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Community and Q&A

Sealing between floors on 1.5 story home?

Zuhl | Posted in Green Building Techniques on

High, I am currently building a 1.5 story home with 2.6 walls that will be insulated with blown cellulose. I am sealing all joints and penetrations with caulk and spray foam, and will doing R-38 in the vented attic.

I am concerned about the open space between joists between the upper and lower floors. I want to know if I should seal off this space so that the hot attic air does not infiltrate between the floors.

The upper floor is only about 1/3 the size of the lower floor and I only need to do supplimental sealing between joists in one directions since the joists running longintudinally block it in the other direction.

I have though about just filling this whole void with insulation, but that is not so simple either. The upper floor level is on top of a ten foot cieling. but some of the rooms under it have 9′ cielings. So there is too big of a hole just to fill and then I have a hot floor on the second floor. I could block it off though with relative ease. In retrospect I should have made all these cielings below it be 10′, but it is too late to fix that now.

Anyway, I wanted to know if there is a standard recommendation for the airspace between floors in a 1.5 story. Seal it or leave it open? If I seal it tight, are there venting concerns?


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  1. GBA Editor
    Martin Holladay | | #1

    Your question is unclear. It sounds like you are describing a Cape style home with kneewalls on the second floor, but I'm not sure.

    In a typical Cape, the second floor area is smaller than the first floor area, and there are triangular attics behind the kneewalls. Is that what you have in your house?

    In any house, of any shape, you can't decide where to air seal and where to insulate unless you know where the thermal boundary of your house is. You need to define the thermal boundary, which must be continuous, without any breaks, gaps, or holes. Once you have defined the boundary, you'll know where you want to air seal, and where to put the insulation.

    More information here: Two ways to insulate attic kneewalls.

  2. Zuhl | | #2

    I guess I never considered this a cape cod, but your description is spot on. I feel like my thermal coundary needs to extend from the upper story walls down to the first story cieling. Basically closing off the air gap in between. I just want to know if this is appropriate. Can I simply seal the area between first story cielign and second story floor and be done with it? Or does that area have to have some kind of ventilation for moisture or whatever reason. I dont want to create one problem by solving another. Thank you.

  3. GBA Editor
    Martin Holladay | | #3

    Every single plane of your thermal boundary -- whether it is a wall, floor, or roof -- needs to include thick insulation and needs to be sealed against air infiltration.

    If you have a Cape house, it makes much more sense to place your insulation in the roof slope, not at the kneewall. Study the article that I linked to in my first response, and my advice should make sense.

  4. Zuhl | | #4

    Thanks. In retropsect I wish I had done a sealed attic, but I am not a huge spray foam fan for cost and environmental reasons. My plan is to seal between the joists with foam sheathing that is caulked at the edges. I will blow the 10-12" of cellulose right up on that foam and that will provide isolation and insulation.

  5. GBA Editor
    Martin Holladay | | #5

    There are many ways to insulate a sloped roof without using spray foam.

    If you are insulating your kneewalls, you'll need to include blocking between the floor joists under the kneewall bottom plate, as you describe. Rigid foam can be used for the blocking, as long as each piece of blocking is carefully sealed at the perimeter.

    Don't forget that you will also need blocking between each pair of rafters above the top plate of the kneewall.

  6. Expert Member
    Dana Dorsett | | #6

    I've starred as the "fool" in this movie a couple of times, and retrofit air-sealing of complicated kneewall situations is most-often a fools-errand, easier to execute during construction than after the fact. Were I to do a return to that role I'd do it with blower-door verification on each kneewall attic space as I went along, and even then...

    On my own home I have three such spaces, and eventually threw in the towel, hired some open cell foam guys to insulate & air at the roof deck, and even THEY missed a huge air leak on one of them (even though I had I told them where it was)- a ~4" x 100" gap below a beam where the porch roof overlapped the main roof. (I went back later and caulked in a cardboard air-barrier, and dense-packed the porch roof with cellulose, promising myself to add rigid foam over the roof deck when I re-roof in short years.)

    In places where R38 is code for attics it doesn't take much spray foam on the interior to be protective if you dense-pack the remainder of the rafter bays- even an inch is enough (though technically a code violation if going to the full letter of the IRC.) Building Science Corp did a pretty good representative set of WUFI simulations on 1" and 2" closed cell flash'n'fill using dense-packed fiberglass (which is less protective than cellulose), and wrote it up here:

    If you look at Table 3 in that document you'll see that with as little as 1" of closed cell foam it works in all of the lower-48 climate zones if you're using a darker composite shingle rather than the worst-case light colored "cool roof" metal roofing paradigm. That is not a huge amount of foam, and if you find an installer using a lower-impact blowing agent (water, or Honeywell Solstice) the net environmental hit is pretty small, and arguably worth it when used in this limited way.

  7. Zuhl | | #7

    Thanks for your comments and ideas. I am confused about the comment for blocking between the rafters of the top plate of the knee wall. I should probably further clarify my house here. The knee walls are exposed to the attic on the backside, but are full 8' tall walls with a flat cieling above them. On the hot side, I am sheathing them with 1" blue foam board, and then filling the cavity with cellulose. On the flat cieling above them, I am blowing R-38 cellulose. I suspect I may have slightly less insulation at the edges where the cornersmeet the roofline, but will at least have teh 6" of insulation there. that can fint under the roof rafters.

    See attached sketch. I am in Houston, Texas. R-38 attic is overkill for this zone, but so is R-19 walls.

  8. GBA Editor
    Martin Holladay | | #8

    Q. "I am confused about the comment for blocking between the rafters of the top plate of the knee wall."

    A. Your "kneewalls" -- which, since they are 8 feet tall, aren't really kneewalls at all -- extend to the bottoms of the rafters, not to the tops of the rafters. That's why you need blocking between the rafters -- to keep the insulation that will be installed above the ceiling of your second floor from spilling out between your rafters into the lower attics below.

    I assume that your upper attic will be vented. That means that you need ventilation air gaps between the bottom of your roof sheathing and the top of the insulation above the top plates of your "kneewalls." Your blocking goes under the ventilation baffles, and needs to be installed in an airtight manner. The blocking goes to the edges of the kneewall top plates, so that the top plates (as much of them as possible) are covered with insulation.

  9. Expert Member
    Dana Dorsett | | #9

    According to the WUFI simulations in that Building Science piece it's perfectly safe to go ahead and go unvented, and insulate with cellulose between the rafters without using any foam. Look at the second column in Table 3, where it was also simulated in a Houston climate.

    Insulating at the roof deck is far preferable due to the better ease and long term integrity of the air-sealing, and brings the attic spaces fully within the pressure & thermal boundary. Insulating at the roof deck is even less material than insulating the kneewalls and mini-attic floors(!).

    In hot humid climates vented attics put more moisture into the attic wood than they remove, and the effect on shingle life of composite shingle is very small (a smaller effect than the color choice), but using a CRRC rated "cool roof" shingle both reduces the cooling load modestly and improves shingle life, whether vented or unvented. (They're not all "white"- the available palette is much broader now after California Title 24 added cool roof mandates than it was just a decade ago.)

    See: (use the search function to find the appropriate products for your roof pitch)

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