Condensation: Why fenestration component selection matters

Improving the center of glass (COG)

The COG performance is driven by the number, position, and type of low-emissivity (low-e) coatings, the size and number of cavities (double-pane versus triple-pane), and if inert gas filling is used.

A typical 25-mm (1-in.) dual-pane IGU with a double silver low-e coating on the inside of the exterior pane (surface #2) and an air-filled 13-mm (0.5-in.) cavity, has a COG U-factor of 1.65 W/m²K (0.29 BTU/F.hr.sf.) Adding argon typically reduces the COG U-factor to 1.42 W/m²K (0.25 BTU/F.hr.sf.) Changing the double-silver to a triple-silver low-e reduces the COG U-factor further to 1.36 W/m²K (0.24 BTU/F.hr.sf.)

Further reductions to 1.0 W/m²K (0.18 BTU/F.hr.sf) can be made by adding a second cavity to create a triple-pane IGU. Triple-pane performance can be enhanced yet further, achieving U-factors as low as 0.74 W/m²K (0.13 BTU/F.hr.sf) by adding a second low-e coating.

Adding a second low-e coating on the room-side surface of a dual-pane IGU—called a fourth surface low-e—provides an intermediate step in performance before moving to a triple-pane, achieving COG U-factors of 1.14 W/m²K (0.20 BTU/F.hr.sf). It does, however, lower the temperature of the room-side glass surface, increasing condensation risk. In this study, fourth surface low-e coatings were not evaluated. Condensation risk should be evaluated carefully, especially for high-humidity applications, when considering these coatings.

Key condensation questions

For the window systems chosen for the condensation analysis (each with different U-factors and CR ratings) the following questions were addressed:

• Under which exterior and interior environmental conditions does condensation occur? And to what extent? How much of the fenestration’s outer perimeter, extending from the outer edge of the frame to the gaskets to the glass surface, is predicted to be covered by moisture?
• On what part of the window does condensation develop first?
• Is there a correlation between the occurrence and extent of condensation, and the CR value?
• Is there a correlation between the occurrence and extent of condensation and the U-factor value?
• How do higher-performing components (warm-edge spacers, frames that integrate wide thermal barriers, improved COG thermal performance) impact the CR value, as well as improve resistance to condensation?