This paper investigates window systems with different heat transfer coefficients (U-factors) and Condensation Resistance (CR) ratings. The objective was to quantitatively analyze under which environmental conditions condensation would occur, to determine the extent of condensation over the interior window surfaces and investigate if CR, U-factors, and extent of condensation are correlated.

THERM software simulations were conducted to calculate the temperature distribution within the window details and the lowest temperatures along the interior edge of glass and interior edge(s) of the frame. Simulations evaluated 11 different window types of various performances, ranked by CR values from 16-72 and U-factors from 0.54 to 0.18 Btu/h-ft2-°F (3.06 to 1.04 W/m2K). Head, jamb, and sill details were evaluated at various interior and exterior environmental conditions leading to a total data set of almost 600 simulations.

Results indicate that condensation occurrence and extent is driven by the aggregate performance of all window components (frame, edge of glass, and center of glass) and their material properties, rather than U-factor or the CR value. Once a high-performing frame was introduced, the presence of a warm-edge IGU spacer compared with aluminum spacer significantly reduced condensation. Additionally, condensation occurred under improbable conditions (or did not occur) for windows where all components were high-performing. Moreover, some windows with similar U-factor and CR values performed drastically differently, exhibiting 80% difference in the extent of condensation under identical conditions. This is attributed to differences in the material properties of their individual components, specifically the spacer in the insulating glass units.