Timber frame cathedral ceiling
I know, I know, it’s another cathedral ceiling question… The specifics: I am building the house myself and the roof structure will be built like a timber frame house, but a shed roof. I am in Climate zone 3 (near the coast in the redwoods (California), and we get A LOT of rain in the winter 60-80 inches, sometimes more, so marine by Joe Lstiburek standards). I am trying to build a good home but not a passive house and will heat with a wood stove (free) and radiant heating, no AC required. This is a forever home, and because of that, and the fact that our climate is mild I am putting durability above energy efficiency when there is a conflict. I would like to avoid spray foam if possible. And lastly I am in the Wildland urban interface which has some vent restrictions but it is possible to buy special vents. These are the two options I have thought of and would like your thoughts on them, however, I could easily be convinced to scrap them and go with a different one. Just for fun the house I am replacing is a 900 sq. ft. cabin built in the 1950s… in the ceiling (R-11) walls (R-0) floor over crawl space (R-0). I know this would not do well in the east coast where most of you are, just a little insight into an old California home. I got the idea for the design with TOPROCK DD from the last Fine Homebuilding concept house but the videos and pictures were a little vague so it may be incomplete. If there is a more detailed drawing of their assembly as well that would be awesome. Thank you in advance.
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Jeroen,
Is the screen shot below what you are talking about? If so, I assume you saw the slide show:
https://www.finehomebuilding.com/2018/11/29/the-california-fhb-house-air-sealing-and-insulation-in-pictures
Martin,
First thank you for the response, its really cool that you answered it and I l love your book and common sense approach
That is the slide show and videos I saw, but it was a little unclear how they attached the standing seam roof to the TOPROCK DD, if they used the TOPROCK MULTIFIX as the top level, or had an additional underlayment above the TOPROCK? or even an air gap between the TOPROCK and the metal roof?
Also I am curious, what you think of the assembly in my climate? or if you would do something different? I am sort of stuck on the Timber frame and 2x6 T&G since I am able to mill my own wood and want the satisfaction of building it myself in this old style. That being said I am not so stubborn as to not take advice on a better way if you think so.
As a side note I would love to see FHB or GBA publish the architectural drawings of the assemblies used in these houses, I love all the stuff you guys put out, but it would be cool to see the actual details. thanks again
Deleted
Sorry, I just realized my drawings (poorly drawn I know) didn't upload the first time. I put one option in the original post (sorry having trouble uploading the second one, but the FHB house is the second option) Ill keep trying to upload it though
You need a different underlayment to get a class A fire rating for the metal roof. It is either a densdeck board or a fiberglass sheet like versashield. I don't know how furring strips can be part of a fire rated roof assembly.
https://www.gaf.com/en-us/roofing-products/residential-roofing-products/roof-deck-protection/fire-resistant/versashield
Jeroan,
A few random thoughts;
- While a self-adhered membrane will air-seal the t&g, the real difficulty comes at the exterior walls. It is very difficult t0 get any effective seal where the boards pass from the interior to the outside. Breaking the span over the exterior walls, and providing an air-barrier in-between is a good idea.
- Ventilation above the sheathing is not as effective as that below. The moisture in the roof structure has to pass through the sheathing through diffusion before it can be dissipated by the vent channel. Why not run the 2"x12"s from eave to ridge, and ventilate below? (And running the framing as you have shown, you will need to block between the rafters at intervals to prevent overturn).
- 2/12 roofs don't ventilate effectively, and clad in metal are at more risk of leaks than those of even a slightly higher pitch. Any chance of making it a 3/12 roof?
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Jeroen,
All of Malcolm's points are good. For information on insulating a low-slope roof, see this article: "Insulating Low-Slope Residential Roofs."
The tongue-and-groove ceiling boards that extend from the interior of your house to the exterior are a nightmare. They create an unfixable problem -- one of the short list of errors that are so egregious that you are really stuck forever with a major air leakage problem.
