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Community and Q&A

Exterior wall assembly

Jayne_Torres | Posted in General Questions on

Hello again!

I am building a THOW (tiny house on wheels) in CZ-3 and have the following plan for the exterior walls:

1/2″ gypbrd
2×4 steel framing, 16 OC with R-13 in the cavities
#15 Black building paper
1/2″ densglass sheathing board
1″ R-Max sheathing
Hydrogap Rainscreen
Combo reclaimed wood and corrugated metal siding

Questions and Concerns:
1. Attaching the two outer layers of sheathing to the framing could create a lot of penetrations into the framing studs.  Should I just prop the dens glass onto the framing with a few fasteners and then attach the R-max sheathing with the advised fastener spacing, running through both layers into the framing?  (I may have just answered my own question)

2.  My siding is a combo of 1/2″reclaimed wood and 1/2″corrugated metal.  I had planned on using 1″furring between the hydro-gap and the wood portion but now realize that this will create an uneven surface for attaching the two materials.  Do I NEED the furring under the wood portion?  The metal is an additional rain-screen by nature.  I don’t think I can overdo the importance of as much rain-screen-ability I can get given the small dimensions of the home (moisture issues abound), and being in a high humidity area.  Do you believe that the Hydro-gap is sufficient?
I COULD add furring strips to the entire envelope but it may well be overkill.    But, tiny home, tiny budget and I have no room for unnecessary expenditure.

I hope this all makes sense.

Waiting anxiously to hear from the experts!!  So glad I found this site.  Many thanks!!!


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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Jayne,

    What you did not mention is how you plan to air seal. Here are some thoughts: First you don't need the "#15 Black building paper" over the framing. Second, you should install the DensGlass based on the manufacturer's fastening requirements.

    Now you have a choice to make, you can either air seal using tape or a fluid-applied product over the sheathing seams (I'm not sure what DensGlass calls for) and install your WRB over the sheathing (in this case, you don't need a drainable WRB) or you can install the RMax insulation, tape the seams and use it for your air- and water-resistive barrier.

    In the latter case, you do not need an additional WRB. I understand that you were planning to use the Hydrogap for some drainability and drying, which would be fine. But furring strips may be less expensive (you'd have to crunch the numbers) and have the added benefit of securing the rigid foam. Of course, if you go without the furring strips, you'll drive your siding fasteners into the studs which will also secure the rigid foam, I'm just not sure how well that works. You can also use adhesive.

    1. Jayne_Torres | | #3

      Thank you Brian. I had planned on covering the Dens seams with the R-max and taping the R-max seams. Had I thought about this earlier, I would have forgone the Hydro-gap and gone with furring as you suggested. I think. At this point, hydro is in route. My biggest concern has been getting enough air behind the wood siding, corrugated not being an issue, thinking I could just furr a portion of the walls, but that is not going to work. Hindsight!

      You say "you can also use adhesive"...meaning to adhere the R-max to the Densglass?

      Another question: The wall assembly was planned by a passive haus practicing architect. I thought it was an odd arrangement---any idea why it would be right against the framing?

      1. GBA Editor
        Brian Pontolilo | | #4

        Yes. You should use cap fasteners to install the rigid foam if you will not be installing furring strips, but you can also add some panel adhesive to the back of the rigid foam to make sure that it stays in place.

        I could certainly be missing something, but I am not sure why the architect spec'd the felt over the metal studs. It doesn't seem to have a purpose in that location.

        1. Jayne_Torres | | #5

          Ha! I contacted him and he said it was an error and should be between the Dens and the R-Max. Necessary or overkill??

  2. Jon_R | | #2

    Consider eliminating interior and exterior gypsum (heavy, not so strong) and just use interior plywood. Also taped polyiso rigid foam (more R value/inch for about the same $/R).

    I believe that Hydrogap is sufficient.

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