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Translating across the Transatlantic: Do North American and European Windows Speak the Same Language?

June blog fig 1

A blog on usglassmag.com by Helen Sanders

 

Last month, I reviewed the important differences between U-factors related to windows: The entire fenestration assembly, the frame, the edge of glass (EOG), and the center of glass (COG). This month, I examine how the COG U-factor varies with cavity dimension and gas-fill type and why the optimum cavity dimension is different in Europe than in North America. The analysis also clearly shows why European and North American U-factors are not comparable–and the risk of inadvertently confusing the two.

The Impact of Cavity Dimension

The cavity dimension is a key determinant of the COG U-factor. The plot of U-factor as a function of cavity width shows that the U-factor starts high for narrow cavities (see Figure 1). This is because conductive heat flow is the dominating heat transfer mechanism. As the cavity widens, conduction of heat across the cavity reduces, but convective currents can start to build and cause increased heat flow through that mechanism.

As a result, there is a “minimum” U-factor which is achieved at a cavity dimension where the combination of conduction and convection is minimized. As the cavity is widened past that minimum point, the U-factor starts to increase because convection starts to increase more than conduction decreases.

The optimum dimension depends on:

  • Gas fill type (e.g. air, argon, krypton);
  • The environmental conditions (e.g., outside temperature, wind speed);
  • The standard used to calculate the U-factor (e.g., NFRC 100 or the European standard).

For North American standard climate conditions (e.g., -18 degrees Celsius, 0 degrees Fahrenheit, wind speed 5.5 meters per second) and NFRC standard calculation methods (NFRC 100), the optimum cavity dimension comes out around 12 to 13 millimeters (mm) for air-filled (0.50 inches) and 11 to 12 mm (0.46 inches) for argon-filled cavities. For krypton, the optimum is closer to 8 mm, which is why it is typically used for insulating glass where thin cavities are needed. The conductivity of the gas is what drives these differences: The conductivity of argon is lower than that of air, and that of krypton is lower still.

FIGURE 1: How center-of-glass U-factor changes with cavity dimension for a dual-pane IGU with a double-silver low-E on surface No. 2 and three different gas fill types. Credit: Technoform.

The minimum in the graph of U-factor against cavity dimension tends to be quite shallow, so the optimum cavity can be widened from 12-13 mm to 15-16 mm without a huge U-factor penalty. This can be seen in Figure 1. Wider cavities are typically desired for acoustical performance, which must be balanced with thermal performance.

This trend in the COG U-factor with cavity dimension translates to a similar trend in the whole assembly U-factor, but the impact on the assembly is reduced by the area-weighted averaging of frame, EOG and COG.

Europe Versus North America

In Europe, the outside standard temperature conditions used in the ISO standard U-factor calculations (EN673 / ISO 10077) are less severe than those used in the U.S., and the calculation standards use different environmental conditions (typically 0 degrees Celsius, 32 degrees Fahrenheit without wind). As a result, the optimum cavity dimension in Europe is closer to 16 mm. This is why windows in Europe tend to be designed around a 16 mm (0.63 inches) cavity, and in North America, they are designed around a 12 to 13 mm (0.5 inches) cavity.

Some practitioners argue that the standard exterior conditions used here in North America are too extreme, are biased to colder climate zones, and are not representative of many of the climates in North America. It is worth noting that the NFRC rating system has been designed to provide comparability between fenestration assemblies, not to demonstrate climate-specific performance. Professionals can carry out climate-specific calculations in the standard simulation software (WINDOW) by changing the environmental conditions if they wish, but those calculations cannot be used for code compliance or for NFRC certification and labeling.

The graph in Figure 2 shows the differences between using the North American (NFRC) and European methods to calculate the COG U-factor. Much of the difference (but not all) is caused by the different environmental conditions. Differences in film coefficients and other parameters also impact the results. An interesting analysis on the impact of exterior temperatures on U-factors calculated with both the NFRC and EN/ISO standards can be seen here.

FIGURE 2: How the North American, NFRC, calculation method for U-factor differs from the European (EN/ISO) method. The IGU design used in the calculations comprises two 6 mm lites with a double-silver low-E on surface No. 2. Credit: Technoform.

European U-factors are Lower Than NFRC U-Factors

It is worth noting that, in addition to a shift in optimum cavity dimension, U-factors (whether COG or full assembly) calculated using the European method are systematically lower than those calculated using the NFRC 100 method. The same window rated using the European method can appear to have better performance (lower U-factor) than if rated by the NFRC method, although clearly this is not the case.

This raises another important point: U-factors calculated using the EN/ISO standards and provided with imported European glass and fenestration systems must not be compared directly with North American fenestration rated using the NFRC 100 method. EN/ISO calculated U-factors cannot be used to comply with U.S. or Canadian energy codes, which require U-factors to be calculated using NFRC 100.

The differences in output between the U-factor calculation methods across the Atlantic give rise to a significant risk of inappropriate product comparisons and performance misinterpretation, especially for projects with international supply chains.

Not only is it critical to qualify what U-factor is being quoted, e.g., fenestration assembly, COG, EOG, etc., but it is also critical to qualify the calculation methodology used to determine the U-factor. It is important to ask: Is the U-factor the NFRC 100 U-factor or the EN/ISO-calculated U-factor?