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Community and Q&A

Reasonable Air Leakage Goals for an Old House

mjacksonw | Posted in General Questions on

Hey All,

We have a 1920s colonial in Eastern Massachusetts (zone 5A). When we moved in, the only insulation on the main part of the house was some vermiculite in the attic, which we had removed. There’s a three-story addition built in the late 1990s which had R-19 in the walls, R-30 batts in the attic’s cathedral ceiling, single-paned windows – seemingly the minimum at that time.

After having the vermiculite removed a few years ago, we went through the Mass Save program and had the unfinished original part of the attic blown in with cellulose, along with blowing cellulose into the exterior walls. The program also provides for free air sealing work, so they went around with cans of Great Stuff a bit. No blower door tests or thermal imaging was done before or after the insulation.

Fast forward a couple years, and we added minisplit heat pumps to the house, and started heading in the direction of using those for heat in the winter over the existing 1940s oil furnace. However, the minisplits have to do a ton of work to keep the house reasonable during the winter, and while expensive clean energy is better than burning oil, I’ve been interested in how we could improve the efficiency further.

We had a blower door test done a couple years ago, and we shot a 10.3 ACH50 (3600 CFM50). Perhaps better than when we bought the house, but pretty leaky. And that was with plastic film over the windows. There were some areas where the insulation was lacking, clearly missed by the contractors during the previous project.

I’ve spent a little time air sealing the newer part of the basement with Intello Plus, adding interior storms to our windows (in place of the film), using a few more cans of Great Stuff in a few key areas, but nothing major.

So, my main questions: what’s a reasonable target to shoot for in our kind of house before things call for heroic efforts, and should I expect more from the Mass Save-funded air sealing efforts?

If I want to be able to comfortably heat our 2600sqft home with our minisplits without using 4000kWh a month, is it going to require exterior insulation retrofit-level efforts, or is a more diligent job of air sealing going to close the gap significantly (see what I did there?)

I’m mostly paralyzed right now by the general direction to take, and whether it’s worth it to knock on Mass Save’s (contractors’) doors again to improve the situation.

Thanks all!

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  1. mgensler | | #1

    Our situation is similar. Old house with a poorly detailed newer addition. About 10 years ago, our blower door was 11ach after some insulation and air sealing. We kicked off a kitchen remodel this year and used that as an impetus for doing a big energy retrofit. We removed insulation, air sealed with foam, reinstalled insulation, redid an external wall with a smart membrane, pulled ductwork out of unconditioned space, installed mini-splits, etc. About half way through, we had a blower door test and it came back at 6ach. I'm thinking we can get to 3ach and still have a couple of vaulted ceilings to redo but those can probably wait as the family is tired of the mess.

    1. mjacksonw | | #2

      Thanks for the reply – do you feel like the work has been worth it? Do you feel like your sub-6ach home is more comfortable (less drafty, evenly-heated, etc)? It may be tough to tell if you did all the work, including the HVAC, in the same project, of course.

      1. mgensler | | #4

        Oh, yes. It's much more evenly heated, less drafty, and quieter. The finished basement is so much better than before as we sealed large holes in the rim joist. The frustrating part was finding contractors to do things correctly even when I gave them detailed specifications. Everyone made big mistakes even the engineer we hired to design the ductwork. We ended up doing (or redoing) about 30% of the work ourselves due to timing or not being able to find the right contractor.

        When we bought the place 12 years ago, the winter utility bills were $1000+ per month for the previous owners. Now we're probably around $300 and that's with two EV's and a hot tub. So a pretty big change plus we've reduced our gas usage by about 95%. The family is happy with the bedrooms being evenly heated as we had complaints of one that was too hot and two bedrooms that were too cold prior.

  2. Andrew_C | | #3

    A few thoughts come to mind. First is that if you do a good job in air sealing, you’re going to need mechanical ventilation. Think about what type of ventilation system you’re going to install and how that project may interact/interfere with other renovation and improvement projects. Off the top of my head, I think if you go below 5ACH50 you are required to have ventilation. This is especially true if you have combustion equipment instead of all electric.
    Second is that you may wish to do blower door directed air sealing, ie, find the major air leaks while the blower door equipment is running. How much a dwelling leaks is worth knowing, but knowing where it leaks so that it can be fixed is worth more. Having said that, you can’t get to everything in an existing house, so there’s a practical limit on what you can achieve wrt air tightness. Unless you use AeroBarrier after plugging the really big holes.
    While having a blower door is great, I think there have been articles here or at least suggestions on how to do your own pressurized leak locating using box fans, etc.


