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Feedback on exterior wall plan for tiny house

michaela_riley | Posted in General Questions on

My husband and I are about to start building an 8×20′ tiny house on wheels, and I’m trying to devise walls that will insulate us well, provide the appropriate air and vapor barriers, and stay as thin as possible to leave plenty of living space. I’d really appreciate any feedback you all can give on our plans! For reference, we are in Nashville, TN, climate zone 4.

From interior to exterior, we are planning:

* Wood tongue-and-groove planks (drywall is too brittle for moving houses)
* 2×4 studs filled with 3.5″ paper-faced polyiso, with spray foam/tape/caulk to seal seams (serving as our air barrier)
* 1/2″ ReWall Essential Board as sheathing
* House wrap (Tyvek or similar)
* Delta Dry barrier for a rain screen
* Cedar planks and shingles for siding

I know this plan doesn’t address thermal bridging, but I would like to keep the walls no thicker than this to leave enough interior living space. (We’re currently at 7′ 3″ wide.)

Thoughts and suggestions? In particular, do you think we need an added vapor barrier on the interior side of the walls? I know that isn’t important compared to an air barrier, but I wonder if the higher humidity in a tiny space like this would necessitate a vapor barrier.

Your advice is greatly appreciated!

Michaela

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Michaela,
    You don't need an interior vapor barrier, for three reasons:
    1. Building codes don't require interior vapor barriers.
    2. Building scientists don't recommend the use of interior vapor barriers except in very cold climates.
    3. Your rigid foam is already a strong vapor retarder (but one that won't cause any problems).

    For more information on this topic, see Do I Need a Vapor Retarder?

    You have chosen the "cut-and-cobble" approach for insulating between your studs. The system can work, but there may be questions about the long-term integrity of the sealing methods used to prevent air leakage (especially if you will be driving around with your tiny home on wheels). You might want to include an interior air barrier membrane (Intello Plus or MemBrain) between the interior surface of your studs and your interior T&G boards.

    For more information, see Cut-and-Cobble Insulation.

  2. Expert Member
    Dana Dorsett | | #2

    Even with all of the can-foam caulk & tape air-tightness of rigid foam between studs can't be maintained if you're moving this house around, and the thermal bridging robs the high-R/inch goods of a large fraction of it's performance anyway. If you use open cell foam it's probably flexible enough to cut it, but otherwise R15 rock wool is probably a reasonable solution.

    Use Intello or MemBrain as a flexible air-barrier & vapor retarder between the framing layer and the interior paneling. The t & g planking may allow a lot of air convection behind the planking, which will end up in the wall cavities. They don't specifiy a vapor permeance for 1/2" ReWall Essential Board, but since it's predominantly plastic it's likely to have VERY low vapor permeance, and there will be NO drying toward the exterior, and wintertime moisture drives from the interior will concentrate in the cool edges of the framing. As long as it's vapor diffusion only through a smart vapor retarder it's not a problem, but with interior side air leakage there is a risk. With smart vapor retarders it will at least have a fairly high drying rate, even if the peak moisture levels in the wood are high.

  3. iLikeDirt | | #3

    Instead of 1/2" ReWall, how about 1" polyiso as the sheathing instead? You'll lose only a grand total of one inch of interior space, and the polyiso will address thermal bridging through the studs much better. It can also be taped to serve as your air barrier or WRB (or both), and then you can fill the studs with mineral wool.

    Polyiso is lighter than ReWall, and mineral wool is lighter than polyiso, so those should help keep your total weight down.

  4. Expert Member
    MALCOLM TAYLOR | | #4

    A disturbing number of tiny houses experience mold problems due to the lack of ventilation to control indoor humidity. It's worth remembering that the occupants generate the same amount of moisture that they would in a larger space.

  5. michaela_riley | | #5

    Thank you, all, for your wise advice! My husband and I really appreciate having this community to consult.

    I've spent some more time looking at mineral wool insulation and external foam board sheathing, and it seems the benefits of less thermal bridging, better fire resistance, and increased environmental friendliness are worth losing a tiny bit of interior space. So, taking into account everyone's suggestions, what about this?

    From interior to exterior:

    * Wood tongue-and-groove planks
    * Air barrier membrane, likely MemBrain
    * 2x4s filled with mineral wool insulation, likely Roxul
    * 1/2" ReWall for structural sheathing (seems important to keep given the fact that we're mobile)
    * 1" foam board insulation
    * Thinner, 1/4" rainscreen, likely a mesh one to allow for shingles as part of the siding
    * Cedar planks and shingles

    To control humidity, we'll also install a mini split air conditioning system with good dehumidifying capabilities.

