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Computer Analysis of the Moisture Performance of Roof Constructions in the U.S. DOE Moisture Control Handbook.


pdf icon Computer Analysis of the Moisture Performance of Roof Constructions in the U.S. DOE Moisture Control Handbook. (3122 K)
Tsongas, G. A.; Thornton, B. A.; Burch, D. M.; Walton, G. N.

NISTIR 5919; 56 p. December 1996.

Sponsor:

Department of Energy, Washington, DC

Available from:

National Technical Information Service
Order number: PB97-132088

Keywords:

air flow; attics; building codes; building research; guidelines and practices; mathematical analysis; mathematical models; moisture; moisture analysis; moisture control; moisture modeling; mold; mildew; roofs; roof cavities; roof ventilation; attic ventilation; site-built housing

Abstract:

A new mathematical model, called the MOIST Attic Model, that predicts the transfer of heat and moisture in ventilated or unventilated roof cavities is presented. The model includes both molecular diffusion and capillary transfer within the materials and also calculates the indoor relative humidity, the ceiling air leakage rate, and the roof cavity ventilation rate at hourly time steps. This computer simulation model can be used to assess the moisture performance of open attics as well as cathedral ceilings. Typical applications include estimating the variation throughout a year of the moisture content of roof wood members, of roof cavity surface relative humidities, and of ceiling heat flux. In the present study, the model is used to predict the moisture performance of a current practice site-built prototype house with 15 different roof designs constructed in compliance with the "U.S. DOE Moisture Control Handbook" in cold (heating), mixed, and cooling (hot and humid) climates. These open attic or cathedral ceiling roof constructions were intended to be the best designs to minimize moisture accumulation, thereby preventing material degradation, mold and mildew growth, and loss in thermal performance. But prior to this study, their moisture performance had not been checked with a moisture model. Thus this computer simulation study of their performance was undertaken. For each of the 15 roof designs, attention was focused on the peak values of the plywood roof sheathing moisture content and the relative humidity at the bottom of the insulation adjacent to the various ceilings where mold and mildew might grow. Findings of the study regarding the moisture performance of the 15 designs, as well as roof design suggestions, are presented along with recommendations for further study.