As soon as the electrical wiring and plumbing was finished, it was time to add the exterior sheathing. Sheathing is simply a 4’x8’ piece of treated plywood screwed into the frame every 12” or so. It adds structural support to the frame to prevent shearing, and also provides a surface to attach the exterior siding.

In this case, since I’m using metal studs, it’s critical to use a layer of rigid foam board between the frame and exterior sheathing to provide a thermal barrier. Otherwise, the insulating R-value of my wall could be reduced by half! After the foam board and sheathing is applied, the wall cavity could be sprayed with a soy-based closed-cell expanding foam.

Dad
sheathing
The finished sheathing. You can see the backhoe we used as an elevated platform at the far right.

After gluing the rigid foam to the 4x8 plywood, screwing the sheathing in was pretty simple – I used some plymetal screws to fasten the 4x8 sheets to the metal studs. They have a little winged head that drills a slightly larger hole through the wood so it doesn’t ride up on you when you begin drilling into the metal. It took anywhere from 5 seconds to a minute to put a screw in, depending on how much force I was able to apply in the physical position I was in.

Rear right sheathing

Since the design called for screws every 6” on the edges and every 12” elsewhere, that tallied up to over 1000 screws! Needless to say, it took over a week of two people working full-time to get it all installed.

Temp support

If your studs aren’t perfectly aligned to 48” boundaries, or if you’re working alone, it can be quite frustrating to get the 4x8 sheets fastened at precisely the right spot. Any gaps would ooze out spray foam and be susceptible to water penetration, so I wanted to get it as tight as possible. I used a few different techniques to position the sheathing before fastening it in place.

front right
sheathing
A hydraulic jack is a precise instrument for controlling the height of sheathing.
temp support

With all those long screws going through the stud, I had to be careful not to penetrate a water line or electrical wire. In contrast, with wood studs you can use nail plates to prevent this much more easily. I also found out if you left foil-faced rigid foam stacked together for too long with moisture, the foil begins to decay!

water inlet

At the corners, I left it at a simple overlap since these will be covered up later with 2x4 cedar trim.

slight warp
The gap was fixed once the screws were in.

Underneath the trailer, I attached four 4x8 sheets of 1/8” aluminum to prevent road debris from ending up in my living room. These also had to be layered with rigid foam – otherwise the bottom of my house would be one giant heatsink.

underbelly shielding
Six hours on my back under the trailer, brushing metal shavings out of my face and hair was a thrilling adventure.

Once the “walls” were up, it really started to feel like a house. Now I had to tow it to get insulated.

Kitchen
sprayfoam

It only took an hour or so with two guys working at it – one spraying and the other cleaning up the excess.

Wall
sprayfoam

I was lucky to find a variant of spray foam made from Soy oil, so it’s non-toxic (but still sticky as hell). Within 3 seconds of being applied, it reaches 95% of its structural rigidity. And this stuff is rock-solid! If I’d known how hard it becomes, I may have designed the frame to be a little lighter.

With
trailer

While I had my house out on the road, I decided to make a quick stop at the scale. With sheathing, insulation, and framing done, I weighed in at about 5,000 lbs! Heavy, but not too bad considering my 14,000 lb limit.

Next is building the front tongue shed!

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