Feedback on Wall and Roof Assemblies
Hey all, long-time listener, first-time caller.
I’m hoping to get some constructive feedback on the attached wall & roof assembly concept. And I have some specific questions on the highlighted labels.
For some context, this is for my own personal home in California. I have meaningful experience in construction, but my only previous experience in building an unvented roof assembly utilized closed-cell spray foam under deck. For this project, I am hoping to avoid spray foam. I am not building this project personally but am working with a builder and sub-contractors. I’m hoping to develop up a design that satisfies my personal criteria, exceeds local codes, leverages the latest thinking in building science, but does not alienate the local labor force that is decidedly unfamiliar with high-performance building techniques due to our historically mild climate.
The project will be built in Napa County, CA.
– We see under 30 inches of rain per year (well under that some recent years), with most of that falling in a few days over a ~60 day period in the winter.
– Winter days are typically mild (~65F) with cold nights (~35F.)
– Summer days are hot (~90F) with mild nights (~60F.)
– We experience very little humidity in the summer.
Homes here are expensive, and performance requirements are low due to the climate. As such, again, I am very sensitive to trade-familiarity with building techniques as well as access to materials.
We are in a wildfire-prone area. My land burned just last year. So I am sensitive to fire-safe building techniques. That said, this home is being built in the middle of a large pasture with acres of defensible space and fire break. So the most major concern has to do with embers finding their way into any vented space (unlikely but possible.)
Questions:
For the roof assembly (10:12 pitch) I’ve specified mineral wool, over Zip, covered with OSB as a nail base for a metal roof. And I’m specifying a 3D mesh like Delta-Trela as a roofing underlayment/venting channel.
The main pro of this approach is that this seems like the simplest option for every trade involved.
My main concerns with this approach are:
(a) Is there any meaningful risk of condensation or water intrusion between the OSB sheets? Would this pose a meaningful problem? How might we ensure any water that may enter this assembly can exit the insulation blocking? (I’ve not seen any common technique resolving that potential problem.)
(b) Is a 1/4″ – 3/8″ ventilation channel sufficient? I’ve heard some claims that 3/4″ is ’required’ on topics here and elsewhere, but I can’t find any engineering to back that claim.
Other options I’m considering are, of course:
(i) Pre-manufactured nail base with venting channels. problem: these do not seem to be readily available from local suppliers and I’ve never seen them used locally. I’m also not clear on how to utilize the ~1″ venting present in these panels, and am somewhat wary of having flammable poly-iso exposed to meaningful airflow. (Though I am investigating a fire-treated nail base panel manufactured by GAF.)
(ii) replacing the OSB nail base with a 2×4 furring strip matrix. problem: this is an atypical approach locally and seems like it would be *a-lot* of labor. The advantage here, of course, is it would let the roof sheathing and insulation dry if water made its way into the assembly. I don’t know how significant that risk is.
I appreciate any thoughts you all may have and am curious to hear your impressions overall of the assembly beyond my areas of concern. Thanks!
Note: Yes, this is being designed as a low/no-overhang building. I know. 🙂 But yes, there will be a gutter system that has not yet been specified.
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Replies
You need a class A roof assembly in a WUI area - Typically for a metal roof over sheathing a underlayment (Versashield, fiberglass based ) or roof type gypsum board is installed to get that class A rating for a metal roof- the Dorken has a class E rating. https://www.dorken.com/media/docs/products/Technical-Data/Technical-Data_DELTA-TRELA.pdf -- I am not sure what a class E rating is, but it seems lower than class A.
I have been required to get a UL rated assembly - roofing mgfs have that info- to get thru plan check.
Any roof venting will need to have Wildland approved vents. Vulcan or Brandguard are two mgf of this tested vent.
Make sure to review chapter 7 in the CA code for the Wildland Urban Interface provisions.
Thanks, Tim. That's really useful feedback. You're absolutely right about the Dorken, I hadn't evaluated it through that lens yet. I have seen some similar 3D mesh products with a Class A rating (Benjamin Obdyke Cedar Breather for example) but I've not found one that integrates a WRB. It seems pairing Cedar Breather with a Class A WRB would maintain the Class A rating of the assembly, but for the question of venting.
