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Community and Q&A

Tinyhouse Floor (SIP over steel frame)

Steven Valenziano | Posted in Energy Efficiency and Durability on

I’m building a tinyhouse and I’m hoping to build on the recommendations from 2 other GBA posts (referenced in the PDF) in order to finalize floor and floor-wall connection details.

I’m hoping Martin Holladay and others can point out any potential issues with my proposed details.

In addition to general sanity-checking / unknown unknowns, I have a couple of known unknowns:

1) What adhesive is recommended between SIP plies?

2) In detail 8 (top-right, detail @ front and back of trailer): Do I need to worry about the fact that the sill plate will only be partially supported between the steel 2x2s?  I anticipate only a couple of studs will fall on these portions of the sill plate, but is there something I should do to better support these studs?

Thanks for your thoughts!
-Steven, 4A

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  1. Scott Wilson | | #1

    This article from the Journal of Light Construction on Tiny House problems may help.

  2. Steven Valenziano | | #2

    What an awesome resource, thanks for sharing! Since my trailer doesn't have a sheet metal deck, I would place the Mento3000 above an OSB or plywood layer, instead of the sheet metal. From what I can tell (from reading JLC's 'Avoiding Wet Walls" and 'Avoiding Wet Roofs' articles), this would be an improvement, because the 38-perm Mento3000 would allow drying towards the outside, in addition to the inside-drying potential in the article you shared.

    Is this correct or am I making a terrible mistake?

  3. Steven Valenziano | | #3

    Well, I think I may have answered my own question. I did some calculations, following the example of BSD-163 (

    Tback of sheathing = Tinterior – (Tinterior-Texterior) * Rbatt/Rtotal

    34F is average low for January, the coldest month

    35% RH @ 65F is a reasonable interior humidity, based on my experience in my current house

    No exterior insulation: 65-(65-34)*1=34F

    Based on this chart (Building Science BSD-163, also attached), condensation will occur on the interior of the OSB in winter months for interior RH above 25%. Not good (right?). If my numbers are correct (and they would be more dire for a northern climate), that explains why JLC is recommending a rust inhibitor on the sheet metal - because it’s going to see a lot of moisture.

    What’s the solution for me? XPS outboard of the OSB? I wonder if the XPS could handle the linear/point loading on the trailer’s steel ‘joists’? This GBA article suggests not:

    Thanks for any thoughts/constructive insults, I'm new at this!

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