One reason people build a tiny house subfloor on top their trailer is to have a complete thermal barrier from floor to wall. We bought a tiny house trailer that would allow us to sink our subfloor within the frame to have extra head (loft) space. While building a thermal barrier for these kind of trailers are very customized, you’ll see building a bulkhead is really not all that difficult, and can be built with minimal lumber and foam insulation. Wheel wells can also be insulated.
Your frame that will hold a bunch of insulated foam board and spray foam consists of pressure treated 2x4s.
Here is the schematic of what we’re trying to achieve. Above the frame we have 3 inches in foam blocked out, with another 1 inch of foam insulation that sits perfectly underneath. This is all covered up at the bottom with pressure-treated sheathing. Any gaps, such as where support braces are, we use open-cell spray foam insulation (like the kind used for windows and doors). Many cases you’ll want open-cell over closed-cell to allow natural expansion/contraction, and a way for any potential water/moisture to escape. More on spray foam differences down the road!
The frame is held into place with wood-to-metal self-tapping screws. We made it to be flush with the angle iron. We welded some additional support brackets underneath to support the weight of the walls (we suggest you do that too- do you really want that angle iron with a few spot welds to be the only thing holding up your walls?). Because of this, we had to customize the fit somewhat. Tedious but worth it!
Your trailer may be set up differently, but parts of our overhang had areas where we could put additional insulation above the frame.
When we placed the first layer of 1 in foam, we allotted room for the support braces, and made sure to place foam behind the support braces too. Foam board of these small sizes you can just score/cut with a utility knife. The supports are screwed in with typical decking screws.
Add your other foam board layers to a tight fit; you may need a block and hammer to tap it in without breaking the foam. Then add within the bottom of the frame another 1 inch foam board that will be covered up.
Final step was to nail gun pressure-treated sheathing to the bottom. Leave room for any wiring!
Our completed thermal bulkhead! Now we have continuous insulation across the floor, removing the disadvantage of a subfloor within trailer frame.
Whenever you get to placing outside sheathing, you will want to cover up the thermal bulkhead. In our case we have thermal structure sheathing. So we dropped down all our sheathing to be almost flush with the bulkhead. The sheathing is about 0.25″ higher than the pressure-treated sheathing on the bottom of the bulkhead to further prevent water from running into that cavity. Let us know if you end up trying a version of our thermal bulkhead!