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Value of air sealing – mostly BS or at least way overblown (pun)?

RedFalcon6 | Posted in Energy Efficiency and Durability on

So I’m a huge fan of this board and of Youtube channels like Matt Risinger.  I’m also building a new house to very high efficiency standards.  I have to confess, I’ve never sat down and done “the math” on the payback for increasing air-tightness from “Ok” to “Pretty Good” to “Awesome”.

So I tried to find a rule of thumb on airtightness alone.  What’s the super rough value here if I take my house down from 3 ACH 50 to 1 ACH 50, all else being equal?  (Assume a 15,000 KWH annual energy budget)?

California has done a study, and in the table it says that the average value for a California home going from “average” to “passive” levels of airsealing would be around 2K kWh annually. (Page 23)  So go go all the way to passive standards only saves an incremental $200 bucks on an average home?  Seems not worth it!

There are lots of energy nerds here (title used affectionately) that will throw up a lot of math and say “it can’t be calculated without specifics.”  That is certainly true.  But since the industry seems to standardize on a measure of airtightness using the blower door test, how much is it worth to really push for truly outstanding results like 1 – 2 ACH 50? 

Doesn’t seem worth it to just reduce my  electric bill by $10-15 a month!

Please prove me wrong!  Am I an idiot for just misreading this study?

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  1. RussMill | | #1

    A whole lot is perceived comfort. Tight houses have far less drats.

    Thats one reason reclaimed foam is pushed, cost savings. I build for a living and i can state for a fact, its hard to justify extreme insulation and air tightness to a customer because our electric rates are so low. They will not be staying that way though. Our small increases caused mass disconnections a couple years ago. Poor state.

  2. seabornman | | #2

    If you live in California you can leave your doors open! Here in the real world (Zone 5) I place air sealing ahead of r-value in home comfort and energy efficiency.

  3. JC72 | | #3

    Remember Passive House concept started in Germany which is a heating dominated climate and free of termites (no worry about buggers using rigid foam as a home).

    Air sealing is much more important in climates with high humidity and/or which are heated/cooling dominated. As for PH standards? Ya, the economics don't work for PH housing in much of California with its temperate, dry, climate

  4. RedFalcon6 | | #4

    Yeah @Joel I'll give you that! But even doubling it is still a far shot. I'm actually building in NH, so the comment is relevant.

    This doc gives annual energy consumption of about 80% more in a very cold climate to "marine" climate. so yeah, absolutely worth more but does it really justify going to extreme levels (which also absolutely require better management of make-up air etc, increasing costs and complexity).

    I'm not saying do old school ACH 10 type numbers. But pushing really hard for anything beyond 3 seems like total overkill to me? You probably experience the max ROI on comfort and energy savings then?

  5. Expert Member
    RICHARD EVANS | | #5


    A specific air leakage number doesn't tell the full story. If the air leaks are all in the center of a wall (around a window for instance), then the heat loss won't be that dramatic. It might be a comfort issue however.

    A house with air leakage at both the ceiling and foundation however will lose a lot of heat due to the stack effect. Even if it's ACH figure is identical to the house with air leakage just at the windows.

    Also, passivhaus institutes have air tightness requirements not just because of heat loss. They care about it because it is a critical metric to determine how 'safe' a super insulated wall or roof is.

    Holes in a super insulated wall can lead to big trouble as there is little thermal energy finding its way through all that insulation and warming up the exterior surface. If warm air does hit that super cold surface (due to hole) then it will condense. Over time, your walls may develop mold or rot.

    Old, leaky homes never had to worry about this. With little or no insulation the sheathing/siding always remained warm from the heated interior space.

    If you are building thick walls then you need to pay attention to air tightness in a cold climate. Pay special attention to the ceiling and bottom plates. Hitting 1 ACH would be my goal and it isn't hard to do at all.

    If you are doing it all right (and depending upon size of house, mini split sizing), then your annual heating bill may be less than $400-$500, even in NH. $10-15 a month is a significant percentage of that figure!

    1. RedFalcon6 | | #6

      @Rick, that's a great point. You do it not only for the value in and of itself, but also because of the benefits to the system. As you point out, a leaky super insulated wall will drive condensation and likely get rot. So if you are going for high efficiency, make sure it all works together.

      That said, I get that it is a big percentage, but as they say in business, "you can't eat percentages".

      (Also to be clear, I'm aiming for ACH 1, but wondering about the value of going for an Aeroseal treatment if I don't hit it. Top and bottom plates all sealed with gaskets, tape, etc. )

      1. Expert Member
        MALCOLM TAYLOR | | #7


        It's an interesting point. The case for further insulation when there are rapidly diminishing returns is hotly debated, but not air-sealing - which for some reason plays into our energy-nerd OCD tendencies.

