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Wall Assembly for “Barndominium” in Hot-Humid Climate

ShannonGoff | Posted in General Questions on
Hello, all.
I am new to the site but have really appreciated the information provided and the building knowledge that is shared.  We will begin construction on a custom metal home (aka “barndominium”) in the next few weeks.  I will have help/advice from my brother-in-law contractor (he only builds traditional homes) but will mostly be my own GC. I have been reading and rereading information regarding the best wall and insulation system to meet our needs and would greatly appreciate professional advice to get it right.
Our house will be a hybrid with a red iron “skeleton” and wood 2×4 framing in between the red iron; therefore we will have both wood and iron studs flush to the exterior.  We live in south Georgia….very hot and very humid.  We only have a couple of months of cold weather with maybe only a couple of weeks of freezing temperatures.  Most people in this area that build metal homes are using spray foam for insulation, but I want to limit the use if at all possible for two reasons:  high cost and the possibility of trapping interior moisture.
Here are my thoughts for the wall system from inside to out:  sheetrock; unfaced batt insulation; wood/metal studs; XPS 3/4 inch rigid foam board (unfaced) that will be taped and sealed completely; vertical commercial grade metal siding.
I would like to have the continuous insulation of XPS foam board to reduce thermal bridging.  I can’t use any more than 1″ because I did not factor this into the equation when we contracted and ordered the metal building, but metal contractor does not believe 1″ or less will be a problem.  He usually installs the MBI but we know we do not want that.  I do not believe other sheathing will be structurally necessary.  Also, the external foam board will help us achieve required R rating with only 2×4 cavity in the walls for other insulation.
We will have a vaulted ceiling and the roof will be traditional metal (NOT standing seam) attached to metal trusses, rafters, etc.  There will not be wood framing in the roof structure.  We will probably use closed cell foam on the ceiling to achieve air/vapor barrier and high R value.  The wings will have attic spaces that will be within the conditioned envelope.
We will research and use the best HVAC system for our home.  I know there are special requirements and plan to have a professional help us with what is needed as far as variable speed, humidity control, fresh air intake, etc.  I do not have enough knowledge to speak to this but know there are special requirements especially if using spray foam, so we will contract with a professional.
My questions:
1 – Will the XPS serve as a sufficient water resistant barrier and air barrier if taped and sealed as required and all windows and doors installed correctly with needed sills, wraps, etc.?
2 – It seems that XPS is slightly vapor permeable which may allow for some drying outward.  Is that correct and will that be helpful in our climate?  Is the XPS enough to limit moisture from driving inward?
3 – I am thinking the corrugation of the metal will be enough of a ‘rainscreen” to allow moisture to drain down without furring strips.  Would this be enough?
4 – I am assuming the metal siding is enough structurally since most metal homes in this area are metal siding with spray foam applied directly to metal without any other exterior sheathing.  Is a separate sheathing needed?
I want to get the wall system right without overbuilding.  I am very concerned with avoiding any moisture problems and achieving a well insulated envelope for efficiency.  Any advice or critiques will be greatly appreciated!  Thanks!

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Replies

  1. Expert Member
    Michael Maines | | #1

    As someone very concerned about our climate crisis, it's always hard to hear about people building with the most climate-damaging materials available. Steel frames, XPS insulation and spray foam insulation are three of the four worst offenders when it comes to our buildings' up-front carbon emissions. It's probably too late, or perhaps you don't care, but there are a multitude of other ways to build that don't contribute disproportionately to the climate crisis.

    Assuming you don't care about that. to answer your questions:
    1. XPS is waterproof and airtight, so it's possible for it to be both the water control layer and air control layer. But it's easier said than done.
    2. At 1", XPS is about 1 perm, which limits vapor movement to some degree but allows the assembly to dry if necessary. In south Georgia, you will probably have inward vapor drive most of the time, with occasional outward vapor drive. My best guess is that 1" of XPS will work well in that situation, but I'm not an expert on wall assemblies in your climate zone.
    3. It depends on how much moisture but my guess is that the corrugations are better than nothing. If possible, I would add a drainage plane--even Tyvek Drainwrap would be better than nothing. Obdyke Hydrogap would be better.
    4. That is a question for an engineer. I would not be comfortable with it and I doubt it would meet code--aka the worst house you can legally build.

