Building a foam and cloth monocoque shell on your trailer

So now we get to the meat of our build: how do we insulate and enclose our homebuilt boondocking trailer? One of the biggest issues with homebuilt dry camping trailers is weight. Most of us aren't welders or metalworkers, so steel and aluminum are not good options. Wood is not nearly as strong and is heavier. Weight determines what kinds of hitches and vehicles can pull your trailer build, and the weight you pile on in your build reduces what you can store inside your stealthly tiny house. In addition, you want to keep your tiny house trailer comfy, and that means insulation.

One simple solution is to go retro and use a monocoque shell on the trailer, and that's what we are doing here. Although insulating foam has very little structural integrity by itself, it is frequently used as the supporting structure for surfboards and model planes that are then covered in fiberglas resin, to build very strong structures. World war I and II aircraft like biplanes and spitfires used cloth and glue over plywood to form extremely light and strong monocoque airframes. This same lost technology is an inexpensive way to build a strong, light, insulating shell for our stealth tiny house.

 So here we have a fairly common 2" foam panel, which I've perforated with a wallpaper removal tool to give the glue a strong bonding surface.

Foam, glue, and a linen bed sheet. Nothing to see here folks. We'll just glue these together and after it dries, I'll beat on it with a sledgehammer....

So when it finally fails, what happens is the cloth and glue shear at the point of impact and the foam blows out under the load. What will happen when the foam is backed with plywood? It will be even stronger. Preparation of the foam surface is critical to getting good results, which is why I used a paper tiger-type perforator to open up the surface. Without good glue adhesion the foam would just pop away from the cloth on the first hit and both would fail immediately. The tiny holes give the glue tremendous gripping strength.

When you are doing entire 4x8 panels, you need a paint roller to manage the gluing process. We used Titebond II by the gallon, about 3 gallons total; I ordered military surplus oversized bed sheets, and we used 12 of these king-size-plus sheets. The linen is tucked behind the top of the panel and smoothed downwards onto the glue, using your shoe to provide tension on the bottom of the linen sheet while you smooth out bubbles and get the sheet glued down. This embeds the linen sheet into the glue and saturates the cloth. You may have to pull the cloth off in places and re-lay it to smooth out some of the bubbles that are trapped under the cloth.
Rotate the panel 180 degrees and glue down what was the bottom edge, and glue the remaining sides, and work out any bubbles you can see. A second layer of glue is then rolled over the linen and it is set aside to dry for a day. Be careful to use spacers between panes that are drying. Even panels that seem dry can glue themselves together if you aren't careful and separating them is a pain in the ass.
Once they were dry, we attached the plain-backed side to the plywood shell of our stealth trailer with liquid nails (heavy duty) and held it on with Lag screws passed through 2" washers. Lags that are removed have expanding foam put into the holes to further tie the panel together and waterproof the shell.

Any exposed edge will get a plastic drywall bullnose glued onto it before the final layer of linen is glued on, but it is remarkably tough and light all by itself.
It's important to note that in our build I used 2.5" lags liberally, and any lags driven into the plywood shell alone were removed after the Liquid Nails dried, and their holes were foamed. Lags into 2x4 lumber were driven until they dished the skin below flush, and thelag head and washer were foamed over. Points where panels met had Gorilla glue injected into the gap and then expanding foam over that to fill in the gaps to the surface. The dried foam was then trimmed flush when it dried so than the last layer of linen would lay smooth over it.

 The completed stealth trailer shell is painted with any heavy-duty high pigment paint; we use concrete paint with good results but pretty much anything waterproof will do. We will focus next on the composting toilet build and our thinking on how to integrate a toilet into a boondocking tiny house.



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