        Where that sweet spot is depends not only on a climate-based assessment of the returns, but also the robustness of the assemblies. Some very fragile ones are extremely intolerant of air-leaks. My own approach would be not to use them, but if you do, they need protection.

  6. walta100 | | #8

    When I run a BEopt model I see $800 a year difference in fuel cost going from 25 to 1 ACH50 given this house model in this location with this HVAC equipment and fuel cost.

    Note BEopt did not add any more cost in dollars for air sealing. It would be a more interesting graph if we enter a cost for each of the 13 levels of air sealing in the graph.

    Of course your numbers will be different and knowing an energy nerds wrote the program.

    My guess is the saving is a lot like insulation there is much more saving changing from R1 to R30 than R30 to R59.

    Be careful when you look at the passive house stuff as that system has no economic component in that your house will use no more than X number of BTUs per X amount of space regardless of how much it cost to build.


    1. Jon_R | | #9

      So using Walter's estimate for his climate, ACH3 -> ACH1 saves $67/year.

  7. Expert Member
    Dana Dorsett | | #10

    Mind you, a lot of tract homes built since 2010 are still a lot leakier than 3ACH/50 (even though that's more of a stripe on the floor to hit rather than a hurdle.) Coming down from some much-higher number to 3ACH/50 isn't usually much of an expense in new construction, but can sometimes be in a retrofit.

    The additional cost of going from 3ACH/50 to 1ACH/50 could be pretty daunting in a retrofit, and may not be "worth" chasing in new construction house that already tests under 3ACH/50. But in new construction just paying enough attention to details to be sure to beat 3ACH/50 often leads to leakage rates that aren't too much greater than 1 ACH/50, often hitting the 2ACH/50 range at no "extra" cost.

    Going from Walter's 25ACH/50 to <3ACH/50 is definitely "worth it" simply on a comfort humidity control basis, even without calculating the net present value of future energy cost savings. But getting hung up on it because it "only " came down to 2.8 ACH/50 or 1.7 ACH/50 (and not 1 ACH/50) would be silly- it's already tight enough to take control over temperature and humidity, and isn't an energy hog.

    The 3 ACH/50 number became enshrined in IRC code because it's pretty easy to meet/beat, and makes a difference in both comfort and energy use compared to typical construction practices in place when there was no standard. Many better-built custom homes in colder climates built prior to that standard would meet that test (though almost none would come in at or under 1 ACH/50).

  8. bfw577 | | #11

    How is this for air sealing savings. Using my FLIR I discovered my furnace return in the stud cavity was completely wide open to the attic. Actually the entire wall was open to the attic. I was drawing in 100+ degree air in the summer and basically outside air in the winter.

    Anyone want to guess on the savings from sealing this up?

    1. Expert Member
      BILL WICHERS | | #14

      This kind of thing is more common than you’d think, but luckily easy to fix. In my own home, I sealed the blocking at the top of the air returns, and the drywall on the sides. Every little bit helps. In your case, you probably want to add some blocking and then seal the edges.


  9. this_page_left_blank | | #12

    Going from 3ACH to 1ACH costs essentially zero. It just requires having proper details and not being extremely sloppy. (Talking about new builds only)

    1. Expert Member
      Dana Dorsett | | #13

      That's right, as long as you're not being super-serious about treating a measured 1.3 ACH/50 as a failure that MUST be remediated to < 1ACH/50. It's just not that hard if the crews are at all conscious about keeping it air tight (something that 20 years ago most were pretty oblivious about.)

  10. Expert Member
    Michael Maines | | #15

    A BEOpt model for a 3000 sf house built to Pretty Good House standards (other than size) with a total annual energy use of about 13,000 kWh shows these results for different airtightnesses, at 16.3 cents per kWh for an all-electric house:

    7 ACH50 (Maine's code minimum): baseline
    3 ACH50: save $280.53/yr
    1.5 ACH50: save $418.59/yr
    1.0 ACH50: save $445.81/yr
    0.6 ACH50: save $467.33/yr

    As others have said, once designers and builders understand how to build relatively airtight, it's really not very hard to get to 1.0 or 1.5 ACH50. Getting to 0.6 (or lower) ACH50 takes more effort and may or may not be worth it to you.

