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Wall Assembly for Home/Shop With Corrugated Metal Siding

Ljung | Posted in General Questions on
Help! My head is swimming with conflicting opinions and details on the following roof and wall challenge:
• Building is a stick-built shop/residence in Montana (zone 6) – 32′ x 80′.
• 12 foot high 2″ x 6″ walls on a mono-slab.
• Gable trusses, 5/12 pitch with a 12′-wide room that spans the entire 80′ length. 100 lbs snow load. 115 mph wind gusts. 7.25” energy heel.
• Must be a warm roof, with conditioned inter-truss spaces, including the long room.
• Roof and wall cladding 24-gauge matte black 7/8s corrugated, run vertically.
• Extremely minimal style. No gutters, no overhangs, no soffits. See similar style pict.
• No drywall in the interior. Just 1/2″ birch ply (fire suppression through sprinklers).
• Although Montana has passed a version of the Uniform Building Code, the only inspections in this county will be septic, electrical, and plumbing.
• Delivery on items such as Zip R are months out. No SIPs available in a reasonable time. Builders in the area are few and far between. The good ones are swamped. Many have not worked with fluid applied WRBs, insulated sheathing, etc.
Questions:
What would a practical wall assembly be?

­Without continuous external insulation (I understand the argument that the studs don’t provide much insulation, and that there are a lot of them in this design).

Sheathing: 3/4 external plywood sheathing with an applied WRB such as that offered by Prosoco. I am thinking plywood based on the opinion that the screws fastening the corrugated siding hold better in plywood than in OSB, especially with the wide swings in temperature in this area. If that isn’t a valid concern, should I substitute standard uninsulated green ZIP which I can find locally? Or is there a plywood product with an integrated WRB?

Internal insulation: Can I obtain an R28-30 within the 6” stud cavity with some combination of rock wool or fiberglass batts and rigid foam, like a hybrid “cut and cobble” polyiso and batt method? The first cold plane would be the inside of the external plywood sheathing, so I am concerned about condensation between the foam and the inside of the plywood. I assume that there is some danger than the dew point will occur within the batt insulation, but won’t that be addressed by the ability of this assemble to dry to the inside. Would it be better to use rock wool rather than fiberglass due to the former’s limited adsorption? Will the foam prevent drying to the inside, resulting in plywood that has absorbed water vapor and won’t dry to the inside?

Vapor retarder: No vapor retarder after batts, allowing the assembly to dry to the inside.

Cladding: Corrugated metal run vertically, with permeable screen “closures” top and bottom between inside of corrugations and the plywood. No rain screen should be necessary provided the corrugated is run vertically, correct?
­With external continuous external insulation
Internal insulation as describe above.
CI: Would ZIP R 6 be sufficient, given the recommended ration of 5 CI and 20 internal? Or do I need to go to ZIP R 12 if I don’t want to use a vapor retarder?
If I cannot get ZIP in time, what could I substitute? Closed cell like Polyisocyanurate despite its performance degradation in cold weather? What R value? What thickness? How do I apply it to the plywood or OSB?
Can I somehow fasten the corrugated directly through the foam and into the plywood? Is this practical considering the fact that I must install screws 12” on center in the corrugated panels?

If I must use nailing strips, what is the alternative to running them horizontally, which will defeat the ability of vertically run corrugated from acting as its own rain screen?

What would a practical warm roof assembly be? 
Thanks for any assistance you might provide. I’m getting up in years, and I remember when stuffing a wall with old newspapers published in Sweden was the standard construction assembly in Minnesota!

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Replies

  1. Andy S | | #1

    A couple practical considerations...
    1/2" Birch ply instead of drywall? I don't know if you've ever done this before, but it can be tricky to install to wind up with any kind of decent appearance. You have to be super particular about fastener location and there really isn't any clean way to fix mistakes or damage like there is with drywall. It also costs way more.
    No overhangs? I know it's a popular designy-architecty look to build a lifesized Monopoly house, but gutters and overhangs are great at keeping siding dry and houses cooler. Not getting the details perfect can be somewhat forgiven when there is an overhang shedding the rain away. Conversely the slightest fault without a sheltering overhang will fail quickly. The corrugated metal siding is fairly robust except at the windows and other penetrations that can be finicky to detail well even by the pros.
    This style of house can be built well, but if you're going to do it you should look at the style's strengths and not ignore its weaknesses.

    1. Ljung | | #3

      Re the plywood panels - yes, we've done it before. Nominal 2'x4' low VOC Purebond 1/2" plywood, plain sliced or rotary cut but laid in strips, hung on track channels or surface fixed. The trick is to make sure the outside surface of the studs is shimmed to level.
      Of course, you're right - overhangs serve a function purpose. Gutters not so much. Our last two modern houses had no gutters and no issues. And yes, it's "designy-architecty", but these types of structures have existed for centuries in wet climates like New Zealand, Scotland, and the Scandinavian countries, etc. I recognize the potential problems, which is why I am doing my research.