    1. ShannonGoff | | #2

      Thank you for your reply. I am trying to consider environmentally friendly options, so that is why I subscribed to this site. From what I have read on this site, It seems that many rigid foam board makers are making great strides to reduce the carbon impact of their production. If I end up going with this option, I will use one of the more green products. And for what it's worth, this will be my one and only build in my lifetime, so I feel like that should count for something as far as being friendly to the environment.

      The only permanent decision is the use of red iron/steel skeleton and siding since that has already been ordered/contracted. The other option that I am considering is the normal OSB sheathing with Tyvek house wrap. Or, I would be interested in the Zip system (or something similar) if I can find it at a reasonable price in our area. What are your thoughts on those 2 options? I believe both of these would give me more options for insulating in the studs as well.

  2. MartinHolladay | | #3

    Shannon,
    You might want to read this article: "Insulating a Metal Building."

    At the end of the article, I concluded: "If you are hoping to build a new building with an above-average level of insulation and airtightness, you’re probably not going to choose a metal building. But if you want an inexpensive building for industrial purposes or storage, metal buildings make sense."

    1. DC_Contrarian | | #4

      Yeah, to adequately insulate and seal a metal building you really have to do a house-within-a-house, which kind of removes the primary appeal of the metal frame, low price.

  3. walta100 | | #5

    You might find this 5-part series of articles interesting.

    https://www.greenbuildingadvisor.com/article/post-and-frame-construction-for-home-building

    Seems to me and most on this board that the pole barns are a great way keep your tractor out of the rain but a total nightmare to convert into a high-performance home. I think if you price out every thing required air seal to under 1 ACH50 R25 walls and R38 ceiling to build your pole barn an and a conventional frame building the pole barn will cost more to finish to that level.

    If you are looking for sympathy ears try posting to the Garage Journal forum.

    https://www.garagejournal.com/forum/

    Walta

  4. seabornman | | #6

    What direction will the metal run? Will you need to run long screws through the XPS? Is the metal building to be wind braced with x-bracing? I've seen more than one metal building have problems with proper detailing of insulation at the eave strut.

    1. ShannonGoff | | #8

      Seabornman,
      The metal is vertical. We have decided to sheath all walls with OSB and house wrap (or possibly something like ZIP). I will be sure to make sure the proper screws are used. I chose to do the hybrid so that all windows and doors are framed traditionally and can be properly installed and sealed to keep water and air out. Since all exterior walls will be framed with 2/4 stud walls, I do not think we will need extra bracing. Do you agree? Thank you for the heads up about the eave strut. We will give special attention to make sure that is sealed and insulated properly.

      1. seabornman | | #10

        If the wood studs are tied to the steel columns, I would guess sheathing with OSB would be all the wind bracing you need (you don't say how tall the walls are). The 1" of foam board over everything is a good choice. You'll have to decide a proper screw pattern for siding as you will only be attached to OSB, not purlins. Don't use black siding; it's alleged that XPS starts to soften at 165 degrees F.

  5. ShannonGoff | | #7

    We have decided to sheath all walls with OSB and house wrap (or possibly something like ZIP). I chose to do the hybrid so that all windows and doors are framed traditionally and can be properly installed and sealed to keep water and air out.

    1. MartinHolladay | | #9

      Shannon,
      You still have to worry about the steel columns (posts). Will you have continuous insulation on the exterior that bridges the intersection of the columns and the wood framing? How will you insulate and finish the steel columns on the interior? How will you handle air sealing of seams between steel and wood?

  6. walta100 | | #11

    Do yourself a favor and do some math the mess you are describing sound likely to cost twice as much as a stick framed building and with all that steel in the wall it is likely to preform half as well.

    Consider sick framing the half that will be conditioned and use the steel for the rest.

    If you are determined to condition a steel framed building the only real option is move most of the insulation to the exterior of the steel frame.

    Walta

    1. DC_Contrarian | | #12

      Or do a house-within-a-house and put the insulation fully within the steel frame.

      Either way it's easy for it to end up costing more than stick framing.

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