Computer Analysis of the Moisture Performance of Roof Constructions in the U.S. DOE Moisture Control Handbook.
Computer Analysis of the Moisture Performance of Roof
Constructions in the U.S. DOE Moisture Control Handbook.
Tsongas, G. A.; Thornton, B. A.; Burch, D. M.; Walton,
NISTIR 5919; 56 p. December 1996.
Sponsor:Department of Energy, Washington, DC
Available from: National Technical Information Service
Order number: PB97-132088
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
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.