U-Factor: Beyond compliance to standards

Referencing the Reference

Most fenestration products are covered by ANSI/NFRC 100 and CAN/CSA-A440.2/A440.3. If specifiers want to understand which standard best applies to a project’s curtain wall or other fenestration systems, they first should be aware that CAN/CSA-A440.2/A440.3 refers to NFRC 100 and to NFRC 102 Procedure for Measuring the Steady-State Thermal Transmittance of Fenestration Systems.

Going one step further, NFRC 100 and 102 also refer to each other for various applicable conditions. These conditions include dimensions, product types, configurations, and procedures. The ASHRAE Handbook–Fundamentals also refers to NFRC 100 and CAN/CSA-A440.2. In addition, ASTM C1363’s standard test methodology is mentioned by both ASHRAE and NFRC 102.

Per CAN/CSA-A440.2/A440.3:



The fenestration system U-factor shall be determined in accordance with NFRC 102.


The fenestration system U-factor shall be determined using the simulation procedures specified in ANSI/NFRC 100, except that validation of the simulations as specified in ANSI/NFRC 100 is not required.

The NFRC Simulation Manual and the NFRC Technical Interpretation Manual shall be used when performing computer simulation. In the event of a conflict with ANSI/NFRC 100 or the NFRC Simulation Manual or the NFRC Technical Interpretation Manual, this Standard shall take precedence. In the event of a conflict between ANSI/NFRC 100 and the NFRC Simulation Manual or the NFRC Technical Interpretation Manual, ANSI/NFRC 100 shall take precedence.

These intertwined standards bring to mind the question, “Which came first, the chicken or the egg?” All of these seem to nest together. In the end, NFRC 100 stands as the ultimate reference regarding the applicable conditions for determining U-factor measurements and the corresponding energy efficiency ratings for curtain wall and other fenestration products.

Figure 3 notes some example fenestration products alongside dimensions the NFRC 100 prescribes for each.

Approved Approaches

NFRC 100 offers three approaches to determine the U-factor values of a fenestration product or a combination of products:

• A U-factor value based on prescriptive methods;

• A U-factor value based on project-specific
conditions; and

• A range of multiple U-factor values that determine the thermal performance limits of the product(s); also known as Linear Energy Analysis For Fenestration (LEAFF).

The top two approaches to obtaining U-factor values have been used for many years, and will continue to be, within the progressive evolution of NFRC 100’s procedures. The third approach was more recently introduced by NFRC to ease the use and understanding of data. The data from these approaches are neither used at the same stages of a project’s development or for the same types of projects.

Nevertheless, all three of these approaches generates data that are in compliance with NFRC 100, but the various data from these different approaches should never be compared, or extrapolated for comparison–such manipulations have been proven invalid.

Specification professionals are challenged to stay up to date on these various approaches and, based on their knowledge, to critically review the U-factor values for curtain wall and other fenestration systems. NFRC 100 and 102 are available free for download. NFRC also offers regular training and webinars on energy efficiency topics. CSA also references a number of NFRC-accepted software for specifier use.


The prescriptive approach leads to a standardized U-factor value appropriate for fenestration products and considers specific criteria defined by NFRC 100. This approach allows for similar product type comparisons and is a good starting point for preliminary selections. It also works well for basic projects that follow the NECB’s prescriptive path.


The project-specific approach leads to a unique U-factor value based on the performance of a fenestration product, possibly in combination with other fenestration systems, to serve a particular project’s requirements. This approach should be considered for any project that exceeds NECB’s prescriptive requirements, or that has special design conditions, such as historically significant buildings.

The project-specific approach can be obtained by either a simulation or a physical test. Numerous aspects can be modified to suit the project’s requirements, including a fenestration product specimen’s dimensions, assemblies, glazing types, interior and exterior temperatures, and more (see Figure 2, on page 20). This allows the U-factor value to be determined according to the project-specific conditions. This unique U-factor is necessary when calculating the building’s energy consumption, and ultimately, for meeting energy codes’ conformity by demonstration paths.

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