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Development of a Humid Climate Definition
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The role of humidity in indoor air quality has become of increasing concern in recent years. High indoor humidities can result in microbial growth on building surfaces, resulting in poor indoor air quality, as well as damage to the building and its contents. In addition to the IAQ impacts, high indoor humidity can cause occupant discomfort. The public review draft of ASHRAE Standard 62-1989R included requirements for installation of dehumidification controls in buildings with mechanical cooling located in humid climates. The draft standard included a definition of humid climate: where, during the warmest six consecutive months of a typical year, the wetbulb temperature is 19°C (67°F) or higher for 3500 hours or more, or 23°C (73°F) or higher for 1750 hours or more. This definition is that used in the 1993 ASHRAE Handbook of Fundamentals to define the humid climate region. The only areas in the continental United States which meet these criteria are close to the Gulf coast, all of Florida, and along the Atlantic coast as far north as southern North Carolina While it is clear that buildings in this humid climate region need to be carefully designed with regard to humidity control, it is also clear that buildings in other areas have an equal need for humidity control. The work described in this paper examines a number of potential indicators of "humid climate" and correlates them with the prevalence of indoor humidity problems in three building types. The FSEC 2.3 energy simulation computer program (Kerestecioglu et al. 1989) was used to simulate the three building types, using weather from 10 cities in the southeastern U.S. The FSEC software was selected because it is capable of accurately modeling moisture transfer within the building space and the dehumidification performance of cooling coils at part-load conditions, and predicting resulting humidity levels. The buildings modeled were a retail store (similar to a K-Mart or Wal-Mart), a large office building, and a fast food restaurant. Existing building models were employed for this study with ventilation rates in accordance with ASHRAE Standard 62-1989. The HVAC systems used were typical for these building types, without any special humidity control measures. The selected indicators of humidity problems are the number of hours per year with space humidity above 60% RH and the number of occupied hours with space humidity above 60% RH. TMY2 weather data (NREL 1995) for 10 cities was used for the annual building energy simulations. TMY2 data was also used to calculate a number of potential humid climate parameters for the same 10 cities. These included: the number of hours and the wetbulb-degree hours above 3 different wetbulb temperatures, the number of hours and grain-hours above 4 different humidity ratios, and the sensible, latent and total Ventilation Load Index (VLI). The VLI is the load (latent, sensible or total) generated by bringing one cfm of outdoor air to space neutral conditions over the course of one year (Hamman, et al. 1997). The ability of each climate parameter to predict indoor humidity problems was analyzed and compared. Implications of using the selected parameters to define a humid climate will be discussed
Hedrick, R. L.; Shirey, D. B. (1998). Development of a Humid Climate Definition. Energy Systems Laboratory (http://esl.tamu.edu); Texas A&M University (http://www.tamu.edu). Available electronically from