My first real physical steps of the building process commenced a couple weeks ago when I decided it was time to build the subfloor of my tiny house. Tiny house builders go about the structure of the subfloor in many different ways. As with most aspects of the tiny house, there are reasons and personal preferences that sway the decision making process, and that’s exactly why building a custom tiny house is so fun and unique. Building codes don’t really exist.
The subfloor design is inherently linked to the structure of the trailer (foundation). Most builders using old fixed up trailers tend to build the subfloor on top of the chassis by framing it in with wood, putting metal flashing underneath, and insulating. Building on top of the trailer frame cuts into your interior headspace. Since we are working with strict width and height restrictions this can turn some people off from this subfloor design. Some trailers are designed in a way that you can build a subfloor frame out of wood and drop it inside the perimeter trailer beams, this saves you headspace and really just depends on the type of trailer being used. My trailer, from Trailer Made Custom Trailers, LLC (https://www.trailermadetrailers.com) has cross beams and metal flashing underneath. This makes it possible to drop in the insulation, cover it in house wrap and screw the T&G OSB (oriented strand board) right into the steel tubing itself. It’s a super fast process unless you go about it the way I did.
A lot of tiny house builders are using various forms of spray foam as their insulation. I have several reasons for why I don’t want to use spray foam, one of which is the toxic off-gassing that takes place during installation, and can continue if it doesn’t cure properly. I also have a personal mission to use most all biodegradable materials when possible or reuse/recycle materials that would otherwise end up in a landfill forever. This means that it might take longer or be more difficult, but so be it.
I had originally intended to use Roxul (basalt rock and steel wool) insulation in my subfloor, which is what I’m using in my walls and roof, however Brian Sisk – my dry-in man – who lives out on Isabelle (coincidence!) road told me that he had a pile of 2″ closed cell foam boards. They had been sitting by the hay barn for 8 years, and said that they would be perfect for layering and insulating my subfloor, or else they were going to be thrown out. The edges were a little moldy, but a couple inches in and they were good as new. We decided to go for it. I was so happy to use this foam that would never biodegrade, but would give my subfloor an R value of 25. We set to work and realized that the rigidity of the foam made it impossible to cut long pieces to slide in and under the top cross bars of the trailer. So, we had to cut each piece individually and hammer them in with a mallet. While Brian scored and broke off the foam, I taped all the metal flashing seams underneath the trailer and in the cavity with this awesome tape I got from Cody Farmer. Cody is a green builder and distributor near where I live. He has turned me on to some pretty effective products because he partners with 475 (https://foursevenfive.com) and caries their products locally. One of which is this durable Pro-Clima tape. Cody’s company MainStream Corporation (http://www.mainstreamcorporation.com) sells this tape that will adhere to basically anything while it seals holes and creates a full waterproof barrier. It was exactly what I needed to completely close the flashing overlaps and it went on really fast.
Once the taping was done we began pounding in the boards of insulation. One layer at a time. As the sun began to set 7 hours (If we hadn’t used the recycled foam, then the subfloor probably would have taken only about 4 hours total) later we had filled the whole cavity with the recycled foam and put sill seal foam onto all the steel beams. I know that there will still be temperature conductivity through the metal beams in the trailer and into my floor, but I wanted to create a small space for transition. We covered up the trailer and the next day put a couple hours in (with help from my Dad) putting house wrap over the insulation and under the OSB. We then screwed in the OSB directly into the steel beams. One screw about every 10″. The subfloor was complete.
One thing I really like about the tiny house project so far, is that each phase is manageable because it’s such a small space. Just as it becomes slightly unbearable, you are done. It should also be noted that once you think you have become a “master”, you have no more to accomplish in that phase of construction.