U-factor: Beyond compliance to standards, Part 2 of 4

Helping decipher thermal performance compliance, standards and specifications, Part 2 of our four-part blog series builds upon the understanding of code considerations and U-factor measurements. In this section, we review the performance procedures for obtaining U-factors, highlight the circular nature of the reference standards, and examine three approaches NFRC 100 offers to determine the U-factor values of a fenestration product or system.

Performance Procedures

Enforced editions and revisions of the National Energy Code of Canada for Buildings (NECB)[2] reference the American National Standards Institute and National Fenestration Rating Council’s ANSI/NFRC 100 Procedure for Determining Fenestration Product U-factors[3] and the Canadian Standards Association Group’s CAN/CSA-A440.2/A440.3 Fenestration energy performance/User Guide[4] as procedures to obtain U-factors.

In the few cases where specialized products may be outside the scope of these two standards, the NECB refers to the procedures described in the ASHRAE Handbook–Fundamentals[5] and ASTM C1363 Standard Test Method for Thermal Performance of Building Materials and Envelope Assemblies by Means of a Hot Box Apparatus[6].

2. NRC, “Codes Canada publications,” https://nrc.canada.ca/en/certifications-evaluations-standards/codes-canada/codes-canada-publications/

3. National Fenestration Rating Council (NFRC), ANSI/NFRC 100-2020, https://nfrccommunity.org/store/viewproduct.aspx?ID=1380591

4. CSA Group, “CSA A440.2:19/CSA A440.3:19,” https://www.csagroup.org/store/product/CSA%20A440.2%3A19%25100CSA%20A440.3%3A19/

5. ASHRAE, “ASHRAE Handbook,” https://www.ashrae.org/technical-resources/ashrae-handbook; accessed July 11, 2021

6. ASTM International, “ASTM C1363-19,” https://www.astm.org/Standards/C1363.htm

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 applies the best to a project’s curtainwall 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[7].

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, Section 5 ¬ Overall coefficient of heat transfer (U-factor):

5.2 Determination of U-factor by measurement
The fenestration system U-factor shall be determined in accordance with NFRC 102.

5.3 Determination of U-factor by computer simulation
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 curtainwall and other fenestration products.

7. NFRC, ANSI/NFRC 102-2020, https://nfrccommunity.org/store/viewproduct.aspx?id=1402095

Approved Approaches

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

1. A U-factor value based on prescriptive methods
2. A U-factor value based on project-specific conditions
3. 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 curtainwall and other fenestration systems. NFRC 100 and 102 are available free for download. NFRC also offers regular training and webinars on energy efficiency topics.

Prescriptive
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.

Project-Specific
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. As shown in Figure 2, 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. 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.

Having reviewed the definitions, standards, reference documents and approved approaches to determining fenestration U-factor values, Part 3 will offer more context and criteria for comparing products and ensuring compliance.