Mathematical Analysis of Practices to Control Moisture in the Roof Cavities of Manufactured Houses, 1996

Authors: Burch, M. D.     Tsongas, G.A.     Walton, G.N.     National Institute of Standards and Technology    

A mathematical model is presented that predicts moisture and heat transfer in ventilated cavities such as attics, roof cavities, and cathedral ceilings. The model performs a transient moisture and heat balance as a function of time of year and includes the storage of moisture and heat at the construction layers. The model includes both molecular diffusion and capillary transfer within the materials. Radiation exchange among the ventilated cavity surfaces is predicted using a mean-radiant-temperature-network model. Latent heat (i.e., the effect of water evaporating from one place and condensing at another place) is distributed within the materials. Airflow from the house into the ventilated cavity is predicted using a stack effect model with aggregated effective leakage areas. Air exchange between the ventilated cavity and outdoor environment is predicted by a semi-empirical model. The relative humidity in the house is permitted to vary during the winter and is calculated from a moisture balance of the whole building.

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.

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