    In my example above, going from 3.0 to 1.0 ACH50 saves $165.28 per year. Even if the associated cost to do so is $1650, that's a 10% return on investment, which is very good, especially since it's virtually guaranteed. For an experienced high performance builder the cost is probably just a few hundred dollars, resulting in ROIs of 50% or higher. Try getting that in the stock market.

    1. Jon_R | | #16

      > going from 3.0 to 1.0 ACH50 saves $165.28 per year

      I assume your model doesn't use a heat pump and it would be more like $55/year with it?

      1. Expert Member
        Michael Maines | | #17

        Why do you say that? The model includes mini-split heat pumps with electric resistance backup.

    2. RedFalcon6 | | #18

      Interesting - super helpful. This is also roughly roughly close to the savings with adding 1Kw of system size to a solar array.

      I think my takeway is:

      1. Shoot for 1 ACH50. The detailing to get here is not that hard when you are building new anyway.

      2. Especially focus on the airsealing around vulnerable integrated systems that may be screwed up if they are getting air / vapor.

      3. Don't sweat it if you are a bit over.

      4. Results of 2+ may point out a bigger problem that needs to be fixed, that could affect other systems above.

      1. Expert Member
        MALCOLM TAYLOR | | #19


        That sounds like a well reasoned summary to me.

        1. Expert Member
          Michael Maines | | #20

          I agree, that sounds like a good plan. I recommend testing at multiple stages, starting as soon at the building is dried in, so you have a chance to close gaps while you have access to them.

    3. maine_tyler | | #21

      I'm not familiar with BEopt so forgive me if this question is non-nonsensical, but how does this account for active ventilation? Does it use an efficiency rating of an HRV or something?

      That's one of the aspects of ventilation/air-tightness that I've always struggled with. I get the reasons to 'build tight, ventilate right' (though somewhat cringe at the saying), but do these upper limit numbers just get thrown in the wash when we consider that we're adding purposeful ventilation anyways? Obviously more so if its not a heat exchange ventilation. In that case, are the effective blower door results changed by magnitudes?

      1. Expert Member
        Michael Maines | | #22

        Tyler, BEopt includes inputs for air leakage, mechanical ventilation and natural ventilation.

        Air leakage is whatever ACH50 number you want to use.

        Mechanical ventilation offers several options for active ventilation:
        Exhaust-only to either the 2010 or 2013 ASHRAE 62.2 standard;
        Exhaust-only to 50% of the 2010 ASHRAE 62.2 standard;
        Supply-only to the 2010 ASHRAE 62.2 standard;
        HRV at 60% or 70% efficiency to the 2010 ASHRAE 62.2 standard;
        ERV at 72% efficiency to the 2010 ASHRAE 62.2 standard.

        Natural ventilation means opening windows when the outdoor weather is within preset parameters for temperature and relative humidity, with options for how often the windows are opened when conditions are acceptable.

        You can adjust the values they use or input your own numbers.

        Not to sound like a BEopt salesperson, but it's free and powerful:

  11. walta100 | | #23

    BEopt is an amazing program. In addition to the leakage options I posted BEopt has 8 more options for mechanical ventilation plus 4 options for natural ventilation.

    Anyone that says BEopt cannot account for some variable is very likely to be proven wrong.


    1. Expert Member
      Michael Maines | | #25

      Walter, so far the only thing I've found that BEopt doesn't do well is certain foundation systems. They don't include an option for sub-slab insulation in a basement, and there is not a way to model daylight or walk-out basements. The results are not as accurate at the output from a PHPP Passive House energy model, but the inputs take about 1/10th the time, and are accurate enough for general use.

  12. AlexPoi | | #24

    Don't forget that air can transport moistures from one location to another as well so it's not just about energy costs. Air leakages depending on your wall and roof assembly can lead to condensation issues so don't just focus on the number but on the whole assembly. For instance, typical double walls constructions are much more susceptible to air leakage problems because the sheating is outside of the insulation.

  13. RedFalcon6 | | #26

    Just to finish up this thread in case anybody was wondering where the story ends - blower door test of 0.7, so we are there! Thanks all for comments.

    1. Expert Member
      MALCOLM TAYLOR | | #27

      Well done!

    2. Expert Member
      BILL WICHERS | | #28

      You should keep an eye on your utility bills over the rest of the season and let us know if you see a noticeable improvement. Ideally you want to normalize the info for weather conditions, which is easier now that many utilities are giving the average temperature info along with their monthly consumption numbers.

      Congrats on your project too! Every time I do some air sealing or insulating work I can usually notice an improvement, and its a REAL improvement, not just a "I did a lot of work so it MUST be better" kind of improvement :-)


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