Delivering building performance: Fenestration education matters
by Helen Sanders, Ph.D
Confusion between 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 oftentimes, are used instead of overall window performance in building energy modeling. In some cases, the COG U-factor is even thought by some 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/oF.hr.ft2, but can have a 33% lower COG U-factor of 0.30 (air filled, dual pane, low-e2 coated IGU).
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/oF.hr.ft2 is used rather than the full fenestration value of 0.45 BTU/oF.hr.ft2 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/oF.hr.ft2) is used instead of the whole unit U-factor (0.45 btu/oF.hr.ft2).