Mobile and manufactured homes have some oddities of construction compared to stick-built homes. One of those, that is often misunderstood, is the issue of the "vapor barrier." Under a conventional home, everybody knows what we are talking about when we discuss the crawl space vapor barrier -- the plastic sheet over the soil.

At a manufactured home, the structure the units rest on can vary from concrete blocks or steel posts on soil with no footings, to blocks on gravel with or without footings, to a concrete slab, to ribbons (concrete strips) that serve as footings for blocks or posts. There should be a bottom board up under the floor of the home. Now, this goes by various names. I have talked to mobile home professionals about this terminology and here are some of the words they use for this barrier: Road barrier, Rodent Barrier, Vapor Barrier, Bottom board. Often, due to past repairs, this undercarriage barrier is cut, not properly repaired, so hanging down.

A misconception with some people is that this bottom board replaces the need for a traditional vapor barrier on the earth. In my experience, usually that is a dangerous assumption for the longterm condition of the home. Now, if the home is up on a neatly poured slab, and that is on a plastic sheet, I think they are okay. But with the other configurations, there is exposed soil. That exposed soil allows moisture to evaporate up. Their thought is "so what" it will be caught by the bottom board and the frame is steel. But think about it, steel rusts and over time rust can put holes in metal or turn it to brown powder. A main cause of high relative humidity in a home is a wet crawl space. Some photos, to illustrate my points, are below.

No conventional vapor barrier on the soil at this mobile

Condition of bottom board

Moisture on the steel frame and heavy rust

Thanks for looking.

Steven L. Smith

Bellingham WA Home Inspector