U-factor: Beyond compliance to standards Part 4 of 4

Concluding our four-part series on understanding and comparing U-factors in fenestration products, this final section demonstrates how changing key components can produce different performance results, and how to ensure these results lead to conclusions that are both correct and compliant.

Validating Variables

Using the same curtainwall system, Figures 5 and 6 demonstrate how altering two variables – glazing type and mullion depth – can affect U-factor values and lead to different performance results.

Figure 5:

This simulation shows the same model of high-performance curtainwall with an integral thermal break and with all the same components in the 2000 by 2000 mm (79 by 79 in) size, divided into two vertical lites, configured as prescribed for curtainwall in NFRC 100. The only difference is the insulated glass unit (IGU). The top example uses a high-performance IGU compared with a low-performance IGU on the bottom. This illustrates the importance of consistency on specification requirements between curtainwall and glass selection. A curtainwall’s capacity to achieve an overall performance required in a specification heavily relies on the performance of the IGU specified.

Figure 6:

This simulation shows the same model of high-performance curtainwall with an integral thermal break and with all the same components in the 2000 by 2000 mm (79 by 79 in) size, divided into two vertical lites, configured as prescribed for curtainwall in NFRC 100. The only difference is the mullion depth. Temperatures observed are superior with the deeper 203 mm (8 in) mullion, which is an interesting aspect to improve the resistance to condensation (I-index). However, the overall U-factor is higher with the deeper mullion because its warmer surface temperature and more interior surface area leads to more heat lost.

The choice of insulated glazing units (IGUs) and the depth of the mullions are outside NFRC 100’s prescribe approach. As shown above, these have a notable influence in the curtainwall system’s U-factor and thermal transmission. It is very important to ensure the framing specification is coordinated with the glass specification and the separate components combined can achieve the desired U-value required for the project.


There are a variety of IGUs that may improve the overall performance of a fenestration product, especially for aluminum curtainwall systems. An overall system U-factor will depend on the choice of glazing. The use of high-performance IGUs may compensate for lower thermal performance elsewhere in the curtainwall system’s design. It is possible to separate the system’s overall performance (Ut) from the performance of the aluminum framing (Uf). Again, keep in mind that the frame’s lower thermal performance may be mitigated by the IGU’s higher performance.


Frequently, the IGU is selected and specified independent of the fenestration system’s framing based on project-specific needs. For optimal performance, these should be reviewed together. It is imperative to ensure consistency between the complete system’s specified U-factor and the IGU’s capacity to support the overall system requirements. For example, specifying a U-factor of 1.65 W/m²•K (0.29 Btu/hr•ft²•°F) for both the IGU and the overall fenestration product is a required performance impossible to achieve.


As for the importance of mullion depth, this also is dependent on the particular needs of the project. Structural performance and safety should always be the first priority. In general, a deeper mullion will have a positive affect on condensation resistance, but it also can have a negative affect on thermal transmittance. This is an important consideration when designing the project and its curtainwall and fenestration systems, as well as in analyzing the data provided as proof of compliance to meet thermal performance specifications, building codes, structural requirements and energy efficiency goals.

NFRC’s LEAFF approach to determining U-factor values introduces new possibilities to ensuring high thermal performance in curtainwall and other fenestration systems. This approach relies on computer simulations to validate the IGU’s performance and then to define the performance intervals of a fenestration product. Because the glazing unit is an essential and influential component of the fenestration system, this method might generate more practical, applicable data about a product’s capabilities.[8]

  1. NFRC, “Linear Energy Analysis for Fenestration,” Sept. 2020, https://cdn.ymaws.com/nfrccommunity.org/resource/collection/B6FBC512-4244-4851-B1BE-D67F3D0B7CD0/2020-09_LEAFF_Trendline_Simulation_Manual_-_Final.pdf


Comparing Curtainwall

In Figure 7, two U-factor simulations are demonstrated. Both are in compliance with NFRC 100, and thereby in compliance with CAN/CSA-A440.2/A440.3. One shows a low-performance curtainwall specimen with aluminum framing that has a basic thermal break, but is optimized with a high-performance IGU and a shallow mullion. The other curtainwall specimen has a high-performance, thermally broken aluminum framing and a deep mullion, but it also has a low-performance IGU with air infill, a metal spacer and no low-emissivity coating.

Analyzing the range in attainable thermal performance, the low-performing curtainwall helps validate the best achievable U-factor, and the high-performing curtainwall assists in determining the worst U-factor. These differences presented by these voluntary choices demonstrate the need to verify beyond the system manufacturer’s promoted U-factor and to affirm compliance with NFRC/CSA standards.

Figure 7:

This simulation shows two different models of curtainwall in the 2000 by 2000 mm (79 by 79 in) size, divided into two vertical lites, configured as prescribed for curtainwall in NFRC 100. The top example is a regular, low-performance curtainwall with a minimal thermal break, a high-performance IGU and a shallow mullion. The bottom example is a high-performance curtainwall with an integral thermal break, a low-performing IGU and a deep mullion. These illustrate how, even when both are in accordance with NFRC 100 prescribed requirements, U-factors can be calculated in a way to demonstrate the best or the worst that a product can achieve. Be aware that U-factor comparisons of products based only on compliance to the standard can lead to incorrect conclusions.

In summary, it is a specifications professional’s responsibility to look beyond conforming to a fenestration standard, to question thermal performance data being presented and to validate the full context in which the U-factor was obtained and is required. The first step is to verify whether the required data are or should be based on project-specific requirements, or if they are or should be based on a prescribed approach. For U-factor values determined according to the standard method, the next step is for a specifier to evaluate which criteria should be considered to adequately compare the thermal performance of curtainwall and other fenestration systems.

Simply stating a U-factor without the compliance requirements is likely to cause confusion and to miss the intended performance requirements. In the aluminum framing specifications, clearly state which compliance path is expected. If following a non-prescriptive path, remember to include all items necessary to determine performance such as elevations, sizes and configurations, materials and components, local climate conditions, and IGU considerations. Coordinating the specifications for the glass and aluminum framing curtainwall or other fenestration systems.

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For personalized assistance in selecting a fenestration system with high thermal performance on your project, please contact your Client Development Manager. For questions regarding this blog series, please contact the author, Jennie Lamoureux, FMPC. She can be reached at j.lamoureux@alumicor.com.