U-factor: Beyond compliance to standards, Part 3 of 4

Now that we understand the definitions, standards, reference documents and approved approaches to determining fenestration U-factor values, Part 3 offers the context and criteria needed to compare products and ensure compliance.

Context and Criteria

The standard method for determining U-factor values is used to establish basic criteria, allowing a comparison of products during the preliminary stages of a project, or to ensure compliance with the NECB’s prescriptive requirements. U-factor data determined by this approach are ubiquitous in the commercial building and fenestration industry. It is imperative to understand the context in which the values were determined to better assess how the results may be influenced.

Two components emerge in the standard method for determining U-factors:

1. The protocol, which is the procedure to be followed to carry out the computer simulation or the physical test; and
2. The specimen, which is the product itself.
It is expected that all parameters of these two components will be exhaustively defined, allowing an equal comparison of fenestration products.

Among the protocol’s many criteria are:

• The calibration of measuring devices;
• The software versions to be used in case of simulation, or the installation of the specimen in the case of a physical test;
• The indoor and outdoor temperature differentials;
• The location of temperature reading points on the simulated or physical specimen;
• The wind speed(s); and
• Any additional reference standards.

As for the specimen, the objective is to measure its performance; however, other factors also have an impact on the resulting U-factor. Some of these factors may not be integral or consistent with the product’s design. The variations in these factors can be especially important in the case of curtainwall systems. Curtainwall offers many options to customize commercial building envelopes’ appearance and performance. Reviewing Figure 2, one can see how non-standard fenestration systems can impact an overall U-factor and why specifiers must carefully review their data.

Defined Dimensions

To provide a framework for specimens’ standard analysis, NFRC 100 prescribes dimensions for each type of fenestration product. As examples, see Figure 3.

Standard dimensions make it easier to compare similar products’ U-factor values. For different fenestration systems, standard results are not a good indicator in product type selection because they cannot be compared on an equal basis. The U-factor performance of a casement window compared with dual-action window serves as a good example.

Compliance with these dimensions and configurations remains important to compare the same type of fenestration products. Particularly in the case of aluminum curtainwall products, the ratio between the framing members and the glass is a key aspect in the measurement of thermal transmittance. Furthermore, to disregard prescribed configurations for curtainwall and only use the overall dimensions for determining standard U-factor would produce an inaccurate result as shown in Figure 4.

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. The difference between these models is that one on the left is undivided due to a mistaken assumption regarding the configuration requirement. The one on the right does is divided in two vertical lites as prescribed by NFRC-100. The difference in results comes from the aluminum mullion division that generates more heat transfer. This comparison is a good indication on how more aluminum mullions on a design can influence the performance.

Building upon a foundational understanding for comparing fenestration products’ U-factor values, Part 4 demonstrates how changing key fenestration product components can produce different performance results, and how to ensure these results lead to conclusions that are both correct and compliant.