Mathematical Analysis of Practices to Control Moisture in the Roof Cavities of Manufactured Houses.
Mathematical Analysis of Practices to Control Moisture
in the Roof Cavities of Manufactured Houses.
(2660 K)
Burch, D. M.; Tsongas, G. A.; Walton, G. N.
NISTIR 5880; 60 p. September 1996.
Sponsor:
Department of Housing and Urban Development, Washington,
DC
Available from:
National Technical Information Service
Order number: PB97-106843
Keywords:
roofs; manufactured housing; moisture; air flow; attics;
guidelines and practices; HUD standards; mathematical
analysis; moisture analysis; moisture control; moisture
modeling; roof cavities; climate; ventilation; ceiling
vents
Abstract:
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.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899