as yours. As an added precaution, I imagine you can balance your ventilation system to slightly pressurize your house, which should reduce infiltration (You'll have to talk to your installer about that)

    All that said, you certainly can use the aerobarrier if you want. It's not cost effective, but if you have the money and it makes you feel better, go for it.

  5. GBA Editor
    Brian Pontolilo | | #5

    Hey W Ramsay,

    I am not sure how much benefit you will get for lowering your ACH numbers by single decimals. That would have to be modeled. But the fact that your insulation contractor is responsible for air tightness raised some questions for me. If the insulation contractor is relying on the spray foam in the flash-and-fill wall assembly for air sealing, has someone looked at the sills, around windows and doors, and what's the air sealing strategy at the attic? Has your contractor tried blower door directed air sealing? Here's an article on that strategy:

  6. tundracycle | | #6

    We had some energy consultants in last week who ran a fog machine, pumped the house up to +50pa and then looked for leaks inside based on incoming air movement. They said that the house looked tight. A few hours later a blower door test indicated 3.32 @ ACH50.

    Our insulation contractor did a bit more work and said that his tests are now indicating 1.8 @ ACH50 so we're getting closer but he added that this is the best he can do and that any better requires the Aerosealing folks.

    So another question is how tight should an insulation contractor w/ closed cell spray foam and other bits be able to get the house? Should they be able to do better than 1.8?


  7. GBA Editor
    Martin Holladay | | #7

    W. Ramsay,
    Q. "How tight should an insulation contractor with closed cell spray foam and other bits be able to get the house?"

    A. The insulation contractor isn't the only person responsible for air sealing. If you want a tight house, you need to have an air sealing plan before construction begins. That air sealing plan will (ideally) involve framers, the sheathing crew, window installers, plumbers, electricians, and other contractors. You can't ignore all the necessary details during construction, and expect spray foam to solve all the defects.

  8. tundracycle | | #8

    Brian, the insulation folks have gone around and caulked any leaks they've been able to find in sills, between adjacent studs, around windows, etc.

    The insulation folks have a blower door and have done some blower door directed sealing. One problem is with current temps they can only do it for about an hour or two in the morning when the indoor house temps are enough above outdoor temps for stuff to show on their IR cameras.

    Some bit on the project and blower door testing:

    I am wondering if their approach of looking for leaks from the inside instead of the outside was perhaps not the best approach based on your link.

  9. tundracycle | | #9

    Thanks for the thoughts on Aerobarrier. Kind of my thoughts as well. I sometimes find myself getting quite anal about hitting certain numbers. Perhaps irrationally anal.

  10. GBA Editor
    Brian Pontolilo | | #10

    Maybe not. Most of the tighter houses that I have known about, say 1 ACH50 or lower have had an air barrier strategy that was part of the design process. Remember, an air barrier is an assembly that needs to be continuous around all six sides of the building. If I were an insulation contractor, I'd want to know a lot of other details about the house before I'd be willing to make any promises on air sealing.

  11. GBA Editor
    Brian Pontolilo | | #11
  12. tundracycle | | #12

    Answering an earlier question. Our contract does not specify 1.0.

    However, when our architect and others were pushing back against exterior insulation one of the things our builder mentioned is that he was confident he could get to 1.0 @ ACH50. Then a few weeks ago the insulation guy twice said that he should be able to get us to 1.0, once during a meeting at our builder's office and a bit later walking the site. Though small compensation, having 1.0 @ ACH50 was a key to my giving up my push for exterior insulation.

    1. Aedi | | #16

      It is always frustrating to have your project undermined by the promises of someone who couldn't follow through. Even if the contractor does not have a contractual obligation to get you to 1.0 ach50, he does have a moral one. Unfortunately, that doesn't carry much weight in our modern economy.

      1. tundracycle | | #17

        Sad and true. And depressing.

  13. Expert Member
    AKOS TOTH | | #13

    This is a rough stab at it, going by 7300sqft with 9' celings [email protected] is 1100cfm. Roughly converting that to ACH natural (this is a very squishy estimate, but at least gets a number), you get around 91cfm.

    Heating 91cfm of air from -10C to 20C takes about (54F delta) 5300 BTU.

    So each 0.1ACH is around 530 BTU of extra heat in the winter. Compared to the overall energy use of the house, this falls into the noise category.

    1. Jon_R | | #14

      > each 0.1ACH is around 530 BTU of extra heat

      So perhaps $20/year per.

  14. tundracycle | | #15

    Akos & Jon, thank you guys! Perspective is amazing.

    I think given all of the energy loss via thermal bridging it may not even be much noise.

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