Cathedral Roof Venting and moisture
I’m building a small (20×14) cathedral roof in Zone 4 – Maritime. Occupancy use is part-time home office with 1-2 people. (no cooking, no showers, perhaps a coffee maker) ie low moisture. HVAC to be a 9k mini-split with an independent ERV moving about 40 CFM. I am taking air-sealing very serious. Ceiling will have may penetrations for lights. Labor is free (cause it’s me)
Walls will be 2×6 (r-21) with ZIP and 1.5-2″ external insulation (material TBD)
Roof will be 2×12 rafters with a 3:12 pitch
Planned roof assembly (top down):
* Composite shingles
* Ice & Water / Synthetic underlayment
* 5/8 CDX
* 1.5″ airgap constructed with 2×3 battens aligned with rafters and vented at high and low soffits
* 7/16 Zip (taped seams and taped to wall sheeting: aka monopoly framing)
* R-38c batt
* Type2 vapor retarder
* GWB
I know the traditional way to build this is to create a channel vent with cardboard / plastic baffles under the one layer of roof sheeting. The baffles are super air leaky.
I was scratching my head with trying to define where my roof/ceiling air-barrier was actually taking place if I have air moving above my insulation and under my roof sheeting It seemed like most assemblies were expecting the ceiling/GWB layer to be the air barriers which seemed less than ideal.
Simultaneously I looking at the “Monopoly Framing” ideas from the usual YouTube / Internet sources which seemed like a robust answer to the air-barrier question, but I was unwilling to invest in all the exterior foam required above the ZIP.
So I came up with the above hybrid which creates strong and consistent air-barrier for the building with the Zip layer. The zip layer is really just an OSB version of the carboard/plastic baffles but better air-sealed. A moisture path out of rafter zone is created via the Zip’s semi-permeability into the 1.5″ air-space. This 1.5″ air space vent will also vent summer solar load on roof.
However, I still have some moisture misgivings about burring my fiberglass batts inside an roof enclosure with no direct ventilation and relying on the vaper perms of ZIP above and Type2 below for drying, albeit the enclosure SHOULD never need drying.
My confidence is then mostly restored when I consider that my proposed roof assembly is really just the same components as a traditional wall assembly just turned horizontal (ie rain-screen, vent space, sheeting over framed batt, vap ret).
Good, bad, ugly?
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Replies
No need to complicate your life with top venting and a 2nd roof deck plus the bolt on overhangs.
The way you get air barrier continuity is by installing a strip of a membrane (house wrap or wide tape) over the top plate to hang to the inside before your set your rafters. You can now tie the ceiling air barrier (drywall or better a membrane under the drywall) to this strip for air barrier continuity.
You can now build a standard vented roof with a gap above your insulation and under the roof deck.
What you propose can work but won't be to code as the vent space is in the wrong location. Zip is also not the very permeable, regular plywood would work better.
The way you can make this work and be code compliant is by using a highly permeable sarking membrane (house wrap can work) instead of the layer of zip. This type of roof is common overseas. This is hard to build over rafters as it is very hard to walk on it while installing the membrane. A better option is to rotate the structure and build purlins as these are easier to walk on. Once the membrane is up install the 2x4 perpendicular to the purlins to form the vent space.
Watching a house being built across the street in Germany this past week. They don't use roof sheathing at all typically.
Tim_O,
My roof is built that way. Eliminating the sheathing removes the material most likely to experience moisture problems, and allows cross ventilation, but it also reduces the shear strength of the structure, and means you can't use asphalt shingles, which are the most common roofing in North America.
This makes sense. What do you use for a roofing material? Does this assembly work with metal roofs?
I think I have yet to see a roof that isn't clay tiles in Germany.
Tim_O,
Yes. Manufacturers all include tables for the spacing of the strapping based on the profile and gauge of metal used.
I’m in the same boat in the same CZ and will be interested in hearing what you finally decide on. Another thing to consider with your proposal is whether or not Huber’s warranty covers this as a few years ago you couldn’t put another deck over. With Risinger making this popular (albeit in CZ 2), they may now cover it (pun intended ;-). I’ve also thought about just doing unvented, but the combination ping-pong and night radiant cooling in our CZ have me thinking twice.
spraggins,
Does Risinger ever use monopoly framing with a vented roof?
Hmm, just looked at his personal house video and it was essentially a homemade SIP, no ventilation. Now I’ve got to go find where I’ve seen the venting between decks before.
Greenland,
I think it was Joe Lstiburek who said: Roofs are not sloped walls - and there is very little transferable about the way they should be built from one to the other.
If you aren't satisfied with the usual way of building these roofs with venting below the sheathing, I'd second Ako's suggestion to use a membrane over the rafters and vent above that. https://www.greenbuildingadvisor.com/article/building-a-vaulted-high-performance-and-foam-free-roof-assembly
I also agree with Akos that creating a good continuous air-barrier at the wall/roof intersection isn't difficult enough to justify the complexity and extra materials necessary for monopoly framing (which if you vent the assembly, makes it non-code compliant.)
If you do decide to go that route, consider something more robust then 2"x3"s extended to support the overhangs, which will have very little strength used on their sides, and provide poor attachment for the fascia and soffit.
Let me start by saying I see cathedral ceilings as an unforced error and a design flaw. If you want a high ceiling make the walls taller and keep the ceiling flat. Then to fill the ceiling full holes for pot light that displace the precious little space you have for insulation, is like shooting yourself in the foot on purpose.
Note R38 is a little skimpy for zone 4.
If you must have a cathedral ceiling pick one of the five in this article.
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Walta
I have done something similar to what Tim O shared in comment 4, called a Sarking Membrane system, using Pro Clima Mento over the rafters, 2x4 battens aligned with the rafters and then either horizontal strapping or Zip sheathing over the battens. I prefer the Zip sheathing even with metal roofing, as it makes a better base for the roofing panels and it prevents condensation on the bottom of the panels from night sky radiation. It does affect the shear strength of the roof but at least in low seismic activity zones where I work there is not a code requirement to have the roof sheathing directly attached to the framing. (Wall sheathing IS required to be fastened directly to the framing, unless you have gone through testing like Zip-R did to prove it's not necessary.)