Tiny House Building Science: The Floor

** NOTICE **
At least one aspect of the following post differs from that which we implemented
in the end-design and/or the actual construction of our Tiny House.
For the most accurate and up-to-date information please refer to our eBook.

Perhaps the largest single issue in the design of our tiny home has centered on how to construct the building’s envelope.  “Envelope” is just a fancy word that describes the shell; the floor, walls and roof that form the parts one sees when looking at the outside of a structure.

This is the 4th of 4 posts detailing the methods we have chosen (after much research, consultation and contemplation) to protect our home from the elements and shield it from the negative results of human habitation.  Read the first post in this series to learn more about Building Science.

Building Science & Tiny Houses:  The Foundation and Thermal Breaks

Our tiny house will sit on a custom trailer that we had fabricated in order to maximize the potential R-value of the floor and the headroom in the main room and the loft.  The combination of 2X6 steel framing and two 6,000# “10 degree-up” torsion axles will allow a decent cavity for insulation plus will allow the trailer to sit low to the ground, thus giving us more vertical room to build on top of it.

Since the wood floor framing is bolted/sistered directly to the steel trailer framing, in order to minimize thermal energy transfer between the steel framing and the plywood sub-floor, we opted to create three thermal breaks:  1) between the top of the steel framing and the top of the wood framing; 2) between the bottom of the wood framing and the bottom of the steel framing; and 3) between the steel framing and the outside air.  It remains to be seen whether these breaks will be enough to limit the transfer of cold air to the 1” thick reclaimed maple floor.  We hope so, since its no fun having “chilly toes.”

We are also considering framing in an insulated hatch in the kitchen floor or a wall to permit the use of a naturally cooled “chest” refrigeration box in the colder months in the spirit of Ben Hewitts’ clever design.  And in the warmer months, we hope to experiment with a naturally cooled evaporative fridge based upon this age-old design recently pioneered by Mohammed Bah Abba in Nigeria (described in greater detail here).

The Floor

The following is a sketch of our envelope, specifically the floor,  along with a detailed description of each layer.

a

All four walls we be constructed of reclaimed native 2X4 framing.

b

We will use ¾ PTS plywood for the subfloor.  It will be adhered to the reclaimed native 2X6 floor joists and nailers using construction adhesive and nails.

c

We found a local metal fabricator who agreed to weld up a custom frame to our specifications.  The fabricator constructed our trailer using 2X6 tubular steel manufactured in Canada.

The trailer is 22 ½’ (instead of the planned on 20’) X 8′ .  As soon as we started to build upon the frame, we discovered that the frame was out of level by up to ¾ of an inch.  To correct for this, we were forced to raise all of the nailers and the floor joist above the level of the steel frame approximately ½” in order to properly level and make the trailer ready for construction.  We’ll further discuss our experiences with the two custom trailer builders we dealt with in a future review.

d

Like in the walls and the roof we will use dense pack celluloseto insulate our floor.

This material is simple to work with, inexpensive and more than adequate, especially when you consider the tiny size of the space it insulates.  Sure, we could have gone with spray foam (for a lot more money) in all of the cavities, but we ran the numbers and found that the payback period for this product for us ran into the centuries.  It seems that when your heating fuel is free (in our case locally harvested “waste” biomass in the form of branches & twigs, etc.) and the space you are trying to heat is so tiny (the annual heatload is estimated at around 6000BTU’s which equals less then half a cord of wood), there is simply no way to justify the toxic footprint of spray foam.  Plus, it would be unfair to future owners of this little building.  Who wants to work around this stuff now or in the future?

e

A couple of years ago we bought about 1500 linear feet of 1” thick and 7” wide maple boards that were reclaimed from an educational facility that was torn down.  They had been originally installed as window and door molding.  We took one look at it and thought “flooring”.  So, in addition to using it for our interior molding, we’ll be using this find for our flooring on the main level and in the loft.  Each board is milled on four sides, three of which have varying degrees of clear finish applied to them, and one of the edges is beveled.  Our plan is to lay the boards bevel to bevel, adhering each to the subfloor with construction adhesive and then face nailing each.  We’ll fill any irregularities with a mixture of sawdust and two part epoxy, sand the entire surface down and then apply several coats of non-toxic sealer and finish.

f

As you will recall, before we install the plywood sheathing on the walls, we’ll be installing ½” thick horizontal strapping to the studs to act as a thermal break and to add a little more depth to the wall cavity.  This layer of the image provides an example of this strapping.

