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Unconventional framing insulation strategy sanity check?

mikeysp | Posted in General Questions on

My buddy Don is building his 24’x48′ house in zone 4a North Central Arkansas.

He called me for a sanity check on his insulation strategy and I told him I was going to ask my vast group of building scientist friends on GBA.

UNCONVENTIONAL CONSTRUCTION DETAILS:
No code where he lives. He milled his own lumber and did not use any wall or roof sheathing. 

Exterior walls, he used 2×4 studs, horizontal purlins 24″ OC, typar, vertical strips for rainscreen 9″ on center, and then metal roof panels for siding. 

His attic is vented. He placed 1″ recycled XPS between roof metal and purlins for a condensation thermal brake.

His HVAC will be mini-splits, so no duct work.

CURRENT INSULATION STRATEGY:
2″ of closed cell in attic floor onto drywall.
1.5″ closed cell in wall cavity against housewrap.
He will have sides of 2×4 studs sporayed as well to get the R5 out of the 2×4 and purlin.
Currently he does not plan to add any other insulation in the cavities.

He said he can ad an inch of spray foam for a $1000 if need be, but he is thinking with the descent air sealing of the closed cell + R10 for walls and R14 for ceiling, he will be ok. This is one point where I disagree, yet concede it will be better than most houses in his area for the simple reason he will have descent air sealing, and most houses there leak like a sieve. 

My opinion was he really need smore R value, but not sure the approach. I thought he might go with only an inch layer of closed cell and add cellulous on ceiling and fiberglass in cavity. This would give him and approx R20 in walls and he could easily get an R30-40 in ceiling.  He hates the mess of cellulose when he has to work in attic.

As for fiberglass, he said it is really not much more $$ to go with thicker closed cell than to use fiberglass batts, so if it is unwise to go with his current strategy, he would rather add a few inches of closed cell to his current pan than to use cellulous or fiberglass. 

Thank you for your help. I will send Don the link so he can read any responses. 

Thank you.

-Mike

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Replies

  1. Expert Member
    BILL WICHERS | | #1

    Does he have any exterior sheathing? If he's spraying foam directly on the housewrap, he'll probably get some bulging that might be an issue for siding. I would use batts in the walls myself. If he's getting a great deal on spray foam it certainly won't hurt, unless he is concerned with the green/not-green aspect of the material -- spray foam is not the most green option out there. He could at least request HFO blown foam though.

    I would not use spray foam on the attic floor. Blown cellulose will be cheaper, and in some ways, better (mostly because you can put in a lot more R value for the same cost). Do the air sealing work with caulk and canned foam.

    Bill

  2. user-2310254 | | #2

    With Bill's approach, he would need to make sure the attic is vented. Can he mill boards for the sheathing? If so, covering it with house wrap and then attaching the furring so the siding can dry sounds better to me. Better still if he can buy some reclaimed rigid foam and put that over the board sheathing (and tape all the seams).

    On the R-values, it sounds like he has been reading some of the misinformation Martin wrote about in this article: https://www.greenbuildingadvisor.com/article/misleading-energy-reports-used-to-sell-spray-foam

  3. mikeysp | | #3

    The critical question is with 2" closed cell (R14) in the wall and 3" closed cell (R21) onto drywall in the cieling is he making a huge mistake? Seems he will have good air sealing, but poor R-Value?

    Spray foam guy told him that any more he would be wasting his money. Perhaps he meant in a return on invesment point of view.

    Don does not want cellulose as it is messy when he has to get in attic.

    Any thoughts.

    -Mike

  4. Expert Member
    Michael Maines | | #4

    Mike, even in places that aren't required to follow the code, the IRC represents a commonly accepted minimum building standard. The 2015 IRC calls for R-49 in the ceiling and R-20 or R-13+5 in the walls for zone 4A. 3" of closed cell in the roof is really about R-18, so 37% of what is recommended. 2" in the walls is really about R-12, about 60% of what is recommended.

    While it's true that spray foam is a good air barrier, it does nothing for the transition areas where most air leaks occur. I doubt your friend will do better than 3.0 ACH50, which all houses should meet with a little attention to detail.

    Spray foam companies love to talk about foam as it it had magical properties. It doesn't. R-value is R-value, as long as the insulation is installed in an airtight cavity.

    I don't think your friend will have damage resulting from going so far below code recommendations. He will spend more on energy than he would with code-compliant levels, and he will have lower levels of comfort.

    Is he planning on spending a lot of time in his attic?

    1. Mark_Nagel | | #5

      "Is he planning on spending a lot of time in his attic?"

      Yeah, I was wondering that myself!

      Why would anyone want to spend any appreciable amount of time up in a non-conditioned space such as an attic? (a couple years back I spent days on end cleaning up in a crawl space- under a manufactured home hitting my head and doing a lot of rolling around to clear obstacles, which I would then hit my head on! - no thank you, which is why I'll be going with a slab-on-grade with no crawl)

      One of the few things I'm pretty much sold on in regards to insulating is raised heel trusses. I see them as something that can become quite widespread.

      1. Expert Member
        Michael Maines | | #6

        Mark, raised-heel trusses are definitely one of those upgrades that don't cost much or change normal practice but make a big difference in most climate zones.

  5. mikeysp | | #7

    If he does a flash and batt, 2" closed cell and fiberglass batts, should the fiberglasss have kraft paper? Seems to my mind they should not have the paper so the wall can dry into house. However, I am an elementary schhol building science student, so I want to avoid assumptions.

    BTW, thank you all. He is now goinng to add more insulation.

    1. Expert Member
      BILL WICHERS | | #8

      The kraft facing acts like an old-school "smart" vapor retarder, so it doesn't hurt to have it in the assembly. It's probably not really needed for moisture control if there is 2" of spray foam, but it certainly won't hurt to have the extra layer of protection. I have a smart vapor retarder (Membrain) AND exterior polyiso on my own home. Belt and suspenders and all that :-)

      Bill

      1. mikeysp | | #9

        Bill, with the closed cell spray foam to the outside preventing any moisture drying to the outside, and the kraft paper stopping drying to the inside, won't that trap moisture in between? I am assuming the faced fiberglass stops moisture, but your saying it is ok, has me thinking it does not trap moisture. If this is so, I need to learn why is there faced and unfaced fiberglass?

        -Mike

        1. Expert Member
          Michael Maines | | #10

          Mike, 2" of closed-cell foam actually allows a small amount of drying to the exterior, when the outdoor dewpoint is lower than the indoor dewpoint. Kraft paper is a variable permeance membrane; when dry, it slows vapor movement, but if the insulation cavity gets wet or even just high relative humidity, it opens up and allows drying to the interior as well.

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