The only other special building consideration, after the foundation and bracing, for a little house on wheels is condensation. Unless they are insulated, sealed, and vented properly, small spaces are prone to a lot of condensation. It simply takes less time to fill the air in a small enclosure with the moisture caused by bathing, breathing, laundry, and cooking than it does to fill a large one. If that warm, moist air comes into contact with a sufficiently cold surface, it will condense into water. That is the reason that cars come equipped with defrosters, and that small houses need to be equipped with the right insulation, vapor retarders, and ventilation.
I used expanded polystyrene foam board as insulation with expanding spray foam in the seams for two basic reasons: 1) It takes a thicker piece of fiberglass batting to get the same amount of insulating power as you get out of a piece of extruded polystyrene. As I didn’t have enough space for eight-inch — thick walls, this would have stood as reason enough for my choice. 2) Foam board is far more resistant to condensation.
With fiberglass batting and other porous insulations, you have to worry about moist air getting into it and condensing when the moisture gets to the cold part of the wall. At that point, the fluffy, pink stuff turns to mush, and mush doesn’t insulate. It rots. To prevent this, you have to use a vapor retarder. This is usually just a large sheet of six-millimeter plastic hung over the inside
surface of the batting and sealed at its edges. If your seals hold and your plastic does not rip, your fiberglass should stay fairly dry.
Expanded polystyrene with an impermeable coating does not need a vapor retarder. Being virtually waterproof makes it its own retarder. I chose the white, expanded polystyrene over the pink, extruded poly because, while I love the pink stuff for its superior insulating qualities, bugs love it, too.
The threat of condensation is also what prompted me to use double-glazed, insulated windows. The glass panes on a little abode can fog up pretty quickly unless they are well protected against the cold. I’ve found that windows sold with gas between the interior and exterior panes work pretty well for this purpose.
The other primary way to eliminate condensation in a small enclosure is by venting it. I installed a fan at the peak of my loft. It sucks moisture-laden air out of my living quarters when I am cooking or bathing and helps keep the place cool during the summer. On cold days, the vent can be sealed with a plug I cut from some leftover scraps of foam board.
