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Community and Q&A

Heated Post Frame Shop

Adam Emter | Posted in Plans Review on

Hi all,

I build small homes and cabins (under 400sqft) on my property. I have room to build a 40x96x14 shop so that I can build the units inside. The shop will be heated to around 55 degrees (never higher than 60) during our winters. I have no plans to air condition in the summer. Live in Zone 6, central North Dakota. I’m leaning toward post frame construction with 5″ slab-on-grade concrete floor. The slab will be insulated below with high-density EPS foam. I will also run 24″ vertical insulation below grade on the edges of the slab and 24″ horizontal insulation, to protect the slab from frost. Heating will be in-slab radiant.

My wall plan (from exterior to interior) is:
36″ ribbed steel panels
1×4 horizontal furring strips
2″ r-tech EPS foam, detailed as WRB with taped seams
2×6 bookshelved girts, filled with r-19 fiberglass
OSB, detailed as air barrier with taped seams

Roof will be:
36″ ribbed steel panels
synthetic underlayment approved for use under metal
5/8″ OSB
blown cellulose on the attic floor
OSB, detailed as air barrier with taped seams

The roof will be vented with passive gable vents. The only openings will be a 18×14 double sliding door and a 36″ walk-through door. There will be no windows and no plumbing in the building. I will be doing most of the construction myself and will oversee the entire project, so I can ensure that insulation and air-sealing details are followed. I know post-frame buildings can be difficult to insulate and air-seal, but I know it’s possible.

My questions are:
Can I use the r-tech exterior foam as an air barrier, in addition to WRB?
Is the 2″ of exterior EPS foam enough to keep the wall dry?
Are there major issues that I’m overlooking?

I appreciate your thoughts!

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Adam,
    If you live in Climate Zone 6 in central North Dakota, you must be very close to the border of Zone 7.

    In Zone 6, the minimum R-value for your wall foam is R-11.25.

    In Zone 7, the minimum R-value for your wall foam is R-15.

    You have specified rigid foam with an R-value of R-7.7, which is nowhere near enough. (This document lists the R-value of R-Tech EPS as R-3.85 per inch. The data sheet has a lot of information that I call obfuscatory; what you are looking for is the R-value per inch at 75 degrees F. It's listed there -- you just have to hunt for it. The number is R-3.85 per inch.)

    For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Adam,
    Q. "Can I use the R-Tech exterior foam as an air barrier, in addition to WRB?"

    A. Probably, although you have to consider the issue of possible rigid foam shrinkage. For more on this issue, see Using Rigid Foam As a Water-Resistive Barrier.

    Q. "Is the 2 inches of exterior EPS foam enough to keep the wall dry?"

    A. As I noted in my first response, the answer to this question is no.

  3. Adam Emter | | #3

    Thank you for the reply, Martin.

    Question regarding minimum thickness of rigid exterior foam: Does the calculation take into account the fact that this shop will be used as a semi-heated space? I will be around 15 degrees less interior temperature and therefore my dewpoint will be lower on the interior. Not sure if that would make an impact on the possibility of warm side condensation.

  4. User avatar GBA Editor
    Martin Holladay | | #4

    Adam,
    You are correct that cooler air indoors, and lower humidity indoors, will lower the risk of condensation problems for these walls. However, here's my philosophy of building design: It never makes sense to choose a risky wall assembly. The building may stand for 60 or 100 years, and you have no idea how the building will be used in the future. It's better to choose robust wall assembly details, in case someone turns the building into a mother-in-law apartment.

  5. Adam Emter | | #5

    Martin,
    Again, thank you. I do agree with your philosophy. At this point, I guess I would have to spec 2 layers of rigid foam, taping the exterior layer as a WRB. I'm not sure this is cost-effective for me at this point, so now I'm thinking about this wall assembly, which is much more simple, but not as efficient.

    36" ribbed steel panels
    7/16" Forcefield sheathing, with taped seams
    2x6 bookshelved girts, filled with r-19 fiberglass
    OSB, detailed as air barrier with taped seams

    I know the efficiency of this wall is much less than that with exterior foam, but do you think it will be acceptable from an air-sealing perspective?

  6. User avatar GBA Editor
    Martin Holladay | | #6

    Adam,
    You can certainly make the wall assembly Passivhaus-tight if you want. All you have to do is to pay attention to potential air leaks, and seal seams very carefully.

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