Glass Versus Window U-factors: The Interpretation Gap

A blog on usglassmag.com by Helen Sanders

I am often asked why glass U-factors don’t include the impact of the spacer. The difference between the center-of-glass U-factor and the full fenestration assembly U-factor is critical to understand, and mixing the two can lead to unintended consequences in building design and construction. In fact, using the center-of-glass (COG) U-factor instead of the fenestration assembly U-factor in building energy simulation can result in substantial underestimation of heating demand.

Let’s Start from the Basics:

• The U-factor of the full fenestration assembly is the area-weighted U-factor of the center of glass (COG), the edge of glass (EOG) and the frame. For the mathematically inclined, the formula is:

  

• Where:
• A_f = area of the frame
• U_f = U-factor of the frame
• A_eog = area of the EOG (this extends 2.5” from the inner edge of the frame into the glass area based on NFRC standards)
• U_eog = U-factor of the EOG
• A_cog = area of the COG
• U_cog = U-factor of the COG
• A_w = total area of the fenestration assembly
• U_(assembly) = U-factor of the fenestration assembly

• The COG U-factor is easy and simple to calculate using LBNL’s WINDOW software. Float glass manufacturers and glass fabricators often use this value to demonstrate their coatings’ performances and compare glass, cavity and coating configurations. COG U-factor is used ubiquitously but must not be confused with the assembly U-factor. This is because:
  • Building energy codes ONLY list the fenestration assembly U-factor for compliance, and they typically require certification of said performance through approved NFRC labels or label certificates.
  • Bad things happen when COG is used in place of assembly U-factor: The assembly U-factor is almost always higher (worse) than the COG U-factor. This is because the conductance across the COG is typically lower than that of the EOG and frame. So, using the COG U-factor instead of the assembly U-factor in energy simulations can result in overestimating energy efficiency and undersizing HVAC systems.
    For example, a perimeter zone energy model of a façade with a 70% window-to-wall ratio in the Minneapolis climate zone shows that using the COG U-factor (0.30 BTU/^ohr.ft²) of a fenestration assembly with a U-factor of 0.45 BTU/^of.hr.ft², underestimates the perimeter zone heating by 28% (see figure below).

• The COG U-factor does not include any influence from the EOG. The COG U-factor purely looks at the performance of the combination of the cavity dimension, glass coatings, glass type (tint, thickness), and gas fill of an insulating glass unit (IGU). There is no consideration of the impact of the insulating glass edge, and no consideration of the frame performance. In fact, it assumes that there is no edge seal.
  There is a minor dependence of the COG U-factor on the glass size used in the calculation (typically standardized at the appropriate NFRC size). This is because an increase in size can change the convective currents and is not because of the influence of the EOG.
• The LBNL WINDOW program also calculates the full assembly U-factor. LBNL’s THERM software calculates the more complicated heat flow across the frame and glass edge. This information is then fed into the WINDOW program, which is used to calculate the assembly U-factor based on the combination of EOG, frame and COG performance. This software combination requires a lot more effort and expertise to use than the COG module in WINDOW. The dual tool set is used by certified simulators to calculate the performance, which is then placed on the NFRC label/label certificate. This value is used for code compliance and for building energy simulation.

How, Then, is the Impact of EOG and Spacers Assessed?

Since the thermal performance of the spacer impacts only the assembly U-factor, the assembly U-factor is where we need to focus (not COG).

Warm-edge spacer, such as a plastic hybrid stainless steel box spacer, typically will result in a fenestration assembly U-factor of 0.02-0.03 BTU/of.hr.ft2, less than if using aluminum spacers in captured aluminum fenestration systems. The reduction can be larger, up to 0.05 BTU/^of.hr.ft², in structural glazing applications. This is because, in the absence of an exterior frame, the EOG becomes the main heat conduction path, and a warm-edge spacer can reduce heat flow much better than a highly conductive aluminum spacer. Stainless steel spacer, since the conductivity is higher than most other warm-edge spacers, typically results in a reduction of about 0.01 BTU/of.hr.ft2 compared to using an aluminum spacer.