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Siding/insulation/WRB/drainage etc. — shorter question

user-2812795 | Posted in GBA Pro Help on

HI

I have a question with somewhat less detail than my last volley 🙂

I am building a new house 100 yards from the ocean – elevated approximately 10′ off grade. fiber cement siding

Connecticut shore – 4+ or 5- mix of humid/warm and first cold zone = maybe 40 rain days… near water – driving rain elevated (somewhat protected by large eaves and gable end over hangs). right on the edge of both zones.

Specs – as they stand right now. Still a few weeks out (can easily change up to application for building permit). From inside out:

3/4″ southern yellow pine bead-board (essentially flooring installed on the wall). Stained and poly coated both sides prior to installation. TG 6″ wide panels. nailed in channel to substrate next layer.

4″ horizontal bars (cut full 8′ length) of plywood fastened to studs (for support to nail bead-board into. This is instead of full ply wood coverage for nail base. will air gap have any effect ? too shallow to use for wiring chase… should we just use full plywood coverage and tape seams inside too?

(insert vapor barrier here?) with external foam, we’ll still need to dry to the inside? Looking at Intello Plus, from 475 (see: http://www.foursevenfive.com) plus, this will surely help with continued efforts to maintain tightly sealed home.

2×6, 24 oc standard dimension lumber framing with Roxul 5.5 inch batts = R23.

Fiber coated gyp sheathing 1/2 or 5/8 (structural engineer still working on sheer loads, etc)

WRB layer (which brand and why?)

Hunter xci ply bonded to polyiso foam = 2″ foam, 3/4 plywood installed with 7-8″ screws into stud One layer – taped at seams. R12 is good for 2/3 in, 1/3 out insulation. Can go to 2 1/2 inches to make R value outside 15 or 16…. ?

WRB layer again – same – which and why.

Trimline plastic 3/8″ furring strips (for drainage plane) tacked on. vertical.

Fiber cement installed over furring directly into 3/4 plywood.

short story long…

a. what WRB and do you agree it is needed in two places?
b. install vapor barrier interior side of insulation?
c. agreement on the amount of insulation as proper ratio of in/out ?

any other thoughts which I missed or screwed up?

thanks!

Patrick

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Replies

  1. mmoses101 | | #1

    my 2 cents, which want get you very far.

    A. First read Martins's "Where Does the Housewrap Go?"(https://www.greenbuildingadvisor.com/blogs/dept/musings/where-does-housewrap-go).
    Bottom Line:
    -If you are installing “innie” windows, your housewrap should go under the foam.
    -If you are installing “outie” windows, your housewrap should go over the foam.

    If you go with it under the foam I like the idea of Wrinkled housewraps. (http://www.finehomebuilding.com/item/23991/wood-siding-best-practice-installation-method)

    I’m not going to call out a specific brand b/c everyone has an option on that and I don’t want to start a food fight. Might want o read:
    http://www.buildingscience.com/documents/insights/bsi-066-holes-and-leaks
    http://www.finehomebuilding.com/how-to/articles/making-sense-of-housewraps.aspx
    and
    https://www.greenbuildingadvisor.com/node/%2019255

    B. No vapor barrio to the inside.
    http://www.buildingscience.com/documents/reports/rr-0410-vapor-barriers-and-wall-design

    C. Ah dew point calculations, fun with math. If you don’t want to do the heavy lifting just look at table Table N1102.5.1. (https://www.greenbuildingadvisor.com/blogs/dept/musings/are-dew-point-calculations-really-necessary) Quick look (Zone 5, 2x6 walls with R-12 foam) you look good with 2”inch.

  2. GBA Editor
    Martin Holladay | | #2

    Patrick,
    Q. "What WRB and do you agree it is needed in two places?"

    A. Almost any type of WRB will work if it is installed well. You need only one WRB, and you had better know where it is, because all of your window flashing has to be integrated with your WRB. If you have two, how will you know which layer is important for integrating your flashing? Whatever WRB you choose, it will perform much better if it has an air gap on the exterior side of it. This method is usually referred to as a "rainscreen installation." For more information on WRBs, see All About Water-Resistive Barriers.

    Q. "Install a vapor barrier on the interior side of the insulation?"

    A. You definitely don't want an interior vapor barrier. (There are no building codes that require a vapor barrier in your wall, thank goodness, although some building codes require a vapor retarder -- a lesser standard.) What you want is an interior air barrier -- especially if you are installing pine beadboard on your walls. The best interior air barrier is gypsum drywall, which should be installed directly behind your pine boards. (It's also possible to install Intello Plus as an air barrier, I think, as long as you pay attention to air sealing all seams and penetrations. I don't know if it will prove to be as durable as drywall in the long run. Remember, it's an air barrier more than a vapor retarder, so pay attention to air sealing -- whatever product you use for this layer.) For more information on air barriers, see Questions and Answers About Air Barriers. For more information on vapor retarders, see Do I Need a Vapor Retarder?

    Q. "Agreement on the amount of insulation as proper ratio of in/out?"

    A. Everything you need to know on this topic can be found here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

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