Water is bad news all around for a tiny house. For moisture problems to occur in houses, four criteria must be present: 1) liquid moisture (rain, condensate, plumbing leak, etc.); 2) a driving force or pressure (wind, gravity, capillary or surface tension); 3) a path (crack, hole, opening, etc.); and and 4)moisture-sensitive material. Wood is a very moisture-sensitive material, and to a certain degree, so is metal. Unless the tiny house is built of stone or concrete, water is a concern.
Movement by liquid. One advantage a tiny house on a trailer has is that it is off the ground by 18-24 inches. Luckily, water can’t jump that high. A solid roof, without complicated roof lines, will help greatly in keeping water from penetrating to the roof sheathing and will direct the water away from the top and upper sides. This is one argument in favor of a simple gable roof versus a shed roof.
At a minimum, a tiny house will be subject to sideways, driving wind, at 60 mph. If its raining during transport, that will amount to a lot more water and wind than many stationary houses ever experience. Something to chew on…
Movement by air flow. The air that can get into a tiny house through gaps and holes will also bring water that can condense and settle inside of walls and floors. Minimizing holes and filling the space around intrusions into walls, roofs, and floors is fairly easy to fix.
Movement by vapor pressure is nowhere near as big a problem as the other two forms of water movement. In fact, most construction details provide for at least some water movement through the walls, depending on the environment (temperature and humidity).
For the Tiny Dawg House, we choose to address water issues by: 1) eliminating any roof penetrations; 2) using closed cell spray foam in the floor for both insulation and water barrier; 3) POSSIBLY using closed spray foam in the walls and ceiling (this is not decided at this point); 4) though we went with a shed roof for ease of building and cost, we left some valuable width space for overhangs at both upper and lower ends of the roof; 5) incorporated two vent fans (composting toilet and kitchen) to remove unwanted fumes and water vapor; 6) wrapped the house in semi-porous water barrier that keeps out rain but allow water vapor to move through the wall. Of course, our house is also off the ground on a trailer so no seeping or leaking from the bottom side.