Back to basics: managing fenestration condensation risk
In the last article, we dove into the basics of condensation. Continuing this theme, we now examine the influences of fenestration design on condensation risk.
Condensation occurs on fenestration when the temperature of any of its surfaces is lower than the dew point of the nearby air. A window’s surface temperature is not uniform, with some areas colder than others based on how large the conduction path is from inside to outside elements.
Condensation Risk
The National Fenestration Rating Council’s (NFRC) Condensation Resistance (CR) and Condensation Index (CI) and the Fenestration and Glazing Industry Alliance’s Condensation Resistance Factor (CRF) are rating systems that help assess a fenestration system’s condensation risk. These systems differ from each other such that comparisons between fenestration can only be made within each rating system and not across systems. The higher the number in each system, the greater the condensation resistance.
The appearance and amount of condensation are determined by the weakest thermal link(s) in the fenestration system: The path(s) of least resistance to heat flow. These paths determine where the fenestration surface will be the coldest and how cold it will be.
In dual-pane glazed aluminum fenestration, there is typically more thermal conduction across the frame and edge of glass (EOG) than the center of glass (COG). This is especially true when the frame does not have a wide thermal barrier or a highly conducting aluminum spacer is used. The frame and EOG exterior surfaces therefore tend to be colder than COG and are where condensation first forms.
Improving the COG U-factor by adding argon, better low-E coatings, or transitioning to a triple pane does not reduce the condensation risk if the EOG and frame have lower thermal performance (see figure above).
Decreasing the COG U-factor would reduce the fenestration assembly U-factor, but in many cases, it has little impact on condensation occurrence. This is why the fenestration U-factor is not well correlated with its condensation risk (see figure above).
Reducing Condensation
The optimum design strategy to reduce condensation is:
• First, improve the frame by adding wide, complex thermal barriers.
• Then, improve the EOG using a warm-edge spacer.
COG improvements can then be used to reduce condensation risk further.