Mathematical Analysis of Practices to Control Moisture in the Roof Cavities of Manufactured Houses, 1996
This mathematical model was used to simulate the performance of a double-wide manufactured house constructed in compliance with the latest HUD Standards. An interior vapor retarder was installed in the ceiling construction and ventilation openings were installed in the roof cavity consistent with the 1/300 rule given in the HUD Standards. The effect of passive and mechanical ventilation, as well as a wide range of other factors on the roof sheathing moisture content was investigated as a function of time. The weekly average moisture content of the lower surface of the plywood sheathing was analyzed in several cold climates, while the relative humidity at the lower surface of the ceiling insulation was analyzed in a hot and humid climate.
The analysis revealed the following: 1) airflow from the house into the roof cavity, as opposed to water-vapor diffusion, was the dominant moisture transport mechanism into the roof cavity; 2) high roof sheathing moisture content occurred in houses having high indoor relative humidity (i.e., high moisture production rate, or tight construction, or both); 3) passive roof cavity vents consistent with the 1/300 rule were found to maintain the roof sheathing moisture content in non-humidified houses below fiber saturation during the winter; 4) the mechanical roof cavity ventilation rate specified in the HUD Standards for removing moisture during the winter was found to be too small and thus needs to be revised; 5) the presence of a ceiling vapor retarder was found to provide very small reductions in roof sheathing moisture content; 6) when an interior vapor retarder was installed in the ceiling construction of an air-conditioned house exposed to a hot and humid climate, the relative humidity at its upper surface rose above 80%, thereby providing a conducive environment for mold and mildew growth; and 7) the use of ceiling vents to provide additional whole house ventilation in cold climates substantially increased the roof sheathing moisture content of a house with an unventilated attic. Recommendations for further study are presented.