DE-93-16-1 -- A Computer Analysis of Moisture Accumulation in the Walls of Manufactured Housing PDF

A detailed computer analysis was conducted to investigate the effectiveness of three alternative practices for controlling moisture accumulation in the walls of manufactured housing during the winter. The three practices included (1) providing an interior vapour retarder, (2) using permeable sheathing and siding, and (3) providing an outdoor ventilated cavity. The current HUD Manufactured Home Construction and Safety Standards do not require a vapour retarder for practices 2 and 3. The analysis was carried out for cold winter climate (Madison, WI), an intermediate winter climate (Boston, MA), a mild winter climate (Atlanta, GA), and a Pacific Northwest climate (Portland, OR). The practice of providing a vapour retarder was found to be effective in all four climates. The moisture content of the siding was always considerably below fibre saturation. On the other hand, the practice of using permeable sheathing and siding and the practice of providing an outdoor ventilated cavity were not always effective in colder climates. Moisture accumulated above fibre saturation, and free liquid water existed within the pore structure, providing a potential for material degradation. A detailed computer analysis was also conducted of moisture accumulation in manufactured housing walls in a hot and humid climate (Lake Charles, LA). During the summer, moisture from the outdoor environment is transferred into manufactured housing by diffusion and, more important, infiltration. As a result, moisture accumulates at interior layers of the construction cooled by air conditioning. When an interior vapour retarder is used in the construction, the relative humidity at the outside surface of the vapour retarder can approach saturation, thereby providing an environment conducive to mould and mildew growth.

KEYWORDS: computers, calculating, moisture, walls, housing, prefabricated construction, winter, vapour barriers, content, relative humidity, moulds.