We had a slight set back before we could really get going on the build, but we have made a lot of progress since my last update!
Just a couple weeks after our trailer arrived, we noticed that there was a significant amount of rust developing on the bottom of some of the pieces:
Needless to say, this was pretty disappointing… we live fairly near the coast, and things rust here quickly, but this was pretty ridiculous. I had to spend several days under the trailer removing rust (with a phosphoric acid-based cleaner). I then sprayed a cold galvanizing spray on the metal (which seemed to have escaped being painted properly) to protect it. Then I learned that only latex based paints can be put over galvanized metal, so I had to do a brushed-on latex primer, followed by a rust resistant black enamel spray.
This was a lot of work that I didn’t anticipate, and I got way too familiar with the view from underneath the trailer (including helpful doggie):
I talked to Tiny Home Builders, who we ordered our trailer from, about this problem… and they were very responsive and apologetic. They gave us a free lifetime membership to their video series on tiny house building (I’ve just a watched a couple so far, but it seems pretty good), and they agreed to pay for the rust removing and painting supplies (unfortunately, I didn’t ask for quite enough; I didn’t realize how many cans of spray paint it would take!). They also said they’ve talked to manufacturer and will be ramping up their quality control measures.
Once that was taken care of, we started on the floor framing. I really didn’t want to do any wood framing, or have a floor frame at all… a lot of people end up putting the insulation between the joists & decking of the trailer, saving a few inches. However, the more I learned about thermal bridging, the worse of a solution this seemed. If you don’t have an insulating layer outside of steel framing members (including steel trailer beams), heat transfer can be really significant, reducing the R-value of your insulation between members by more than 50% . There’s no real way to isolate the steel trailer from the inside of your house without doing some kind of framing and insulation above the deck — at least nothing reasonable I could come up with — so I succumbed to the idea of a wooden floor frame (thermal bridging exists with wood, but is much less severe).
We’re using 2×3 members for the joists, saving an inch over regular 2x4s. They seem to be plenty stiff enough @ 16″ on-center, and we’ll be putting 2 1/4″ of polyiso insulation between them, giving us a floor R-value of 14.4. We built three main pieces – one on each side of the wheel wells, and one in the center:
The extra piece that sticks out a few inches near the wheel well will support our front door — since we’re doing thick continuous insulation outside the framing, and we’ll have an out-swing door, this will allow the door to be flush with the siding.
I’ve got to say, working with wood is a little bit of a PITA — a lot of the pieces were twisted, not straight or level, etc… I’m hoping that the steel studs will be easier to deal with in this respect.
We’re almost finished with the aluminum flashing and attaching the frame permanently to the trailer — I’ll have more updates on that later this week!
 IMPROVING ENERGY PERFORMANCE OF STEEL STUD WALLS, ORNL