You Get What You Measure: Implications for Window Performance

By Helen Sanders

As the common management theory goes, you can’t manage what you don’t measure. On a similar note, there is also a theory that “you get what you measure.” For example, if manufacturers focus on volume alone, that’s what they get. But at what cost? The unintended consequence might be a gain in output at the expense of product quality, or even safety.

Window Performance

Similar unintended consequences happen in the world of codes and standards. One example that has driven fenestration design, and caused a divergence between U.S. and European markets, is how the criteria for window performance are set. In the U.S., thermal performance requirements in codes and standards for fenestration systems specify overall fenestration U-factor. On the face of it, it seems to be a good measure; after all, the overall thermal transmittance of the system is what matters in the building. How-ever, since the system U-factor is an area weighted average of the frame, edge of glass (EOG) and center of glass (COG) U-factors, the COG U-factor can be used to drive overall performance. This can lead to an imbalance in the window performance.

A window with a non-thermally broken frame, aluminum spacer and triple-pane, low-E coated, insulating glass is a rather extreme example, but does exist in the National Fenestration Rating Council-approved product database. With a high performing COG and a poor performing perimeter, the window may achieve code compliance, but thermal bridging in the frame and EOG will cause poor condensation resistance and thermal discomfort for occupants. Without a requirement for condensation resistance, there is no counterbalance to these potential unintended consequences.

Overall Performance

In Europe, fenestration codes have limits on the U-factors of each of the fenestration zones: frame, COG and EOG. While this could mean a more balanced and higher performance window, there could be unintended consequences. Improvements are made to individual components without the perspective of their impact on the overall system performance.

The graph offers an example of the effect of different EOG solutions (see box). Significant performance increases can be achieved by moving from a 100-percent metal spacer solution. Hybrid plastic or non-metal systems all have about the same impact on overall U-factor: 0.02-0.03 btu/ Incremental improvements from this basis, while changing the EOG U-factor, don’t result in a significant change in overall system U-factor, and maintaining the durability of the edge seal becomes a primary consideration.

Fenestration should be designed from a balanced approach from holistic whole system performance. That involves U-factor and condensation resistance, as well as a component level evaluation to ensure thermal performance and durability across the EOG, COG and frame.

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