Back to Basics: Don’t Mistake Center of Glass and Complete Assembly

By Helen Sanders

One of the easiest mistakes to make is confusing center of glass (COG) thermal performance (U-factor) and solar heat gain coefficient (SHGC) with those of a complete fenestration assembly.

The assembly U-factor is the area-weighted average of the U-factors of the frame, edge of glass and COG. The assembly SHGC is calculated similarly.

COG U-factors for insulating glass units (IGUs) typically are significantly lower than those for a full fenestration assembly because the frame and edge of glass have higher conductivity. COG U-factors for 1-inch low-E IGUs usually range from 0.24 to 0.30 BTU/°f.hr.ft², depending on coating and gas fill. In comparison, U-factors of curtainwall assemblies containing such insulating glass may range from 0.46 BTU/°f.hr.ft² to 0.30 BTU/°f.hr.ft² depending on thermal barrier performance and IGU spacer used.

COG values are available more readily than assembly values because the COG performance of multiple configurations of glass tint, coating type, cavity size and gas fill can be easily and rapidly calculated using LBNL’s WINDOW software. The implications of mistakenly using COG glass performance instead of assembly values are significant in building energy performance modeling. This mistake may contribute to the “performance gap” widely reported between as-designed and as-built.

Illustrating the impact, a prototype building’s perimeter zone (16 feet deep x 25 feet long x 10 feet high) in Minneapolis (climate  zone 6) with 70% window area was simulated using the Department of Energy’s EnergyPlus building energy modeling software. The first simulation used a window assembly with a U-factor of 0.45 BTU/°f.hr.ft² and the second used its COG performance of 0.30 BTU/°f.hr.ft² to represent fenestration performance.

The results show that assuming the perimeter zone is the same on all four sides of the building, using COG U-factor causes the simulation to underestimate the perimeter zone energy use intensity (EUI) by 15% and the heating energy by 28%.

The whole building’s EUI is the sum of the core and perimeter zone EUIs. In buildings with large core zones, the total EUI over-estimate will not be as large as in skinnier buildings where the perimeter zone comprises much of the floor plate. No matter the building size, occupant comfort in the perimeter zone will likely be poor because the mechanical system will not have been sized appropriately.

The takeaway is that when receiving or providing a U-factor, it is critical to confirm whether it is for the COG or assembly.

Helen Sanders is the general manager at Technoform North America Inc. based in Twinsburg, Ohio. Read her blog each month at usglassmag.com/insights.

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