    1. chicagofarbs | | #8

      Rule of thumb is below 3ach50 is when you should have mechanical ventilation.

      I’d track CO2 regardless to make the final determination.

  3. Expert Member
    BILL WICHERS | | #5

    It's probably worth doing some blower door directed air sealing, which is basically pressurizing or depressurizing your house with a blower door, then going around with a smoke pen looking for leaks. This will help you to pinpoint the problem spots. Classic spots are recessed can lights and plumbing vent penetrations, also attic hatches.

    When you put up that window film, did you check if the frames themselves were leaking (unless you filmed over the entire frame)? Double hungs especially are prone to leaking. Rope caulk is your friend here, although small foam backer rod can often work as a nice removable gasket material too.

    I would think the Mass Save people would have checked this, but rim joist areas are also classic places for air leaks.

    If you have some windy weather, go around some of these "classic trouble spots" and feel for drafts with the back of your hand. If you feel air movement, you know you have some work to do. This is an easy and cheap way to find big leaks without bringing in any contractors.


    1. Expert Member
      NICK KEENAN | | #7

      Note that you don't need a full blower door setup to do this, you just need a way of pressurizing the house.

    2. mjacksonw | | #11

      Good call on the window frames. From our IR images, you can definitely see that they're not well-sealed, so that might be some low-hanging fruit.

      The rim joists were Great Stuff'd by the Mass Save contractors, but still showed some leakage during the IR segment of the audit. It's definitely on the list to spray those with a few inches of insulation as part of a broader basement project.

      I am such a measurement-centric person, and it's kinda frustrating that blower doors aren't cheaper or more readily available. I'd love to fix, measure, repeat and gauge the progress-to-effort ratio.

  4. Jon_Harrod | | #6

    I sealed my 1200 sf house from around 11 ACH50 to 5 ACH50. I'm happy with the results, but it was a lot of work. If I had it to do over again, I wouldn't worry about the walls and windows and would focus on the attic and basement. I'm guessing that would have got me to around 6-7 ACH 50 at much lower expense.

    Nate Adams has written that about 1 CFM50 per sf of conditioned floor space is a good retrofit target. That's the level at which he's found that indoor temperatures really even out and you start to feel like you have good control of the interior air. That would require you to cut about 1000 CFM50--an ambitious goal, but doable. That's about 100 square inches worth of air leaks.

    If you decide to pursue this route, it's well worth getting a really good diagnostic audit. Not just a blower door but an infrared scan, smoke test, and zone pressure diagnostics. This requires an experienced auditor and goes beyond what most no-cost audits will provide. But zone pressure diagnostics can tell you a lot more about where the leaks are happening and what CFM50 reduction you can expect from different work scopes.

    It's also really important to remember that, because of stack effect, leaks at the top and bottom of the house have a disproportionate effect on comfort and energy bills.

    One last point to consider: Are you sure your minisplits are working well? There are a couple ways to check. One is to do some performance testing on the systems themselves (I describe this process in detail here: The other is to look at your former oil bills and the increase in your electric bills with the minisplits and do some math. It will requires some spreadsheet work, but you can get at least a rough idea as to whether your minisplits are delivering a decent coefficient of performance.

    1. mjacksonw | | #10

      Thanks for the response! I follow Nate, but hadn't heard the 1 CFM50/sqft metric, which is super helpful.

      The diagnostic audit I got a year and a half ago was a paid audit, and was blower door + infrared, so I do have some insights there, and that's what drove the interior storms (since it was low effort, would replace the window film, and would have an overall aesthetic improvement). But it showed plenty of work to do on rim joists, band joists, sill plates, and the doors (which apparently cannot be weatherstripped).

      I'm not sure I have an anemometer and thermohygrometer handy, unfortunately. I did some of the calcs on the minisplits about a year ago, but I didn't have a full year of data at that point and should revisit it. I have moderate to low confidence in the details of the performance, since our installer has ghosted and vanished, but the early numbers I looked at weren't totally unreasonable. But still worth an update.

      Thanks for the food for thought!

  5. GBA Editor
    Kiley Jacques | | #9

    Allison Bailes addresses that question in this post: How Much Air Leakage in Your Home Is Too Much?

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