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Community and Q&A

Floor insulation for a tiny house on wheels

faoinaslinn | Posted in General Questions on


I’m beginning to build a 24′ tiny house on wheels, and I was hoping that someone here might be able to answer a few questions I have about the floor framing and insulation. I’m trying to devise a floor system that saves on height (I’m limited to a max of 13’6” to be road legal) and avoids thermal bridges. I plan on insulating within the trailer frame with 3″ foam board between the metal runners (with aluminum flashing on the underside). However, that still leaves about 3” along the perimeter where metal would be in direct contact with the plywood subfloor. Here’s my question:

Can I simply layer 1” of foam board between the trailer’s metal deck (see photo) and the plywood subfloor without using 2x4s to create a floor frame? I’m worried about the unsupported weight of the house bearing directly down on foam board. I’ve read on other posts here that the foam board might also compromise my ability to create a secure connection between the house frame and the trailer. And there’s also the issue of the foam being exposed along the edges…

If this isn’t a good idea, would it be possible to build a 1.5” deep floor system by using 2x4s (not on edge). Would this be structurally sound? Could this sort of floor system be made to cantilever about 1.5” off the trailer’s flanges on each side to gain a bit of extra floor width?

I’m new to all of this, and I really appreciate any advice more experienced builders might be able to share with me. Thanks!


(I’ve attached photos of the trailer for reference)

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  1. GBA Editor
    Martin Holladay | | #1

    Two choices: (1) Stop worrying about a minor thermal bridge, and allow the plywood to be indirect contact with some of the steel framing, or (2) Make a site-built SIP consisting of plywood or OSB, a continuous layer of rigid foam, and a second layer of plywood or OSB.

    Yes, you can cantilever your floor framing framing, but a flatways 2x4 can't go very far -- I'd say maybe 2 inches (but I'm not an engineer).

  2. Yamayagi1 | | #2

    If you live anywhere near a Structural Insulated Panel (SIPS) manufacturer, you might check with them on the availability of "seconds." Some of the manufacturers make "grands" which can be a single SIP up to 8' wide and 24' long. No thermal breaks there! (most are 4' wide x up to 24' long.) Haul the trailer to the plant and have them drop it right on! Find companies on, the site for the Structural Insulated Panel Association. For a "Tiny Home," I would put either a 4 1/2" or 6 1/2" floor down on the trailer fame. Foard Panel in West Chesterfield NH has a very cute SIPS "tiny home" out back of the plant that is used as a drive-up takeout Thai restaurant. Very cool! EPS insulation is the least expensive and most common. They also do XPS, which costs more but is higher perfoming in both thermal resistance, structural rigidity and vapor transmission, especially useful values for a thinner floor such as a 4 1/2." Build on... Martin's suggestion of a site-built SIP is probably the cheapest. But maybe not, if you can get seconds SIPs. Let's see... 12 sheets of OSB, 6 sheets of 4'x8' XPS foam.. Check out recycled foam insulation suppliers. With all the work you will be doing, I would go thicker than the 1", though.

  3. AlanB4 | | #3

    I assume there is way to calculate the effect of this thermal bridge and i'll guess its much less then $5 a month in energy. You can also go with thinner foam, less R value but more then omitting it.

  4. faoinaslinn | | #4

    Thank you to everyone for your suggestions! Guess I am being a bit overly worried about a minimal thermal bridge. The SIP idea is one I hadn't considered, and seems like the solution I was looking for. James - Thanks for letting me know about the place in West Chesterfield. I think I'm going to make a homemade SIP as Martin suggested for the sake of cost efficiency. I really appreciate all the help in figuring this out!

  5. TinyCobalt | | #5

    Hey Fallon, just wondering how the build was going. Did you end up using the homemade SIP, and/or do you have pictures of what you did do? I'm also around this same point (worrying about a thermal bridge) but I think I have it figured out. By the way, is that a trailer from Tinyhousebuilders (Dan Louche)?

  6. faoinaslinn | | #6

    Hey Damon,
    The trailer is from Tinyhousebuilders. So far, so good. I ended up doing a sort of 2 layer floor system: 1st I flashed the underside of the trailer and insulated between the trailer ribs (in the long direction). Then I framed an actual subfloor and insulated between the joists. I didn't want to risk the moisture issues of having that much metal in direct contact with my floor sheathing. And SIP panels would have ended up taking too long for my timeline. Doing it this way also allowed me to eek out an extra inch on the width, though I had to be really careful in adjusting my plans to ensure that I came in at exactly the legal height (13'6"). Good luck with your build! I got as far as getting the roof up and everything dried in before breaking for the winter. Feel free to get in touch if you have other tiny house questions. And thanks again to everyone who helped answer my original questions!

