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Timber-Frame Roof With Exterior Insulation

Bill_Whitman | Posted in Energy Efficiency and Durability on

We are in the planning stage for a single level timber frame house that will largely be an owner build, I’d appreciate hearing perspectives and critique on the enclosure, especially concerning the proposed roof assembly.

For context: the house will be approximately 28’ x 28’ with ~ 3.5/12 pitch shed/single slope roof. We’re building in a forested hilltop location in Nova Scotia, Canada, climate zone 6. This will be our age-in-place home and durability is as important as energy efficiency. For the enclosure we want to have a continuous external insulation layer and a continuous WRB/primary air barrier situated roughly mid assembly following a REMOTE/PERSIST approach.

The characteristics of mineral wool fibre (i.e., vapour open, dimensionally and thermally stable, no offgassing, easy to work with, fire resistance) are really appealing and ideally I’d like to avoid foam if at all possible (although a couple of the roof options proposed below use polyiso)

The wall will be approximately R30 using Rockwool Batts in a 2×4 stud cavity and exterior Comfortboard 80. The roof will be entirely externally insulated and I am asking for opinions, pros and cons, red flags on three variations to achieve ~ R48. I’ve done considerable reading on GBA, FHB, JLC and other sources and Rob Myers’ timber-frame house build blog has been particularly helpful and inspiring https://www.greenbuildingadvisor.com/article/a-timber-frame-house-for-a-cold-climate-part-3

I’ve attached a conceptual sketch of the roof and wall assembly which I hope is helpful and legible. Note: 24 “ add-on overhangs will be integrated into the 2×4 strapping roof strapping (detail tbd) . For the roof, inside to out:

2×6 T&G roof decking over the timber frame (this to be the exposed ceiling)

Self Adhered/Peel and Stick Membrane (tbd)

Built up layers of rigid insulation to R48+ (see below)

2×4 strapping on flat running from soffits to ventilated ridge

2×4 strapping running horizontally

metal roof

Proposed roof insulation options to achieve R48:

1) My ideal (if I could afford it) would be to use Rockwool Comfortboard entirely, however, the 12” built up thickness required for R48 would require costly and rather daunting 16” screws to fasten down the assembly!

2) Second choice would be a hybrid roof using a base 4” layer of Polyiso with a 6” layer of Rockwool Comfortboard above it. The objective here is to keep the polyiso warmed and nearer to its nominal R6/inch with a more fire resistant outer layer of rockwool.

3) Third choice would be an 8” total thickness of staggered layers of Polyiso

Do any of these make a better roof for long term durability or make more sense for other reasons? Thanks, Bill

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Replies

  1. John Straus | | #1

    Hi Bill -
    Have you considered framing a roof over your timber frame.? It is possible that another set of rafters (possibly I-Joists filled with cellulose..) would be more user friendly and cheaper than all of that Roxul and long screws.

  2. plumb_bob | | #2

    I like the design, and it looks like you will have a beautiful house. Couple of thoughts:

    I know there are plenty of details for thick insulation and LONG screws, I do not feel comfortable with the bending potential on these fasteners. Too much leverage can be applied to them.

    Like John mentioned above, a rafter system might make sense. Just like you are building a wall external to the timber superstructure, building a roof on top of the timbers may be practical. Ijoists have low thermal bridging. You could extend these rafter tails out to give the overhang, but this would be at the expense of continuous insulation.

    How are you going to run electrical in the roof?

    What is the spacing of your timber rafters, and are there purlins tying them together? Just trying to figure how you will fasten down your 2x4 strapping running top to bottom.

    For structural decking, 2x6 T&G can only span so far. 3x6 T&G is often spec'd.

    The 2x4 on-the-flat roof overhangs will not be up to much of a snow load, and you will need to build something more robust. Especially if you go with an aesthetically pleasing and super practical large overhang, not the typical 24". I would suggest 30-36", this will look better with the proportions of a thick roof build-up. I like my rafter tails to be level with the tops of the windows.

    Cheers!

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