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Climate Zone 3C: Crawl space, roof, and wall systems review

Phil Boutelle | Posted in Green Building Techniques on

I’m in the design phase of a remodel of an existing 2 story house, CZ3 coastal, zip code 95062. The scope includes a new foundation and gutting the house with some interior walls moving, but for the most part keeping the existing exterior shape and roof line. Goal is a high performing house (well insulated, air sealed, mechanically ventilated, high IAQ and minimal heating and cooling loads). I’ve attached a line drawing of the shell for reference.

The foundation will be perimeter with pier and post, and proposing an unvented crawl space with supply and return registers from the HRV. Vapor barrier on the ground (possibly a rat slab poured on top), 2″ of polyiso on the stem walls, closed cell spray foam on the rim joist.

Do I need to air seal in between the crawl space and the house interior? Does the HRV supply/return meet the requirements of the ‘conditioned’ crawl space? For the spray foam, should I make sure it doesn’t cover up the mud sill, and do I leave a gap between the polyiso and the spray foam, exposing a bit of concrete and the mud sill? In my CZ, do I need to bother with insulation under the vapor barrier and rat slab?

The proposed wall system, from inside out, is as follows: drywall –> 2×4 wall with dense pack cellulose –> plywood sheathing –> peel and stick vapor-permeable air and water barrier (Blueskin VP100) –> 1-1/4″ Roxul –> 1×4 vertical furring strips. On the 1st floor, we plan to install stucco, with paper backed lath attached directly to the furring strips. The entry area will have vertical siding. On the second floor, we plan to install a fiber-cement product, either shingles or sheet siding. In between the floors would be belly band trim.

Does this wall system seem reasonable? Can I install a continuous rainscreen, with furring strips all the way up the building, and vary the exterior cladding?

The proposed unvented roof system, from inside out, is as follows: drywall –> 2×6 wall with open cell spray foam –> plywood sheathing –> peel and stick vapor-permeable air and water barrier (Blueskin PE200HT?) –> 3″ of polyiso (2 layers, staggered and taped seams) —> Second layer of plywood sheathing –> 30# felt paper –> standing seam metal roof. The soffit will be open to air.

Does this roof system seem reasonable (to get to R-38)? Am I placing the peel and stick membrane at the right level, or should it go on top of the foam, and wrap around the fascia? Could I then tape the plywood seams to act as an air barrier, or should I use something in between the rigid foam and the lower layer of sheathing?

Any feedback is appreciated, and apologies for so many questions at once (I’ve been saving them up).
-Phil

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Phil,
    Q. "Do I need to air seal in between the crawl space and the house interior?"

    A. Not necessarily, although worries about the quality of crawl space air might convince you to do so.

    Q. "Does the HRV supply/return meet the requirements of the 'conditioned' crawl space?"

    A. Not the way I read the code. (An HRV introduces cool air in winter and warm air in summer, so it can't condition a space.) For more information, see Building an Unvented Crawl Space.

    Q. "For the spray foam, should I make sure it doesn't cover up the mud sill, and do I leave a gap between the polyiso and the spray foam, exposing a bit of concrete and the mud sill?"

    A. From a thermal perspective, covering everything up with insulation (either polyiso or spray foam) is best, but termite worries in some sections of the country preclude the best insulation details. In termite-infested areas, you may be required to leave a bare inspection strip. If you are unsure on this issue, consult your local code enforcement officer.

    Q. "In my climate zone, do I need to bother with insulation under the vapor barrier and rat slab?"

    A. No.

  2. GBA Editor
    Martin Holladay | | #2

    Phil,
    Q. "The proposed unvented roof system, from inside out, is as follows: drywall --> 2x6 wall [did you mean rafters?] with open-cell spray foam --> plywood sheathing --> peel and stick vapor-permeable air and water barrier (Blueskin PE200HT?) --> 3" of polyiso (2 layers, staggered and taped seams) ---> Second layer of plywood sheathing --> 30# felt paper --> standing seam metal roof. The soffit will be open to air. Does this roof system seem reasonable (to get to R-38)?"

    A. That will work, although you could probably save money by substituting a different insulation -- cellulose, mineral wool, or fiberglass -- for the open-cell spray foam

    Q. "Am I placing the peel-and-stick membrane at the right level?"

    A. Yes. I'm not sure why you want the peel-and-stick membrane to be vapor-permeable, though. It doesn't have to be.

    Q. "Could I then tape the plywood seams to act as an air barrier?"

    A. You could, although the most important air barrier is at the peel-and-stick layer (directly above the lowest level of roof sheathing).

