Designing for Better Buildings: Education Ensures High Performance Systems

By Helen Sanders

Confusion among the center of glass (COG) performance, thermal performance (U-factor) and whole fenestration unit performance is, unfortunately, all too common in the architectural design and energy modeling communities. COG values are so easy to calculate and obtain from glass manufacturers and are often used instead of overall window performance in building energy modeling. In some cases, the COG U-factor is even thought to be “close enough” to the whole unit U-factor to be a reasonable approximation in building energy modeling. But this is not at all the case and bad outcomes can arise if this mistake goes undetected.

Because the edge of glass and frame generally have higher thermal conductance than the center of an insulating glass unit (IGU), the fenestration edges increase the overall U-factor of the window system. Sometimes, this increase can be large, especially if the frame and edge of the glass are not well thermally broken.

For example, even a typical thermally broken fenestration system may have an overall U-factor of say, 0.45 BTU/°², but can have a 33% lower COG U-factor of 0.30 (air filled, dual pane, low-E² coated IGU).


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 analysis¹ shows that the calculated perimeter zone energy use intensity (EUI) is 15% too low if a COG U-factor of 0.30 BTU/°² is used rather than the full fenestration value of 0.45 BTU/°² across all elevations.

The heating energy is underestimated by an even larger proportion (28%), assuming the same window area on all elevations. This 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. 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.

Even if the disconnect is uncovered during the bidding and/or construction phase, significant budget, design and schedule problems can result.


As an industry we must continue to reinforce education to the architectural and engineering community regarding the use of whole unit U-factors. We must also provide the tools the community needs 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.

1. EnergyPlus from the U.S. Department of Energy

Helen Sanders is in strategic business development for Technoform North America Inc. in Twinsburg, Ohio. Read her blog each month at

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