g

In order to encapsulate the trailer’s tubular steel framing, we’ll be wrapping the edges and the underneath of the trailer with ½” of either fiberglass-reinforced paper-faced polyisocyanurate or XPS, whichever we can find.  Doing so will cut down a little bit on the thermal bridging effect that would ordinarily occur between the cold outdoors and the wonderfully conductive steel.  Since we also plan to bolt or screw the wall sheathing to the sides of the steel frame (to serve as the hard connection between the structure and the trailer), we will utilize squash blocks everywhere we install a mechanical fastener to prevent the plywood from deforming and crushing the rigid insulation underneath.  To clarify, around the perimeter of the trailer frame, there will be a ¼” of space between the sheathing and the rigid insulation.

h

This image is representative of the native reclaimed 2X6 framing that comprises the wooden portion of the floor structure.

i

This image is representative of the ½” plywood wall sheathing.

j

To fasten the ½” plywood wall sheathing together with the ¼” Masonite, we’ll utilize corner blocks spaced appropriately to screw each edge together.  All joints in the Masonite will be caulked with removable caulking and each panel will be screwed to the joists and nailers with gasketed roofing screws.

k

In most places around the perimeter of the frame we’ll insert shims on top of the steel frame to support the 2X4 bottom plate (and, by extension, the wall).  The gaps elsewhere in the frame will be left open to somewhat offset the effects of thermal bridging through the steel.

l

This image is representative of the rigid foam insulation that will encapsulate the sides and underside of the trailer frame and floor structure.

m

In order to slightly increase the depth of the floor cavity, slightly mitigate the effects of thermal bridging and provide flush nailers to fasten the Masonite to, we will install strapping perpendicular to the floor joist and along each joist nailer.

n

To cover and protect the rigid insulation and close off the floor cavity, we will install ¼” thick Masonite panels underneath the entire 8’ X 22 ½’ trailer.  These panels will be removable should we ever need to access the floor structure.

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That’s about it as far as the envelope of our tiny house goes.  Now its just a matter of applying these methods and materials to the real thing.  With any luck we’ll get a thaw in the coming weeks so I can get outside and work on implementing some of our ideas.

This is Part 4 in a series of 4 posts in which we examine building science as it pertains to our tiny house.  

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6 comments to Tiny House Building Science: The Floor

  • If I had only read this post first. I especially like item d.

    Thank you for such thorough posting.

    Please feel free to come visit our r(E)volution anytime. We are moving a little slower as we have land to clear, gardens/farmland to establish, etc. before moving into our house and onto our land.

    • Hi Andrew,

      Glad you found Kai’s “Building Science” series of posts. We had a hard time finding any information on this stuff ourselves so he thought it might be beneficial to share what we’ve learned along the way. Of course, the real test will be to see how our chosen methods fare over the next few decades.

      We follow your Tiny r(E)volution blog and love what you two are doing. We believe there really is a revolution beginning out there – bringing better ways of living to the forefront. Good Luck with your building – we’ll be following your progress and rooting for you from here. 🙂

  • Hey there Sheila. Thanks for writing back. You are so right. There was so little information on the actual construction before the few of us came along. It seemed until then Tiny Houses were little more than finished product images and some “maybe we’ll do this or that” type posts. But now it is time to get down to brass taxes, if you will. In fact, I am not sure if you saw but I am going to be guest blogging “How-To’s” on the tinyhouseblog.com for Kent Griswold every other week. I hope to really break through with some real solid, construction, information.

    Thank you so much for your kind words and if even just our two families can show true success, then the r(E)volution will have begun!

  • Breh

    Thank you for putting together such a thorough and informative post. There are some clever ideas here that I had not considered. Speaking of clever ideas for constructing a floor for a tiny house on a trailer and attaching the walls, I have to recommend Dee Williams’ e-book titled Go House Go. Her method also involves building the floor into the frame, so as to provide more room for insulation and more head room within legal road height. However, instead of the 2×6 tubular steal frame joists, she uses smaller dimension steel ribs which are attached under the 2×6 tubular steel trailer frame and provide support for the 2×6 lumber joists which will sit on top of them. Besides being (I imagine) less work to install the floor — because you can assemble the wooden floor frame and just drop it in without having to bolt on all those nailers — There is a lot more room left for insulation and not so many steel thermal bridges.
    She also provides the best/most secure method I’ve seen for attaching the walls to the floor/trailer — although it may have to be modified slightly for larger and custom trailers.
    Definitely worth a look for anyone considering such a project.

    • Hi Breh,

      Thanks for the reference – we love Dee. We’ll have to check out her book when we have a chance. It sounds interesting, although we can’t wrap our heads around how you could not attach the frame to the trailer at all….

      Be Well. 🙂

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