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Parametric Study of Wall Moisture Contents Using a Revised Variable Indoor Relative Humidity Version of the "MOIST" Transient Heat and Moisture Model.


pdf icon Parametric Study of Wall Moisture Contents Using a Revised Variable Indoor Relative Humidity Version of the "MOIST" Transient Heat and Moisture Model. (1242 K)
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, 1995.

Keywords:

heat transfer; moisture; humidity; walls; computer models; MOIST

Abstract:

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.