Delivering building performance: Fenestration education matters

Twinsburg, Ohio
energy modeling

Impact on building energy performance

Consider a prototypical building located in Minneapolis with a 16-foot-deep by 25-foot-wide by 10-foot-high perimeter zone with 70% window area. A building energy analysis  shows that the calculated perimeter zone energy use intensity (EUI) is 15% too low if a COG U-factor of 0.30 BTU/ is used rather than the full fenestration value of 0.45 BTU/ across all elevations (see graph).

The heating energy is underestimated by an even larger proportion (28%), assuming the same window area on all elevations, which could result in significant under-sizing of heating system capacity. These results translate to the total EUI of well daylit buildings, where the perimeter zone dominates the floor plate.

With large buildings, where the perimeter zone is smaller compared to the core, the overall impact on EUI will be reduced somewhat, but the potential for substantial discomfort for those occupants sitting near the fenestration remains.


Based on this analysis, using COG U-factor in the building energy modeling rather than the whole unit performance can result in significant gaps between as-built and as-designed energy and occupant performance if the error is never caught. This is a risk not just for the design team, but also for the glazing contractor, and for our industry in general. In the battle for the wall, it is in no one’s interest for windows to be blamed for uncomfortable or gas-guzzling buildings because of this preventable error.
Even if the disconnect is uncovered during the bidding and/or construction phase, significant budget, design and schedule problems can result. Higher performance fenestration would be required to meet the modeled energy performance, or alternatively substantial changes in the building design or performance targets would need to be made. This is also not a good outcome.


Firstly, as an industry we must continue to reinforce education to the architectural and engineering community regarding the use of whole unit U-factors. However, we must also provide the community the tools they need to (i) more easily derive the whole unit U-factors and (ii) specify the performance of the frame and edge of glass (e.g. a better definition of “warm-edge”), as well as is currently possible for the COG. This will help ensure that the window and IGU constructions installed are consistent with the overall window performance specified.


Figure caption:  The calculated underestimate of the energy use intensity and heating energy usage of a perimeter zone of a prototypical building with 70% window to wall ratio in Minneapolis when the center of glass U-factor (0.30 btu/ is used instead of the whole unit U-factor (0.45 btu/

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