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Community and Q&A

Value to Air Sealing Between Floors?

johnstith | Posted in Energy Efficiency and Durability on

In a two-story house of average air-leakiness, is is possible and valuable to slow down the stack effect by air-sealing the upstairs and downstairs from each other? Weatherstrip the stairwell door, air-seal any chimney or plumbing chases at the upstairs floor, etc.? 

Would different HVAC configurations (zoned vs. unzoned, ducted vs. unducted) change the answer?

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  1. Expert Member


    That approach is sort of analogous to finding a window open in a house, and instead of closing it, shutting the door to that room. The same effort put into air-sealing around where the infiltration is usually highest - that is around the rim joist, and the top floor ceiling - will yield much better results.

  2. johnstith | | #2

    Thanks Malcolm and I hear what you’re saying, but if I’ve done all the envelope sealing I personally can do and can afford, and there are cheap-and-easy floor-to-floor sealing opportunities, should I spend my time doing them too? Is that a significant way of reducing air leakage at all?

    In our case our house will soon have insulation under the upstairs cathedral ceiling and all the professional air sealing of the envelope we can afford. Even after that, we’ll still be somewhat leaky.

    1. Expert Member
      Michael Maines | | #3

      One interesting thing you learn doing or following along on blower door tests is that whether doors are open or closed usually doesn't make much difference on overall air leakage. If the doors were weather-stripped with a good seal on all sides, that would make a difference but it's pretty rare to see inside a house. Air-sealing between floors can help with sound transfer and it might have a tiny effect on energy performance but I don't think it would be enough of an impact to focus on it. I'd put that extra effort into the envelope. Why do you think you will still have a leaky house after weatherization? Are you planning to use a blower door to help you find leaks? Have you considered using the Aerobarrier system?

      1. johnstith | | #4

        Hi Michael, I was hoping my original question could be a general building-science question, not a let’s-diagnose-this-one-house thread. I read dozens of GBA pieces about air sealing and couldn’t find anyone addressing this, so I thought I’d add to the overal GBA knowledge base. Sometimes the “Basics” GBA articles still aren’t basic enough for newbies like me.

        But it sounds like I’ve got my answer — floor-to-floor air sealing matters very little. Thank you for that — nice that it also crosses some house tasks off my list!

        I’m certainly happy to have specific advice too about our house! We live in a 1600 sq ft (of living space) Cape Cod built on a slab in 1947 in Maryland (climate zone 4A). Our blower door came in at about 5100 CFM/50 and our contractor has proposed air sealing to get it to about half that. Between that and the new insulation, the suggested project is about all we can afford. I don’t know where the remaining air leaks will be coming from or why they can’t be fixed too. When they do the final blower door, it will be a lot more interesting than the initial one!

        1. Expert Member
          Michael Maines | | #6

          John, assuming your ceilings average 8', you have about 12,800 cubic feet of volume in your living space. As noted above, in most cases, basements are part of the air volume which might add another 6500 sf of volume for a total of roughly 19,300 cubic feet.

          5100 cfm is 306,000 cubic feet per hour; divided by 19,300 cubic feet, that works out to almost 16 ACH50, which is extremely high. And being a Cape-style your ceilings might be lower, on average, which means an even higher ACH number. Do you have a couple of windows open or other large holes in your building envelope? Getting down to 8 ACH50 is a good start but it should be possible to get closer to 3-5ACH50 without unusually complicated methods.

          1. johnstith | | #8

            Hi Michael -- no basement here, our house is built on a slab. I think our ACH50 is about 26! Yes, we most certainly have huge holes in our building envelope. There's a section of kneewall missing, where someone made a closet and never re-built it after placing our air handler behind the kneewall. Missing kneewall plus a ridge vent means air has nothing stopping it. We also have significant holes at the bottom of our kitchen wall, which seems to be an old thick drywall with a brick veneer, so the stack effect is on overdrive here. In our neighborhood, entirely built in 1947, I suspect such situations are all too common.

            It's a good question why our contractor is only thinking they can halve our ACH50 rather than getting it down to 8 or 3-5 like you say. Thank you for that point -- I will ask them that. Hopefully they are just being very conservative in their claims, because there is a utility subsidy involved, and they don't want to overpromise what the subsidy will ultimately calculate out to.

  3. walta100 | | #5

    Cape Cod!!!! OOOO NO

    Ask them to calculate you ACH50 number. 5100 CFM does not mean much without knowing how big the house is.

    You may want to read these articles

    If you can’t bring yourself to move away from this problem then spray foam is your last hope.
    I don’t recommend spray foam often but yours is one of the most desperate cases imageable.


    1. johnstith | | #7

      Yes I've benefitted from those two articles and several others! Once I ruled out insulation on top of the roof due to cost and the challenge of finding a knowledgeable contractor, I concluded insulating in and under the rafter bays made the most sense, thanks to Martin Holliday's It also has the benefit of preserving the "triangle" as storage space, which this built-on-slab house desperately needs more of. We have a reputable contractor ready to do either option.

      I know spray foam would likely air seal better, but cost, fumes, blocking future access, etc.

      I would never leave this house -- it's the planet's good fortune that someone who cares about insulation ended up in this disaster Cape Cod! Of course the planet would benefit even more if we had more money to spend, but hopefully over time we'll get there.

  4. walta100 | | #9

    Since you are committed to this house and making better without foam. I think it will be almost impossible to find contractors willing to do the tedious work required in the miserable work spaces. My guess is if it happens it will be because you made the choice to do it yourself.

    You could duct tape a few box fans in the windows to depressurize the house and use incense sticks to locate the leaks and caulk them shut.

    Ask them to calculate you ACH50 number. 5100 CFM does not mean much without knowing how big the house is.


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