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Energy-efficient envelope advice

David Kenesson | Posted in Energy Efficiency and Durability on

I’m building a high efficient cabin in climate 6 (dry) and have got a ton of helpful info from this site. I was hoping the the experts that frequent the site could make comments or give advice to improve the design. Also, let me know if there is something that doesn’t mesh with the current building science. Any input would be helpful, thanks.

Foundation System:
Sealed crawl space with vapor barrier and 4” rigid foam insulating interior foundation walls.
Ground Floor System:
I joists with R30 batt insulation.
Above Grade Walls:
Double stud wall (exterior to interior): wood siding, 1/2” rain screen (1” x 3” furring strips), plywood with air and moisture barrier, 2×4 wood studs, 2-1/4” gap (to accommodate 2×12 bottom plate), 2×4 wood studs, wall cavity filled with dense pack cellulose, smart vapor retarder, gyp. board.
Windows and sliding doors:
Zola ThermoPlus Clad triple glazed windows.
Cathedral Roof & Ceiling:
Roof assembly exterior to interior: pre-finished standing seam metal roof, roof underlayment, plywood sheathing, 6” rigid foam, plywood sheathing, 12” batt. insulation, T&G wood ceiling.
Mechanical System:
HRV in crawl space. Three ductless mini-splits.

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  1. GBA Editor
    Martin Holladay | | #1

    A few observations:
    1. If you are creating a sealed conditioned crawl space with insulation on the crawl space walls, then you don't want to install any insulation in the crawl space ceiling (the framed floor assembly above the crawl space). So you don't need R-30 insulation in your I-joists.

    2. 1x3s will create a rainscreen gap that is 3/4 inch deep, not 1/2 inch deep. Most builders find that 1x4 furring strips are less likely to split than 1x3s.

    3. Most importantly, you have a layer of rigid foam on your roof that isn't thick enough to keep your roof sheathing dry during the winter. For more information on this issue, see Combining Exterior Rigid Foam With Fluffy Insulation.

    To solve this problem, you either have to make the rigid insulation thicker, or the batt insulation thinner. In Climate Zone 6, the rigid foam layer must provide at least 51% of the R-value of the total R-value of the roof assembly. If your rigid foam has an R-value of R-4 per inch (typical for EPS), then the rigid foam layer represents only 35% (rather than the required 51%) of the total R-value of the assembly.

    If your rigid foam has an R-value of R-5 per inch, then the rigid foam layer represents only 40% of the total R-value of the assembly.

    4. You should install an air barrier on the interior side of the fiberglass batts in your roof assembly. Tongue-and-groove boards are not an air barrier.

    -- Martin Holladay

  2. David Kenesson | | #2

    Thanks for the response Martin, your input is invaluable. That is an excellent point about the crawl space. Do you think 4" rigid foam is enough on the crawl space walls? Anything you would recommend on the floor of the crawl?

    I got the design/roof detail for the cathedral ceiling from one of your articles, it stated that the assembly should dry to the inside and not to use an interior vapor barrier. I guess that's different than an air barrier? Should I use a smart vapor retarder like I've used in the wall system?

    Thanks for your time and input.

  3. GBA Editor
    Martin Holladay | | #3

    Q. "Do you think 4 inch rigid foam is enough on the crawl space walls?"

    A. In Climate Zone 6, most building codes require a minimum of R-15 wall insulation for basement walls or crawl space walls. So 4 inches of rigid foam will work.

    Q. "Anything you would recommend on the floor of the crawl?"

    A. Yes: at a minimum, the dirt floor needs to be covered with a layer of durable polyethylene. More details here: Building an Unvented Crawl Space.

    Q. "I got the design/roof detail for the cathedral ceiling from one of your articles. It stated that the assembly should dry to the inside and not to use an interior vapor barrier. I guess that's different than an air barrier?"

    A. Yes, an air barrier is different from a vapor barrier. For more information, read these articles:

    Questions and Answers About Air Barriers

    Vapor Retarders and Vapor Barriers

    Do I Need a Vapor Retarder?

    Q. "Should I use a smart vapor retarder like I've used in the wall system?"

    A. No, your ceiling doesn't need an interior vapor retarder (as long as you have installed an adequate thickness of exterior rigid foam). However, you still need a durable air barrier (for example, taped drywall) on the interior side of the fiberglass batts. If you want to have tongue-and-groove boards as your finish ceiling, the boards can be installed on the interior side of the taped drywall.

    -- Martin Holladay

  4. Expert Member
    Dana Dorsett | | #4

    It's a moot point if you've made the decision to insulate the crawlspace walls & over the dirt floor, but in general batts and I-joists are never a good fit: The joist webbing is about 0.5" thick rather than 1.5" thick, so the cavity is about an inch wider than what the batt was designed for. You could use batts designed for steel stud framing, but they don't come in higher-R, and since they're designed for a cavity about a half-inch wider than I-joist bays they don't always fit without a bit of wave/wrinkle without some massaging it a bit.

    Even in cold, dry US climate zone 6B climate where summertime condensation & mold on an uninsulated vapor barrier floor isn't much of a concern, it's still financially rational over the longer term to put a couple of inches of EPS under the vapor barrier on the floor (or even three, if you're heating with something expensive such as propane or resistance electricity). If you can find a source of used/reclaimed EPS, even 4" or more isn't out of the question.

    Going with 3.5-4" of polyiso (R20-R24)on the crawlspace walls instead of 4" EPS (R16-R17), also financially rationale in your climate. See the zone-6 recommendations for whole-wall R in Table 2 in this document (treating your crawlspace as a basement.)

    At those levels it's pretty easy to hit Net Zero Energy with a PV array that fits on the house, using cold-climate mini-split heat pumps for heating.

    A 2x12 bottom plate for a double studwall is just a big thermal bridge to increase the heat load. Only the exterior wall is structural, and using separate bottom plates is higher performance, lower cost. A bit more performance can be had if you stop the subfloor at the interior studwall, the cu in a strip of foam-board of the correct thickness to be flush with the top of the bottom plates thermally breaks the bottom of the assembly, and removes the thermally bridging subfloor too. The top plates of the two walls with half-inch plywood gussets, or (if it makes dense packing easier), half-inch low density fiberboard sheathing (asphalted or not) to cap off the wall cavity. Typical fiberboard sheathing typically runs about R2.6 per inch, more than twice that of dimensioned lumber or plywood, and nearly 3x that of MDF.

    See the discussion of double studwalls starting on page 38 of this document:

    To hit R35 whole wall recommended in Table 2 of that document you get there at about 10.75-11" of cellulose in a double studwall (that's thinner wall than the 2 x1 2 dimension!), but only if you pay attention to the thermal bridging stuff- get RID of the 2 x12 studwall plates!

  5. STEPHEN SHEEHY | | #5

    David: Why a crawl space, as opposed to slab on grade? What's the benefit?

    If you put the HRV in the crawl space, will you be able to access it to change filters? Where will the supply and exhaust go? You'll need to run them up inside a wall before they go out anyway (to stay above the snow), so why not locate the HRV in a mechanical room?

  6. Kapriver | | #6

    Is there a vapour barrier in your ceiling?

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