U-factor: Beyond compliance to standards, Part 1 of 4

Commercial building designs incorporate large spans of glass, creating a modern appearance with transparency, daylight and a sense of connection to the larger world. Aluminum remains the most commonly used material for framing these expansive views and vision areas in building envelopes’ curtainwall, storefront, entrance, window and other fenestration systems.

Aluminum is light weight and easily fabricated into versatile, durable products that require little maintenance throughout their long lifespans. Curtainwall and fenestration systems’ framing members can be manufactured with recycled aluminum content and recycled at the end of their use on a building. The benefits associated with expansive, aluminum-framed fenestration systems also include supporting occupants’ well-being and health, and building owners’ lease rates and property values.

As buildings are designed with larger and larger vision areas, it is essential to remain aware of curtainwall and other fenestration systems’ impact on the thermal performance and overall energy efficiency of the building envelope.

This four-part blog series helps to ensure that both the design intent and performance requirements are achieved for each project. The following sections will assist building teams in evaluating and selecting curtainwall and other fenestration products, and remind all parties to be alert for disparities in thermal requirements and data analyzed.

Part 1

Consider the Code
Understanding U-factor

Part 2

Performance Procedures
Referencing the Reference
Approved Approaches

Part 3

Context and Criteria
Defined Dimensions

Part 4

Validating Variables
Comparing Curtainwall

Consider the Code

When analyzing curtainwall and fenestration systems’ energy performance criteria, first check on the applicable codes. The most recent, national model building and energy codes may not be the most current ones, or modified versions may be enforced by the authority having jurisdiction over the project in question.

Provinces, territories, municipalities and some self-legislating authorities, such as First Nations, retain responsibility for how code editions and modifications are adopted and enforced except for federal buildings, where the most recent model code is automatically applicable. The National Research Council Canada (NRC) provides a high-level list of code adoption and enforcement throughout the country.[1] Additional verification at a municipal and project-specific level also is strongly recommended.

In all known Canadian codes and their provincial adaptations, each refers to “U-factor” or “U-value.” This is the industry-accepted measurement indicating the rate of thermal energy transmission in a fenestration system.

Government of Canada, National Research Council Canada (NRC), “Model code adoption across Canada,” https://nrc.canada.ca/en/certifications-evaluations-standards/codes-canada/model-code-adoption-across-canada

Understanding U-factor

U-factor measurements consider the combined role of the glass, opaque panels and framing members, and take into account three different ways a curtainwall or fenestration system transfers energy: convection, conduction and radiation.

Figure 1: U-factor, the thermal transmittance, is the inverse of the thermal resistance measurement used in the insulation industry, which is commonly expressed as effective R-value or RSI. When it comes to fenestration, it is not about measuring how well it insulates, but rather about measuring the total heat transfer through a system including convection, conduction and radiation under specific environmental conditions. The lower the U-factor, the less heat will be transferred. There are different procedures and methods to determine U-factor.

With a better understanding of code considerations and U-factor measurements, we invite you to continue reading Part 2. In this section, we review 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.