Exterior insulation details for a laminated timber gothic arch cabin
nyruralhomebuild
| Posted in General Questions on
I’m planning to build a small home (24×32) in climate zone 6A, and one option I’m exploring is using laminated timbers to create a cabin with a gothic arch shape, which will give more usable space than an A-frame. I’m trying to figure out the best approach for the roof/wall assembly, but the curve is making things more complicated. If my roof needs to be R-49, is there any way to do that with all exterior insulation on a curved roof? I’d rather not have to hide the arched timbers inside the wall assembly in order to insulate between them. Would love any advice!
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Spray foam!
It is used on geodesic domes ...
~ former spray foamer
begreener,
Can you flesh-out what the roof assembly would be from inside to out?
Spray foam makes me really nervous! From what I've heard, getting the mix just right out in the field doesn't always happen, and I'm a bit paranoid about moisture issues as my area gets both really damp and really cold. Is there a certain type of spray foam that would be best or a particular wall assembly that allows for drying as needed?
Lot of it depends on how tight of a curve. Thin foam (~1.5) especially unfaced will follow a gentle curve without too much persuasion. You can also take something like paper faced roofing polyiso and give a bit of water spritz and form it similar to drywall.
I would clad your arches first in a layer of plywood (if curve is not too tight, regular stuff, if tighter, bending ply), tape the seams (or full peel and stick) as your main air barrier and cover with roofing underlayment. This would get you under-roof and dried in while the exterior rigid is installed. Install the rigid and cover with another layer of plywood for your roof deck.
You can then finish the inside with T&G nailed to the interior plywood.
Since you are using continuous exterior insulation witch works better, I would also check your local code as most allow U factor based compliance. This might mean only R38 or R42 of rigid.
A simpler option might be to cut blocking out of larger dimension lumber to the curve of your beams and have a straight edge to use as roof deck. Depending on how tight of a curve, this might be only 1/2/3 straight section to approximate the curve. Bonus is you can run your services between two layers of this blocking above your beams. This would let the beams be fully exposed and if the blocking is inset a bit and painted to match the ceiling, it will disappear into the shadows.
Thanks for your advice! I'm attaching a drawing that shows what the curve would be (roughly). It seems like a gentle enough curve to me to be able to bend some materials to fit. I didn't think about code compliance using U factor -- I'll look into that. Wrapping my head around how to anchor the roof deck/siding to the house through such a thick band of insulation has been tripping me up, so if it can have less depth, that sounds great (Though if I create a flat roof, I could use a Larsen truss).
The idea of creating blocking is really fascinating... my initial thought is that I would be sad to lose the aesthetic of the curve on the exterior, but if we did the blocking with a straight roof line for only the upper portion of the curve the lower portion would still be visible. It would provide a natural transition from roof levels of insulation to wall levels, as well as giving the opportunity to create an overhang. This might make the house more cottage-like and adorable -- squat little walls bulging out a bit with a big hat on top!
Okay, I'm just now learning more about Larsen trusses and I had only seen examples using I-joists. But... I've gotten ideas from looking at the drawing for "The Original: 2x2s and plywood gussets" in this Fine Homebuilding article: https://www.finehomebuilding.com/2024/01/10/understanding-larsen-trusses
Would it be feasible to build my own Larsen trusses by bending or laminating the 2x2s and cutting the plywood gussets to match the curve? I could also ask the company making the laminated arches to make pieces for the Larsen trusses, but I'm trying to keep costs down and do what I can myself. It looks like they'd need to be 12" or so to get the needed r-value with dense pack cellulose (haven't found anything yet that helps me determine U factor).
For the rest of the wall assembly I'm looking at the I-Joist Outrigger ebook from 475 High Performance Building Supply.
I think there are many ways of skinning that feline.
You can skip any framing, install say 2x6 T&G directly over the arches, peel and stick over it as the air barrier. Layers of rigid for insulation followed by plywood and roofing/siding. You'll have to install some bracing somewhere, might need to add a layer of plywood directly over the T&G the corners.
If you don't want to use so much rigid, you can install horizontal 2x6 grits between the trusses and fill those with batts. In zone 6 you need about the same R value as exterior rigid for condensation control, so 1/2 of what you would need for an all rigid roof. Bonus is you can now run wiring inside this wall.
If you want a more traditoinal outside look, you could modify the arches a bit to reduce the curve at the roof line (the overall shape of the arches can be anything as long as the bends are not tight).
The arches can then support a large ridge beam which you can set rafters onto to and standard stud walls on the bottom of support. From the outside it would look like an A frame with shorty walls. This would mean pretty much standard construction and insulation with no rigid needed. Bonus is you can increase the arch spacing which can save some cost. The arches would still be exposed on the inside and you can use the gap between the roof/walls and the arches as a location for some nice indirect lighting.
Most of the buildings I have seen, and the house I used to own, built with laminated arches have a flat roofing surface. The only houses I have seen with an actual curved roof are boatbuilders houses which there are a few of in town, and they are of more standard construction
One of the companies I got a quote from for the laminated timbers actually did the curved beams 40 years ago for my neighbor's boat-hull roof! I've seen the arches that are curved on the inside face and straight on the roof/wall faces, but I'm not a fan of them -- they remind me too much of a particular style of church that I'm guessing were built in the 70s maybe?
I don't think you lose the aesthetic by making the walls straight. People are still going to say 'wow' when they walk in.
This room[originally the whole house] is 35 feet across about 17 feet tall at the peak