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Helping my insulation contractor make the right decisions. :)

Norman Bunn | Posted in Energy Efficiency and Durability on

CZ 3A

Good insulation contractors around here are hard to find. A lot of people are doing the work without understanding many of the techniques shared on this site and others. I talked to the person I am considering for my new house and I think I could use your help in educating him a little.:)

1) Radiant barrier in attic
He is touting the advantages of an RB in the attic space. However, since my attic will be vented, have no HVAC duct work, and will use foil faced roof sheathing, I see no need for an additional RB. I know that even the RB sheathing maybe questionable in my zone, If its only a few dollars more a sheet, I’ll probably do it, since it will tend to lower the temp of the attic space, which will result in a lower delta t during the summer.

2) Taping the wall sheathing
He wants to foam the OSB sheathing instead of taping it. I don’t think this is a good solution long term and want to “prime” the joints with contact cement followed by 3M 8067 tape with a j-roller. This will be followed by a HydroGap WRB,1/2″ foil-faced polyiso with taped seams that are offset to the sheathing, a rain gap, and then the exterior finish (brick, stone veneer or engineered siding).

3) Crawlspace insulation
He is proposing a 12 mil liner and 2″ of EPS held by plastic anchors to the concrete/CMU. I am wondering if the anchors are adequate long term or should some foam board adhesive be added to make sure.

4) Air sealing<.b>
He proposes caulking and foaming. I am leaning toward Knauf Ecoseal Plus, which should penetrate the gaps more deeply and provide an extra layer of protection long term. I am fine with foam on the larger gaps.

Replies

  1. Jon R | | #1

    Ask him to demonstrate an effective air seal between two sheets of OSB as the gap between them cycles between zero and 1/8 inch. Even when the foam was applied when it was zero.

  2. Expert Member
    Dana Dorsett | | #2

    You're right that foil faced sheathing would be "worth it", but with no HVAC in the vented attic, and a code-minimum R38 or more of fluff on the floor there's no rationale for additional RB. There might be a rationale for going for better than R38 though.

    Are they talking can-foam in the seams OSB (not so great), or is it open cell foam in the cavities (pretty air-tight, if you caulk the framing seams)?

    I suspect the plastic fasteners will go the distance for the crawlspace wall foundation. It's not a difficult or expensive repair if they start to fail in 40-50 years anyway. I'm not convinced foam board construction adhesive would be even as reliable as the fasteners over a handful of decades. But I have no data on this- it's just a best-guess.

    Can foam and polyurethane caulk are pretty good. Can't say for sure that Ecoseal is really an improvement in initial air tightness OR long term reliability though it may be easier to apply than some other materials.

  3. Norman Bunn | | #3

    Dana,

    He plans to use can foam on the seams and use dense pack cellulose in the stud bays.

    The YouTube video on Ecoseal Plus vs caulk is what is leading me to go in that direction. (I would post it but the site's spam filter keeps rejecting it. tinyurl dot com/Ecoseal)

    Price-wise it is similar to the cost to elastomeric paint, though more than elastomeric roof coating.

    Jon R. - good point

  4. Expert Member
    Armando Cobo | | #4

    Based on my experience in CZ3A:
    1.- I never spec RB. If my attic floor is sealed, I have full R38+ insulation (always heeled framing), no HVAC in the attic, I could care less how hot the attic gets, assuming is well ventilated. It’s a lot cheaper to blow R50 if you want to.
    2. and 4 - Always tape the wall sheathing on the outside and caulk plate, corner, joint and opening seams for air sealing. We install ½” or 1” taped rigid foam on top of sheathing, to avoid thermal bridging and get a WRB, and HouseWrap to protect the foam from the inclement. Polyseal around windows and doors only! Ecoseal or Energy complete is AWESOME!
    3.- Crawl space has taped and sealed 40 mil liner (12mil gets tore up quickly). Install 2” Polyiso glued and anchored. Dont forget to ventilate.

  5. Steve Knapp CZ 3A Georgia | | #5

    Armando,

    On #3, are you recommending a ventilated crawlspace? If so, why?

  6. Expert Member
    Armando Cobo | | #6

    No. All sealed crawl spaces require ventilation. IRC recommends 1 cfm / 50 sf

  7. Norman Bunn | | #7

    Plan to use supply to ventilate the crawlspace. HVAC is in the crawlspace.

  8. Expert Member
    Dana Dorsett | | #8

    The 2" of EPS on the crawlspace wall aready beats the IRC code minimum continuous R5 by ~1.7x, and Armando's recommended 2" polyiso would be ~2.5x code.

    While 2.5x code in the crawlspace might appropriate for a high-R house, in this case the additional crawlspace-R money (like the radiant barrier money) might be better applied toward something fatter than 1/2" for the polyiso sheathing, or deeper fluff on the attic floor. The cost adder for R50 instead of R38 in the attic is remarkably small if you already have the vertical space for it.

  9. Andy CD Zone 5 - NW Ohio | | #9

    Crawlspace liner: 12 mil is certainly too thin, but 40 is overkill, if only because of expense and it's just way too heavy to work with. I've had great success with 16 and 20 mil, which seems plenty robust for crawl traffic.

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