  2. Jon R | | #2

    I'd adhere to the 2021 IRC code for walls (and roof). Excepting "approved design/hygrothermal analysis", it requires some class of vapor retarder (vs barrier) for all walls.

    https://www.continuousinsulation.org/content/2021-ibc-and-irc-adopt-improved-vapor-retarder-requirements

  3. Ljung | | #4

    Thanks for the link. But what this source highlights are the differences between the IRC, the UBC, the IECC, and ASHRAE specifications. For example, with 11.25 FPIS and internal cavity insulation, there is disagreement on whether any kind of vb is needed. Moreover, I find the wall calculators difficult to use.
    I was hoping for very specific design and directions, e.g., use this design for no CI or this design with CI, complete with manufacturer's products. But even when I spoke to Huber, they were reluctant to endorse any specific design that incorporated their products. Very frustrating.

  4. Expert Member
    Malcolm Taylor | | #5

    Ljung,

    If you are running the corrugated cladding vertically you can get away with no rain-screen strapping, but I would be careful to vent both the top and bottom of the cavities, as the cladding itself is very low-perm. You want to use perforated flashing, not permeable screen closures for that.

  5. Expert Member
    Akos | | #6

    In zone 6, you have two options for your walls.

    Include enough exterior rigid for condensation controls (R11.25 for a 2x6 wall) and skip the interior vapor retarder or use less/no exterior insulation and include a vapor retarder. Both assemblies still need a decent warm side air barrier, which your wood paneling won't do. You can skip the warm side air barrier if you bump up the exterior rigid, I think it is something close to R20 in your climate.

    Unless you are going down the thick exterior rigid route and you won't have drywall as your warm side air barrier, I would include one of the smart membrane products detailed as your warm side air barrier. These membranes also limit moisture movement into the wall but still allow for drying to the interior.

    For an unvented roof, your two options are going with exterior rigid over the roof deck (R25 above, R25 fluffy bellow) or with closed cell spray foam under the roof deck.

    1. Ljung | | #9

      Akos, what would the roof assembly look like if I were to use Hunter Cool Vent or an equivalent product by Atlas? I am still not sure what purpose a vent-over roof serves in a warm roof. Is it just to help stop ice build up due to condensation on the under sheathing?

      1. Expert Member
        Akos | | #11

        The vent over an un-vented roof is only really needed in heavy snow country. You can read more about it here (figure 8):
        https://www.buildingscience.com/documents/insights/bsi-046-dam-ice-dam

        Hunter also makes standard nail base panels without a vent gap, these work the same way as any exterior rigid insulation setup would. Depending on your local code, this means you need either R25 rigid above/R24 fluffy bellow or R18 rigid/R19 fluffy bellow. The R18/R19 combo gives you about an R38 assembly which meets most codes based on U factor. When it comes to expensive roofs with rigid insulation or SPF, there is no ROI on high R value assembly. In most climates even an R30 assembly is good enough.

        If you are going with nailbase plus batts, make sure the top cord of your trusses is either 2x8 or 2x6 to fit the required insulation batts.

        If you are going with SPF, go for 2x4 as you want the spray foam to cover the top cord to reduce thermal bridging.

        1. Ljung | | #13

          Thanks Akos. I won't have issues with building codes or inspections in my locale. If I were to go with the R30 assembly (rather than the recommended R 49) using 6" top cord trusses (needed according to the truss manufacturer), what would my assembly be? Could I get by with creating a completely conditioned attic liveable space with just internal insulation, with no vapor barrier or membrane under the insulation, allowing the assembly to dry to the inside? Or am I creating a problem with the moist internal air hitting uninsulated sheathing? Would I install a smart membrane on top of the sheathing in such a case? Or an impermeable product? With the corrugated metal applied directly to the plywood sheathing.

          1. Expert Member
            Akos | | #14

            As I noted earlier, for an unvented roof, you have two options:

            Rigid above plus batts below or ccSPF/ccSPF+batts bellow. Both of these create a fully conditioned attic space which is great if you want to run your ducting in there.

            Which path you choose depends on local costs and installer comfort. Not the greenest or most energy efficient option but in most cases this tends to be cc SPF. If you do go down this road, make sure the SPF fully covers your top cord to reduce thermal bridging, also talk to your truss supplier about going to a 2x4 top cord.

  6. plumb_bob | | #7

    -Large box with 12' walls
    -High wind loading
    If a project like this came across my desk it would have walls designed as shear walls (interior ply is good), the ceiling would be a plywood diaphragm, and there would be some serious hold-downs attaching the framing to the foundation. Among other details.
    FYI

    1. Ljung | | #8

      Thanks PB. Yes, we are working on creating that diagram with special plates fastened into the concrete below the mudsills and onto the verticals, with the same plates at top plates to verticals.

  7. Tim R | | #10

    The off the shelf " special plates" are for wind uplift forces. The diaphragm is created by the plywood sheathing & detailing the connections. I think plumb_bob is indicating that it seems the building is beyond the IRC prescriptive design & should be engineered.

    1. Ljung | | #12

      Thanks Tim. I am having trouble locating a skilled insulation engineer consultant in Montana. Can anyone recommend one in a Zone 6 location?

  8. Jason S. | | #15

    This article may help in determining your insulation options and R value ratios:
    https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies

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