Last December, a young couple contacted us for assistance with their three-year-old tiny house in Burlington, Vt. Mushroom blooms had appeared at the base of the home’s interior walls, which led to their discovering that the wood-framed deck built on the lower level of the home’s metal trailer had rotted. When we arrived on site, we probed the single-part spray foam at the bottom of the wall near the kitchen cabinets and were able to dig out sopping wet Rockwool batt insulation (installed on the steel deck of the trailer between 2-by sleepers) and open-cell foam in the wall bays, as well as rotted exterior OSB sheathing.

The cozy structure was part of an HGTV program presenting an idyllic, storybook look at the tiny house movement, but the ending proved otherwise for the young homeowners. We felt bad for them. They were a cute couple, recently married, and their first investment in a new home turned out poorly (they purchased the structure from a Connecticut builder for $70,000, arranged by an HGTV freelance production company). While living “small” with an active outdoor-oriented lifestyle is admirable, the realities of marrying a small wooden structure to a steel trailer without considering basic physics is fraught with danger. Building science principles must be respected whether you’re building a mobile home or a traditional structure—or a tiny house in which a young couple will be living with two cats and a dog.

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