You can't install fluffy insulation like fiberglass or rockwool under your roof sheathing (in your suggested detail, that would be the 1/2 inch CDX plywood) unless either: (a) you install a continuous layer of adequately thick insulation (usually rigid foam, but possibly mineral wool) on the exterior side of the roof sheathing, or (b) you install an adequately thick layer of closed-cell spray foam on the interior side of the plywood roof sheathing (the flash-and-batt approach).
Thanks, I agree with all your points and am leaning towards the second option (being the style of the FHB House). In this case would I put something like the VersaShield over the TOPROCK? and is the a reason to create some airflow under the standing seam in this assembly? As for the air sealing, what if I ended the T&G at the outside of the walls and wrapped the Self-Adhered membrane down the wall tying it into the house-wrap/zip/etc and then clad the bottom of the eves separately?
Jeroen,
Q. "I am leaning towards the second option (being the style of the FHB House). In this case would I put something like the VersaShield over the TOPROCK?"
A. I've never built a roof assembly like the one in the slide show, but the caption to the photo I copied and reproduced in my Comment #1 reads as follows: "The metal roof can directly overlay the [TopRock mineral wool] insulation without an additional layer of plywood over the insulation." So you don't need anything between the mineral wool and the metal roofing -- assuming that the manufacturer of the metal roofing you have chosen agrees with this approach.
Q. "As for the air sealing, what if I ended the T&G at the outside of the walls and wrapped the Self-Adhered membrane down the wall tying it into the house-wrap/zip/etc and then clad the bottom of the eves separately?"
A. I can't visualize your approach; perhaps you can create a sketch and post it. If your tongue-and-groove ceiling boards penetrate the air barrier, the only way to seal up the leaks is to wrap some type of air barrier material (as you say, a self-adhered membrane) totally around the cantilevered boards. I'm not sure you want to do that -- it might be easier to stop the ceiling boards at the exterior walls, and to build attached overhang assemblies that are separate from your ceiling boards.
Martin, Thank you again, it is awesome how responsive you all are, and for all the money I have spent so far on the planning of the house GBA has been the best investment so far.
Your second answer above is what I was trying to describe (just not as effectively as you did) basically end the ceiling boards at the exterior, wrap down the exterior with an air barrier, and then worry about and clad the overhangs.
In your Detail Library I found this... which I think best shows what I am imagining for the insulation except the air barrier would be above the ceiling boards and I would use the TOPROCK product instead of foam. Do you think that makes sense?
(Metal roof // unvented// drain through battens // exterior 6″ rigid insulation)
https://www.greenbuildingadvisor.com/cad/detail/metal-roof-unvented-drain-through-battens-exterior-6-rigid-insulation
Jeroen,
The detail you linked to shows battens (purlins) above the continuous exterior insulation. That should satisfy any metal roofing manufacturers who are nervous about using mineral wool on the exterior side of the roof sheathing.
That said, it never hurts to investigate the installation instructions provided by your roofing manufacturer to make sure that your method of installation doesn't violate the manufacturer's recommendations.
Jeroen,
Your air-sealing plan will work. It's the continuity of the t&g when people want it to form both the ceiling and 0verhangs that causes the problems. If you use the same material for the soffits, no one will even know they aren't continuous.
Thanks, I think you are picturing what I am, and I don't mind if the boards match for outside to inside. and on some days I even think about putting some sort of Hardiplank Soffit boards outside in order to do away with any maintenance and bug problems.
I know we don't all always love HGTV, but this is the house that kind of inspired the "shed roof timber frame" idea for me, except I want solid beams and the T&G inside, since they are more traditional and I can mill them myself, which brings me satisfaction.
https://www.diynetwork.com/blog-cabin/2015/backyard-patio-pictures-from-diy-network-blog-cabin-2015-pictures
Also I am having trouble uploading the second picture, it is a JPG and well under 3MB?
I see this as a rustic cabin so let turn the roof inside out by exposing the rafters with the bead board on top of the rafter, then your air barrier recycled foam insulation if you can get the required fire rating if not then Rockwool, plywood, felt, air gap and steel.
I do think you would spend less money and have a better building if you skipped the cathedral ceiling and make the walls taller with a flat ceiling. But that would give the room a different feel.
Walta