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Wall assembly detail question

mikeysp | Posted in General Questions on

Hi.

My buddy is building his house in the southern end of the Ozarks zone 4 and has no code requirements.

His wall assembly outside to inside is:
1. 26 gauge tin
2. 1″ spray foam
3. actual 1x horizontal girts (rough sawn 1x)
4. 2×4 studs
5. cavities filled with r-13
6. drywall

He wanted to know if he is making any big mistake?
Is there any reason for housewrap between the metal and the horizontal girts.

I did not say anything as I am unsure, but I thought that if he ever needed to change his metal siding, it would be a whole lot easier if that housewrap was in place, so they are not glued together. 

Thank you for your advice.

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Replies

  1. Expert Member
    Zephyr7 | | #1

    Why spray foam and not exterior rigid foam? I think rigid foam would offer a lot of advantages here, and probably some cost savings over using spray foam. It’s pretty much impossible to get a flat surface with spray foam if you’re applying the sprayfoam on the exterior before putting the tin up. Rigid foam can also double as a WRB in many cases if properly detailed. I’d look at using polyiso here instead of spray foam.

    Bill

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Mike.

    That's an interesting wall and I'd love to know why he is building like this, but I'll just ask one question: has he consulted an engineer to make sure that the framing plan has enough shear resistance without sheathing?

    To address your specific concern, I agree with you. Not only will spraying the foam against housewrap (installed over the girts to decouple the siding and the foam) make remodeling and repairs much easier down the road, but it will protect the girts from water that gets behind the siding. I think that would be a good decision and wouldn't add to much cost to the build.

  3. Joel Cheely | | #3

    Also, the spray foam is broken up by all of the girts. There's bound to be shrinkage (of the foam and the girts) and movement that results in air leakage. Is this post frame (pole barn) construction?

  4. mikeysp | | #4

    Ok gents. He is building this on an old set of doublewide trailer frames, so he can move it in the future with some ease. So he wants it to be light weight and of course, cheap to build, so he thought to scrap the use of plywood. If needed he can put in some diagonals, but he "thought" the metal would suffice for shear. He definitely is not consulting an engineer, unless one chimes in on here :) He is my best friend back in Arkansas and I am doing this to help him avoid any errors that will haunt him or be very expensive to remedy. He does not want any regrets with a problematic detail, so he asked me if I would find out if it would work after hearing me champion building science. He is building his own trusses and will make them so he can support each side and cut dead center (12ft) to make the two halves for transport if he ever moves the house. I recommended he make raised heal trusses so he can have good insulation at the eves. He is planning on the roof being nearly identical to the walls. Walls 16"O.C. studs with 1x girts 24" OC and the roof with 24" OC trusses and 1x 24" O.C. I need to ask why he does not use 24" OC studs? He will be using multiple mini splits, so no ducting will be in the attic area; therefore, I am going to recommend he use blow in cellulose above ceiling with proper percentages of ridge venting and greater area of eve venting to avoid vacuum effect as well as drive home importance of air sealing. I am just not sure about his wall assembly as it is not anything I have seen. I see how foam board on the outside would be a great way to get a thermal break vs spray foam which might crack with expansion and create air leaks, but what about shear? If he put in a couple cut in diagonals on each side, maybe that would be a solution.

    Thanks for the comments. I am going to send him the link, so he can read your advice.

    -Mike

  5. Joel Cheely | | #5

    You got me curious. What is the diaphragm capacity of ribbed metal siding? I thought I had seen a chart at Fabral.com but couldn't find it. My neighbor just built a 22 ft. eave height pole barn with only the typical knee braces for wood bracing. The siding must provide the real diaphragm to combat wind loads. So your buddy's modest project is probably ok from a structural standpoint. There's plenty of mobile construction sheds built the same way and not-so-gently moved around. He should pay attention to truss bracing, since he won't have wall sheathing that extends up the ends of the raised heel to give some resistance to racking. He's sheathing the roof or using metal on purlins? Some cross-bracing is in order. Store-bought trusses are cheap and they provide a drawing showing required bracing. Good luck!

  6. mikeysp | | #6

    Thanks. Turns out he can't do raised heel trusses as it will make it to tall for moving later.

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