Do you know of a continuous venting solution like CORAVENT that is WUI compliant?
I've noticed quite a few homes in the area utilizing a mesh rain screen detail on the wall assembly (but roofing is typically affixed directly to the deck.) I'm curious what is typically used for wall assembly venting. (My guess would be these are typically open-matrixes with a metal bug screen.)
I'm also wondering if it's even worth attempting to vent the roof given the WUI requirements and inherent risk in doing so.
It's very difficult to find “high performance” building details that have been utilized successfully in CA so I appreciate your wisdom here.
Answering my own question, it appears that Vulcan VSC2120R could be a rough equivalent to CORAVENT.
https://www.vulcanvents.com/wp-content/uploads/cut-sheets/Vulcan-VSC2120R.pdf
Only at $15/ft instead of $1.5/ft. ~$10k for soffit vent screens. 😅
Some of the plastic roof vents are approved, I don't remember which brand. I don't know if they are allowed to be used as soffit vents.
I don't know if it is really work the effort for a rain screen in Napa Area. Long dry summers to dry out. I have not had a project where a rain screen was proposed in a WUI.
How will your wood siding go up? A lap siding is somewhat self vented.
I looked at your details- Zip R is not approved for high seismic zones. see the ICC report. https://www.huberwood.com/uploads/documents/technical/documents/Code-Recognition-for-Insulated-ZIP-System-R-sheathing-ESR-3373.pdf
Looking at the attic detail - why not just blown in cellulous and a vented attic? You can blow in r60 an not have a problem. Simple & cheaper than exterior insulation. Just put your ducts & HVAC in the bldg envelope.
Metal roof like a plywood substrate due to the screws that hold the panels down - plywood has a much higher screw pullout & shear value than OSB. OSB just needs more screws to do the same job.
Also a double stud wall is easy cheap. No foam.
This is all very insightful, thank you.
I did not expect to be second-guessing exterior insulation and a rainscreen after posting on GBA but here we are. 😄
→ How will your wood siding go up?
The design is currently such that part of the structure will have cementitious Hardie Board & Batten, and another part of the structure will have vertical plank vulcanized “Adobo” wood siding.
→ Why not just blown in cellulous and a vented attic?
A few reasons. (1) Some of the attic area (outside of the vaulted ceiling main living area depicted in the above drawing) will be usable space. (2) I'm wary of vented attic assemblies in a WUI area. (3) I like the idea of an air-tight home (particularly given our prolonged summer “smoke season”) and I'm wary of moving air-sealing details to the interior rather than the exterior as I feel this adds even more complexity. (4) I am just emotionally attached to the idea of a clean, conditioned attic space.
→ Also a double stud wall is easy cheap. No foam.
This is definitely an option to explore further. However upon initial calculations this ended up fairly expensive due to the effect on all trades (larger slab, slightly more complex framing and structural engineering, slightly more complex window/door detailing [as compared to the hope Zip-R could be used, at least])
- - -
At this point I'm inclined to forgo the rain screen / drainage mesh.
For the roof I'm inclined to maintain the sandwiched mineral wool detail, removing the 3D mesh and attaching the roof directly to the nail base. I think I could delete the blocking at the roof edge to reduce my fears of water pooling.
The technical Zip-R bulletin is above my pay grade. I was all but certain I've seen it used around town. If not a feasible option I'm inclined to forgo a thermal break for the wall studs. I'm not sure the performance gain offsets the costs of a true exterior insulation detail. 🤔
Here is the NAPA County WUI guidelines https://www.countyofnapa.org/DocumentCenter/View/8241/Wildland-Urban-Interface-WUI-Fire-Conformance-Checklist-PDF
Seismic Designations from the County - It shows Seismic Design D - See Zip R ICC ESR Report https://www.countyofnapa.org/DocumentCenter/View/3638/Basic-Design-Criteria-For-Conventional-Construction-PDF
From this site https://www.countyofnapa.org/1890/Building-Documents#tab624dfb5d-3590-4a1f-84bf-d9ca3dc5856d_12
Search for California Residential Code section R806.5 Unvented Attic and Unvented Enclosed Rafter Assemblies for how to make a unvented roof assembly. I don't know the answer for your proposed assembly.
