Parametric Study of Wall Moisture Contents Using a Revised Variable Indoor Relative Humidity Version of the "MOIST" Transient Heat and Moisture Model.
Parametric Study of Wall Moisture Contents Using a
Revised Variable Indoor Relative Humidity Version of the
"MOIST" Transient Heat and Moisture Model.
Tsongas, G. A.; Burch, D. M.; Roos, C.; Cunningham, M.
U.S. Department of Energy (DOE); Oak Ridge National
Laboratory (ORNL); American Society of Heating,
Refrigerating and Air-Conditioning Engineers (ASHRAE);
Building Environment and Thermal Envelope Council
(BETEC). Thermal Performance of the Exterior Envelopes
of Buildings VI Conference. Proceedings. Thermal VI.
December 4-8, 1995, Clearwater Beach, FL, 307-319 pp,
heat transfer; moisture; humidity; walls; computer
The present 2.1 version of the "MOIST" software predicts
wall moisture contents and associated parameters using
an assumed indoor relative humidity input that is
constant for the duration of the simulation period. The
authors modified the model to calculate the hourly
indoor relative humidity during the heating season as a
function of outdoor weather conditions, indoor air
temperature, building size and airtightness, and indoor
moisture generation rate. These changes were
accomplished by incorporating within MOIST an indoor
moisture balance and a single-zone infiltration model.
The modified version of MOIST allows the summer indoor
relative humidity to either float to simulate open
windows/doors or to be fixed to simulate air
conditioning. The new version has the advantage of
incorporating many more inputs that influence the indoor
relative humidity and construction-layer moisture
content results. The development and details of the
revisions are described. This enhanced version of MOIST
was subsequently used to investigate moisture
accumulation in a 5-cm by 15-cm (2-in. by 6-in.)
wood-framed wall exposed to a number of different winter
climates. Predictions with a constant indoor relative
humidity were compared to those with a "floating" or
variable indoor relative humidity. The results
generally are different, with the results of the revised
version agreeing closely with field measurements. In
addition, the variable indoor relative humidity program
was used to analyze the effect of building airtightness,
the indoor moisture generation rate, and the existence
of exfiltration. The need for an interior vapor
retarder in walls exposed to cold climates also was
examined. Moreover, the effects of exterior insulating
sheathing and an exterior vapor retarder were modeled.
Results and findings are presented along with pertinent
conclusions regarding appropriate building construction
techniques in winter heating climates.