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Condensation resistance of fenestration systems (windows and doors) is determined from the temperature distribution on indoor fenestration surfaces and the indoor air dew-point temperature. For many years, condensation resistance was determined from hot box measurements of indoor fenestration surface temperatures, but more recently, computer modeling of the temperature distributions has emerged and is currently being validated. Depending on the method used, the temperature considered is either the calculated coldest indoor fenestration surface temperature or a measured average temperature for several predetermined indoor fenestration surface locations. It is known from experience with U-factor determination that computer models lower the overall cost of determining thermal indices (e.g., U-factor, solar heat gain, condensation resistance) and provide more consistent results. Computer models capable of predicting temperature distributions on fenestration surfaces are more complex than models used only for the calculation of U-factors, which is the main reason that they did not appear until recently. An ASHRAE research project was initiated several years ago to validate existing computer models, and the first phase of the project included seven different insulated glazing units (IGU) for which computer modeling (using standardized overall surface heat transfer coefficients) and infrared thermography measurements were performed. The results of the research project indicated that computer models can offer a viable alternative to actual physical measurements. This paper shows that further improvements in calculated indoor surface temperatures can be achieved if more realistic boundary conditions are used. Since IGUs are flat products, no significant improvement is achieved by including more accurate radiation models; however, for fenestration products with frames, improved radiation models are useful. The inclusion of local convective heat transfer coefficients, which are geometry as well as temperature dependent, provide more accurate boundary conditions for both IGUs and fenestration products. Local convective surface heat transfer coefficients were obtained from detailed numerical calculations and from detailed measurements. The agreement between measured and calculated results showed significant improvement over previous studies, indicating that future condensation resistance models need to incorporate improved boundary conditions. This can be done for many fenestration system geometries; however, for more complex geometries, additional detailed numerical computational fluid dynamic/heat transfer research efforts are required.

AUTHOR: Dragan Curcija, Ph.D., Yie Zhao, Ph.D., William P. Goss, Ph.D., P.E.
CITATION: Thermal Performance of the Exterior Envelopes of Buildings VII
KEYWORDS: December, Florida, 1998
YEAR: 1998