  3. GBA Editor
    Martin Holladay | | #3

    Phil,
    Q. "The proposed wall system, from inside out, is as follows: drywall --> 2x4 wall with dense-packed cellulose --> plywood sheathing --> peel and stick vapor-permeable air and water barrier (Blueskin VP100) --> 1-1/4" Roxul --> 1x4 vertical furring strips. On the 1st floor, we plan to install stucco, with paper backed lath attached directly to the furring strips. The entry area will have vertical siding. On the second floor, we plan to install a fiber-cement product, either shingles or sheet siding. In between the floors would be belly band trim. Does this wall system seem reasonable?"

    A. It will work, although there are cheaper ways to get an R-18 wall.

    Q. "Can I install a continuous rainscreen, with furring strips all the way up the building, and vary the exterior cladding?"

    A. I don't see why not.

  4. Phil Boutelle | | #4

    Martin,

    Thank you for these responses, and for the helpful links. I do live in a termite heavy area, so I will leave an inspection gap. For crawl space ventilation, I understand the code interpretation that an HRV doesn't condition the space, but isn't there an alternate approach that allows continuous mechanical ventilation, and would an HRV meet this, as long as flow rates were adequate?

    There are cheaper ways to get to the roof/wall R-values than what I am proposing. I was given budget pricing that dense pack insulation was about the same cost as open cell spray foam. I prefer something sprayed on/blown in, versus a batt installation.

    I sketched up the attached detail for the plan set.

    Thanks again.

  5. GBA Editor
    Martin Holladay | | #5

    Phil,
    Q. "Isn't there an alternate approach that allows continuous mechanical ventilation, and would an HRV meet this, as long as flow rates were adequate?"

    A. Not that I know of. It's worth pointing out, however, the building codes are often amended by individual states and even municipalities. So if you want an interpretation of your local code, the only person who can help you is a local code official.

  6. Phil Boutelle | | #6

    I'm revisiting this roof assembly to avoid foam, both sprayed and rigid, if possible. A roxul tech recommended using Roxul Rockboard 60 rigid insulation board in place of polyiso on exterior roof insulation. In the rafter cavity below, I have this drawn as R-22 batts, but in this roof I'd prefer to use a blown-in insulation, maybe fiberglass, to fill all the voids. Will this work as a compact unvented roof?

    The second question is, what if I can vent part of the roof? The house has a clerestory, and the north facing roof plane will be mostly over an attic space (dropped ceilngs of bedrooms), with a bit over the upstairs landing/hallway that is a cathedral ceiling. If I fur down with 2x2, I can create a vent space in the hallway ceiling, and use R22 batts. In the attics, I would use blown-in loose fill cellulose/fiberglass, but no batts in the rafters, just open venting. There is still rigid insulation (Roxul??) on the roof deck. I would insulate the short sections of wall in between the attic spaces and the hallway. Would this venting strategy work?

    Two construction details attached. Any feedback appreciated. Thanks!

  7. GBA Editor
    Martin Holladay | | #7

    Phil,
    If you want to create a vented roof assembly, all of the insulation has to be on the interior side of the vent channel. If you invite outdoor air between two layers of insulation, as your detail below shows, you are making the insulation on the exterior side of the vent channel worthless (because in the winter, you've invited cold air into the middle of your sandwich).

    .

  8. Phil Boutelle | | #8

    Thanks Martin, that makes sense. Because the cathedral ceiling section doesn’t have enough room to furt down beyond 2x2, I’ll make the whole roof invented and keep the insulation on the deck and in the rafters. In the three attic spaces above the bedrooms (created by angled roof above, flat ceiling of bedroom below), should I include a supply and return to the HRV, maybe with registers between each space?

    If I use the first detail, with rigid Roxul above the roof and air permeable fiberoua insulation in the invented rafter cavity (installed flush to the bottom of the roof deck), where isn’t the best place for an air barrier in this assembly? Do I need a vapor barrier anywhere? It appears that my roof could dry in either direction. I’ve read about the intello smart vapor barrier but don’t understand if/why I’d need it in this assembly in my CZ.

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