Vapor and Air Considerations Tiny House Build
From the articles and FAQs I have read on this site, I believe that the materials I listed below are generally vapor-permeable (mineral wool, wooden panel/boards) or at least semi-impermeable (water-based paint). I am in Climate Zone 4 where we have cold winters and hot, humid summers.
My question: Is Certainteed MemBrain a good choice to place between the wood wall panels/boards and the insulation/studs? I know poly is an enemy…but the internet has made me feel some kind of a plastic layer is mandatory. I guess I am looking for someone (hopefully Martin Holladay) to let me know if I can do without it?
Order of wall materials for my tiny house shed-build from outside to inside:
– water based paint
– plywood
– 2×4 studs
– mineral wool insulation between studs
– MemBrain would go here
– (hopefully soon) wood panels
– (hopefully soon) wood stain / sealant (?)
I have a similar question for the flooring with materials listed below.
Second question: Can I rely solely on the OSB for moisture management and not worry about MemBrain? So long as I seal it super well? (tips on that would be lovely as well).
Order of floor materials for a tiny house build from outside to inside:
– treated plywood
– 2×4 floor frame with sill gasket stapled to the ‘floor side’ of each 2×4
– mineral wool insulation
– (hopefully soon) OSB
– (hopefully soon) subfloor sheet
– (hopefully soon) hard wood flooring (and sealant?)
I am concerned about the water based paint on the exterior walls causing condensation to build in my studs/insulation. I am concerned that sealing floors with poly will cause a similar issue closer to the interior of the space. A friend of mine just finished a home with a tin roof (which I also have) and found condensation on the insulation between the purlins and roof metal and so I am feeling quite worried. Martin Holladay, if you’re out there, I would be so happy to pay you for a short phone call to discuss this.
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Replies
I would say that the first step is to determine your air sealing technique. Where is your primary air barrier? Do you want to treat the plywood as your primary air barrier, or do you want the primary air barrier to be on the interior. With a vapor open assembly, you could tape the sheathing to create a wind resistant barrier, and detail your Certainteed Membrain to be an interior air barrier. Then, you could heat your tiny house with tealights...
Are you using your plywood as siding?
MaddJenk,
Matt is correct. You are thinking too much about vapor diffusion and not enough about air leakage. Your first job is to come up with a plan for air sealing. Your house needs an air barrier. For your walls, that will probably be your plywood sheathing. But you have to install the plywood with the knowledge that you are creating an air barrier -- and that usually means tape.
The second question concerns your climate. Is this tiny house on wheels? Will you be living all over North America, from Minnesota to South Texas? Or will this house have a foundation in a particular spot? Climate matters.
Matt and Martin,
Thank you for your thoughtful responses. I would like to treat my exterior plywood as my primary air barrier. When you all talk about taping, do you mean using an air/water proof tape around all seams and potential leakage spots on the outside of the structure? I was planning on doing another coat of water based paint this spring to boost that seal. I have also used an outdoor grade spray foam to seal various spots that weren’t tight, such as tiny sports where the tin roof meets the top of the walls.
My tiny house is stationary and currently on concrete blocks above ground. Some have advised using spray foam insulation on the underside of the tiny house, but I would love to not do that if possible. The tiny house will stay in climate zone 4 in the Ohio River Valley.
Thanks!