You will need a title 24 energy calculation of a permit. You might want to talk to them sooner than later to get the assemblies correct.
Who ever help draw you plans would be a good asset to figure this stuff out,
Thank you, Tim. Looking at the Napa WUI checklist it has this clue:
705A.2, R337.5.2: “If [space proposed between the roof covering and roof decking], the spaces shall be constructed to prevent the intrusion of flames and embers, and be fire-stopped with approved materials, **or have one layer of No. 72 ASTM cap sheet installed over the combustible decking.**”
The metal roof can use the 72 lb cap sheet or a tested assembly. The metal roof mfg have rated assemblies - they have ones using Vershield a fiberglass underlaymentit is not 72 lb/100sqft.
The part I don't know is the external roof insulation issues, of permeability and how to build it to meet the CA code. Also the issues of getting a Class A rated roof with a unvented assembly. I don't have the answer.
Great point. Most homes in the area are being built with metal from Taylor Metals, on top of Mapei Polyglass underlayment as recommended by Taylor Metals.
This led me to find that Mapei has a UL-listed Class A assembly incorporating their Polytherm rigid foam insulation. It suggests the metal cladding would be fastened *through* the rigid insulation into the roof deck/framing rather than adding a secondary nail base. I've not seen this done before.
This system comes with a 30-year warranty if two-ply's of Polyglass are used.
If others could help on needing a rain screen in California, that would be helpful. Also how to build it to meet the wildland fire provisions.
I'll let others chime in, but I'd note the following: 1.) If you're specifying 2x6 blocking above the roof deck, I'd go ahead and fill the full depth to the extent possible with mineral wool. With only two 1.5" layers, your total nominal R-35 value is below code. 2) To my knowledge, you can still use Zip R in high seismic zones, but you'll need your structural engineer to spec the fasteners/schedule. You just can't use the prescriptive fastening schedule in Huber's report. 3) I don't think you need any sort of mesh product or ventilation gap above your upper layer of OSB, just an underlayment that meets that manufacturer's requirements (e.g., Grace); the assembly can dry inward. And without such a gap, the roof assembly shouldn't have any problems meeting WUI requirements.
Thank you for your insights.
On (1) great catch. The lower r-value of mineral wool relative to some rigid foam boards presents a new challenge there.
On (2), that would make sense given that I've seen R-sheathing installed locally and it is stocked at building suppliers. I'll run this by our structural engineers.
On (3), great to hear. Grace is an underlayment I see used quite often around here too.
Thank you again.
Howdy - I’m learning all OSB related products are have super low perm ratings because of all the glue. Using a vapor permeable mineral wool between two relatively impermeable OSB sheets would concern me too.
For a ventilation layer I’d take a look at VentGrid since it can ventilate on both the x and y axis.
Thank you. I'm curious: What about the permeability specifically of mineral wool do you think might contribute to issues when sandwiched between impermeable layers of sheathing? Or, why might the impermeability of foam enable this type of assembly?
Notably: Sandwiched foam boards seem quite common. But the only place I've seen sandwiched mineral wool boards is on a recent episode of This Old House with an install by Sweenor Builders:
https://youtu.be/LFT27wKjsK8?t=89
The limited precedent here definitely gives pause. But I can't come up with a scientific hypothesis as to why this would be common with rigid foam while seemingly uncommon with mineral wool.
Hi Cadam
What was the final detail you used. I just posted on another thread and am in Sonoma with same requirements and similar wants thoughts on not having a vented roof due to fire, using mineral wool because its great for fire, but permeable which needs to be mitigated depending on application. I am also doint standing seam metal roof with 10:12 pitched roof and lots of vaulted ceiling / attic area that would be used as mechanical roof and drywall would be used for vault ceiling.
Thanks