    How does this sound now? Any further suggestions?

    Again, thank you SO much!

  6. dburgoyne | | #6

    You may want to consider a welded tube steel frame (especially to provide lateral strength in the8 ft dimension.). You could use 2x3 framing and gain a couple inches back on your interior. Maybe even a more wind resistant siding? (Hardiplank?)

  7. iLikeDirt | | #7

    Hardiplank would probably be too heavy for a mobile tiny house. Come to think of it, vinyl siding might be a better idea than cedar for the siding for reasons of cost, long-term durability, and weight.

    You could probably save some space, money, weight, and complexity by omitting the ReWall, using non-structural polyiso sheathing, and adding structural stability with diagonal steel braces.

  8. michaela_riley | | #8

    @Nathaniel, this being my first building project, I guess I'm a little paranoid about skipping structural sheathing, especially since this will be driven down the highway. That being said, I do like the idea of saving space and cost. :-)

    Can anyone else weigh in on the possibility of skipping the ReWall and instead using some sort of bracing?

  9. Expert Member
    MALCOLM TAYLOR | | #9

    Michaela, I'd stick with using the sheathing to provide shear strength and tie everything together. I don't know enough about Rewall to comment on how it perform with repeated movement. 1/2" plywood would be my first choice

  10. STEPHEN SHEEHY | | #10

    Michaela: how about installing the t&g planks diagonally? Would that provide the structural strength you're concerned about? Another alternative might be to use beadboard sheets instead of separate planks. You'd probably need to screw or glue to get the shear strength you want.

  11. user-4053553 | | #11

    Roxul or dense pack cellulose are good ideas for bay insulation since they need to have some give and not settle when moved/vibrated.

    You may want to use hurricane clips to hold the roof onto the frame and use larger well secured plastic/metal for the roof, shingles will start flying off on long drives.
    Look into very small HRVs for air circulation, a tight house especially such a small one will build up in co2 concentration very quickly. I remember seeing a very small Panasonic model for a few hundred dollars on amazon a few years back, though i never bought it.

  12. Yamayagi1 | | #12

    Have you considered SIPS- structural panels? Incredible strength, and almost no thermal bridging. I believe R-Control is near you in Knoxville. Foard Panel in New Hampshire will make the panels as thin as 3", and will also insulate with higher r-value XPS if you choose. Don't know what R Control options are offered. Worth looking at, though...

  13. Expert Member
    MALCOLM TAYLOR | | #13

    I helped my cousin's nephew build one last spring. By the time you get all the plumbing, electrical and ventilation into the exterior walls (as there are usually few interior ones) there isn't much room left. We also used a lot of Simpsons connectors, so I don't know how well SIPS would work. Structural screws work better than nails in this application for their superior pull-out strength.

  14. michaela_riley | | #14

    Again, many thanks, everyone!

    @Alan, yes, we will definitely use hurricane clips! And many thanks for the HRV suggestion. I knew we'd need to pay attention to ventlation, but I hadn't found a solution yet. That little Panasonic model looks like just the right option; I just found it on Amazon.

    I've looked into SIPs, but just as Malcolm noted, they didn't seem like the best option given everything we need to build into the walls.

    @Malcolm, if you have any other tips from your experience with your cousin's nephew's house, I'm all ears!

  15. Expert Member
    MALCOLM TAYLOR | | #15

    Micheala,

    General: Calculate the weight of your proposed house very carefully. Small house sites are full of optimistic rules of thumb which may make it impossible to move. Make sure your dimensions allow you to use your local highways. Include any overhangs in this equation. Don't compromise your design by trying to use "green" alternative materials. The main benefit of small houses is how little resources they consume. My relative insisted on using lamb's wool insulation and "letting the walls breath". By not air sealing his place already has moisture problems and probably consumes as much heat as my whole house. Think through the location of vent openings, including those for your roof so that they don't take on water when moving at high speed. Provide attachments for outside canopies to expand the space once you get settled, and storage for things, like wet boots you don't want in such a small place.

    Structure: The sheathed walls and regular shape of the house makes it very rigid. The weak point is the connection to the trailer frame. We used: http://www.strongtie.com/ftp/bulletins/T-ANCHORSPEC12/T-ANCHORSPEC12_p14-15.pdf

    Services: Choose connections that will allow you to use both RV and house services. Include an electrical sub panel which offers some protection. Include good, quiet ventilation, both to keep down humidity and cooking fumes, especially if you use gas. Think through your septic and consider including an RV style holding tank.

    Like boats these small houses rely on very high quality finishing for their viability. Don't skimp on interior finishes.

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