  7. Mortiferon | | #7

    The issue is not the thermal bridge itself, but what it leads to - condensation (whenever warm and cool air meet, that's what happens), and in the case of lumber, rot. The best way to head off issues would be to have a trailer custom made or else modified to have its cross rails mounted flush to the main frame, 6" deep and at 24"O.C. You'd drop in the rigid, Roxul or spray-in insulate (all of which are impervious to water/rot) directly into those cavities, then you'd glue over all of the top-side exposed steel with sill-seal/gasket (it is an adequate thermal break unto itself... it is used on all slab-on-grade structures for the same purpose), and after that you'd put down a poly barrier, and finally over all of that you'll lay your T&G ply substrate. The condensation may still happen, but it will NOT be able to get beyond that poly barrier, therefore it won't affect/rot your floor, and wick out to whatever else it's directly adjacent to. And that method (if you are not going to spend a fortune on spray-in foam) will demand that you do not entirely air-seal under the trailer (the flashing, and which you'll have to do first, to hold the insulate), so that the moisture that gathers in there, will have an out (keep your trailer from rusting out prematurely). You will need to also make the same considerations if you are building around the wheel wells, very important not to neglect them either. A sort of SIP with ply and some spray-in will be efficacious, but you should also utilize the poly around their perimeter wherever they meet the interior framing envelope for the same reason as the sub-floor. You DO NOT want condensation/rot in there, either. It doesn't take much R-value to head it off, around R5, and air-sealing to thwart drafting (which is what allows the condensation in the first place), should do. I know this is latent for the OP, but hopefully it will help others.

    1. The_Wagonista | | #8

      Hi, I am also thinking about building a tiny house but have no experience in construction except for what I have read online. I have a plan in mind for the floor assembly and was wondering what people think. The trailer will have drop axles and the crossmembers will be flush with the bottom of the trailer.

      From the bottom up:
      Metal flashing under the trailer, parallel to the crossmembers, shingled with rainscreen at the seams to allow moisture to evacuate
      Housewrap (air/water barrier) tucked into the bays between the crossmembers and trimmed long to wrap up around the bottom plate, subfloor, and exterior wall sheathing and fixed under the wall housewrap
      3" Roxul between the bays
      3" polyiso on top of crossmembers completely eliminating thermal bridging
      3/4" ply
      Cork underlayment
      marmoleum click tiles

      My big questions are:
      Will there be enough structural support if the ply is not directly on the crossmembers? how strong is the polyiso?
      Also, is the polyiso enough of a vapor barrier if i tape it? Would I also need a 6 mil poly sheeting or would that be overkill?
      Is there anything I am just not thinking of that would make this assembly impractical or not feasible?

      Thank you!

      1. 1869farmhouse | | #9

        Resurrected thread! Polyiso is definitely a vapor barrier, especially since it’s normally faced - but I wouldn’t drive around with anything that had 3” foam between the frame and floor assembly.

        The sq footage is so small, I’d focus on controlling the condensation that could occur directly against the wood and forget about the thermal bridge. The energy we’re talking about is so small... you’d offset it by eating a couple extra burritos a month.

        1. The_Wagonista | | #10

          Thanks for the reply! Would something like Roxul's 3" ComfortBoard be sturdy enough to go above the crossmembers and support the ply subfloor? I mean, it goes under concrete foundations, right? That insulation would also prevent condensation... Any other recommendations for preventing condensation? A previous poster suggested gasket/sill seal and I saw somewhere else wooden shims to create an air gap (but would that compromise the structural integrity?).


          1. 1869farmhouse | | #11

            I thought more about this. I still think 3” is asking for a lot from any hardware you used to attached the floor assembly to the frame. But if you were willing to compromise to something like 1.5-2”, you could use rubber body mounts as an isolator between the two. These are meant to attach the chassis of vehicles on body on frame assemblies. They’re extremely rigid and meant for (more or less) exactly this purpose.

            Technically, there would still be a thermal bridge through the bolt itself, but you’d still be isolated through 98% of the assembly.

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