“Condensation Study of Windows: Comparative Analysis of Different Window Systems under Various Exterior Conditions” discusses a simulation study of different window systems, where heat transfer simulations were performed to investigate thermal performance and the potential for condensation. Particularly, window systems with different heat transfer coefficients (U-factors) and Condensation Resistance (CR) ratings were investigated. The main objective of the study was to quantitatively determine the environmental conditions under which condensation in various window systems would occur and to predict the extent of condensation along the interior surface of investigated windows.
Using THERM software, simulations were conducted to calculate the temperature distribution within the window details and to determine lowest temperatures along the interior surfaces of windows. Simulations evaluated 11 different window systems of various performances, ranked by CR values from 16-72 and by U-factors from 0.54 to 0.18 Btu/h-ft2-°F (3.1 to 1.0 W/m2K). Head, jamb, and sill details were evaluated for three interior dry bulb temperatures and relative humidity (RH) conditions (70oF/21oC and 30% RH, 70oF/21oC and 50% RH, and 70oF/21oC and 60% RH) and six exterior dry bulb temperatures (30oF/-1oC, 20oF/-7oC, 10oF/-12oC, 0oF/-18oC, -10oF/-23oC, and -20oF/-29oC) under a constant exterior relative humidity (50% RH) and exterior wind speed of 12.3mph/ 5.5m/s. This resulted in the total data set of almost 600 simulations, which were analyzed and evaluated.
Results indicate that the conditions under which condensation initiates, and the extents of condensation on window surfaces are driven by the individualized performance of each window component (the frame, the edge of glass – including the spacer, and the center of glass) and their material properties, rather than solely the U-factor or the CR value. For window systems with low-performing frames (non-thermally broken or minimally thermally broken), condensation began to occur along the interior edges of the frame. For window systems with higher performing frames with wider thermal breaks, condensation always occurred along the interior edge of glass, pointing to the edge of glass and its spacer as the weakest link. The presence of a warm-edge spacer, compared to aluminum spacer, reduced condensation potential. Additionally, for window systems with all high-performing frame, edge of glass, and center of glass, condensation occurred under very extreme and unlikely conditions (or did not occur at all). Surprisingly, window systems with similar CR values performed drastically differently from each other, in terms of the extent of condensation under identical environmental conditions. This is attributed to differences in the material properties of their individual components and identifies a major short coming with the CR rating.
Citation:
Aksamija, A., Milosevic, S., Sanders, H., and Blakeslee, A., (2022). “Condensation Study of Windows: Comparative Analysis of Different Window Systems under Various Exterior Conditions”, Proceedings of the Facade World Congress 2022, Los Angeles, CA, October 12-13.