By Alexandra Blakeslee and Helen Sanders, PhD
Condensation is an important, but sometimes an overlooked failure mode when specifying glazing. Condensation can be an issue in any climate or building where the interior surface falls below the dew point. This is especially important in areas where outdoor temperatures are low, such as in Northern climates in winter months, and where there are high levels of humidity, such as hospitals, laboratories, museums, and multifamily housing.
The condensation resistance (CR) rating, developed by the National Fenestration Rating Council (NFRC) has long been used as a metric to assess condensation risk for fenestration. Theoretically, as fenestration thermal transmittance (U-factor) reduces, the improved thermal performance should help maintain higher interior surface temperatures above the dew point, preventing condensation.
However, it is well established that U-factor and CR do not perfectly correlate. This is because condensation is typically driven by thermal bridging at the fenestration perimeter—the frame and edge of glass (EOG). The fenestration U-factor, being the area-weighted average of frame, EOG, and center of glass (COG) U-factors, can mask perimeter thermal bridging using high thermal performance in the COG.
In addition, CR values do not predict how much or where condensation could potentially occur for a window assembly under specific environmental conditions. The CR value, which is a number from one to 100, provides only a qualitative way of comparing different fenestration systems and their relative risk of condensation. This can be a challenge for the specifier who needs to better quantify condensation performance.
In 2022, the authors collaborated with researchers at the University of Massachusetts, Ajla Aksamija, PhD, and Suncica Milosevic, to quantify the potential for condensation on a variety of window systems.1 Eleven different aluminum architectural window systems spanning a wide range of heat transfer coefficients (U-factors) and CR ratings were investigated. The research results and implications for fenestration specification are summarized herein.
Strategies for improving fenestration thermal performance
Before diving into the drivers of condensation, it is important to understand the ways in which fenestration thermal performance can be improved. Many of these strategies are used to create the range of window systems used in the condensation evaluation.
