Timber-Frame Roof With Exterior Insulation
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
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.
Hi John, Sorry for my delayed response. I was more or less out of commission for a bit, but diving back in now. I welcome your expanded thinking on options for a timber frame roof assembly. While I'm comfortable with my proposed wall assembly, I admit I'm a bit daunted by the prospect of dealing with exceptionally long screws to fasten down a Roxul or Polyiso roof assembly.
If anyone including GBA editors or experts can point me towards a dense-pack cellulose cathedral roof assembly (assuming vented assembly would be the ) that might work well in conjunction with my desired 2x6 T&G boards over timber frame rafters I would greatly appreciate it? Thanks
I did recently see Ben Bogie's GBA blog article and video on a super insulated project using TJIs and dense pack cellulose and it made me wonder if I could adapt for my situation, maybe with 12 inch TJIs or custom built larsen trusses sized to accommodate ~ R49 insulation levels. https://www.greenbuildingadvisor.com/article/framing-for-superinsulation . I'll comment further below, in response to plumb_bob.
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!
Plumb_bob, thanks for your comments and as I said above sorry for my delayed response.
"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."
As I commented to John, I'm open to other roof assembly solutions. That said, I really like the idea of continuous external insulation over the whole structure and now wondering if a TJI or Larsen Truss secondary roof with dense pack cellulose could work if I could extend Rockwool Comfortboard on the wall exterior up to meet or overlap the TJIs and cellulose. If this was achievable, I could still conceivably add- on overhangs without any major breaches in the thermal envelope. I see in Ben Bogie's recent project referenced above (about 3.5 minutes into the video) the TJIs stop at the outer wall plane.
"How are you going to run electrical in the roof?"
Good question. This is still undetermined, however, we're anticipating minimal ceiling lighting and possibly a ceiling fan or two. I've seen one solution in other timber frame builds where a shallow surface channel is used to accommodate wiring at ceiling level.
"What is the spacing of your timber rafters, and are there purlins tying them together?"
The 6x8" timber rafters are on ~ 3 feet spacing with post-supported purlins at the midway point along the slope. As mentioned before, there will be 2x6 T&G roof decking on top of the rafters. The frame plan is designed and will be engineer stamped by a timber frame specialist.
"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."
Appreciate your thoughts on the overhangs. I'm at a good point to still reconsider how much overhang I want. 24 inches is actually a lot compared to most of the houses, old or new, around Nova Scotia. Hah, I think the winds off the Atlantic Ocean drive the precipitation sideways here more often than not. To be clear, I wasn't thinking 2x4 solely on the flat. For sure they will have to be reinforced somehow, perhaps with a 2x6 rafter tail affixed to the underside of the flat 2x4 (a T in cross-section view). Your suggestion for a greater overhang is something to consider for aesthetics as well as function.
As I mentioned to John, if you or any GBA editors or experts can point me to potential construction details or further information I'd welcome it. Thanks
You are also in a hurricane zone in NS. Hurricane-force winds put high uplift stresses on long overhangs. Your engineer will definitely have to account for that. This is one of the reasons that overhangs are relatively short in NS. Easier to hold them down.
If you wrap your air barrier around the top of your wall, it's relatively easy to integrate the wall air barrier with the P&S on the ceiling. Then, your I joists can extend out to form long, strong rafter tails. Or, you can fasten lumber tails to the I joists. You also don't need dense pack cellulose. With tall I-joists and a relatively low pitch, you can just fill the cavities with cellulose and leave some space at the top for ventilation. Use cardboard, thin plywood, anything fastened to the bottom of the top flange of the I joists to keep the vent channel open while blowing the cellulose.
Each of these options has its pros and cons. While option 1 is ideal, the cost and difficulty with attachment can be a disadvantage. Option 2 is a good compromise to increase fire resistance and keep the polyisol closer to its intended R-value. Option 3 can also provide sufficient insulation.
When considering long-term durability, I suggest evaluating factors such as moisture resistance, ventilation, and potential for condensation. It may be worth consulting with professionals or local experts with experience. You should also have your framing checked for durability, and if you need engineered wood, you may want to contact Scotts Timber Engineering. I wish you all the best on your project.