Challenges of Meeting 2015 IECC U-Factors with An Off-The-Shelf Curtain Wall System

Many buildings have non-load-bearing exterior cladding components as a feature of their design. This cladding is attached to the structure of a building to provide protection for the building’s interior against the elements such as air, water, and heat or cold. Energy standards for these systems are defined in the International Energy Conservation Code (IECC), a set of design and construction guidelines used by many states and municipalities in the U.S.

IECC compliance is a critical consideration in determining appropriate Curtain Wall Systems for each project and climate zone.

Cladding Component Configurations

Exterior cladding components can be split into different categories based on their configuration. Curtain walls typically span multiple stories on a building, bypassing and anchoring to the perimeter of the slab. Window walls generally run between floors on a building and anchor to the floors at the top and bottom of the glazing system. Punched openings—what people would typically call a “window”—are surrounded by an opaque wall such as brick, stone, metal panels, precast, etc. and anchor to the support framing of the opaque wall assembly.

Many projects are budget driven, thus requiring the use of “off-the-shelf” glazing systems in lieu of custom higher performing systems. The challenge that building owners face is that these systems may not meet IECC standards.

Understanding the IECC

Two factors used in the IECC are what are known as the R-value and the U-value. R-value describes how well an object (in this case, the exterior cladding) resists the flow of heat. The higher the R-value of a substance, the better its thermal insulating properties. U-value refers to the overall rate of heat transfer. It is derived by dividing the transfer rate in watts by the difference in temperature across the structure.

The IECC provides two methods by which a building can be analyzed for energy code compliance. These methods are the “prescriptive” and the “performance” methods. The prescriptive method can be used when the building meets requirements outlined in the IECC. These requirements include items such as 1) the vertical fenestration area (not including opaque doors and opaque spandrel panels) shall not be greater than 30% of the gross above-grade wall area (2015 IECC), and 2) the skylight area shall not be greater than 3% of the gross roof area (2015 IECC).

The code outlines different climate zones around the country. The higher the number, the colder the zone. These climate zones range from 1 to 8, with one “marine” zone. As the climate zones get colder, the performance requirements for the different cladding components become more stringent. For example, the prescriptive U-value requirement for fixed fenestration in climate zone 3 is 0.46; the prescriptive U-value for fixed fenestration in climate zone 6 is 0.36.

If these requirements can be achieved, the IECC provides minimum U-values and R-values that the glazing systems, opaque walls, and roofing assemblies must meet. Most off-the-shelf glazing systems have a U-value in the 0.48 to 0.52 range, with some systems being higher or lower. This is where the challenge arises on many projects when trying to follow the prescriptive design in colder climates.

If these requirements cannot be met, the performance method is to be followed. The performance method requires that an energy model, or total building model, be developed utilizing the actual U-values and R-values for the assemblies that are to be provided on the project. The off-the-shelf concern is less critical when following this design path because the lower performing glazing systems can be offset by the opaque wall and roof assemblies by utilizing more insulation where required.

Finding the Right Balance

The increased performance requirements outlined in the IECC require building owners and architects to make decisions focused on achieving a desirable cost/compliance balance. For example, higher-performing systems traditionally cost more than the typical off-the-shelf system. Even when following the performance design path with off-the-shelf systems, the cost may be increased in the opaque wall and roof assemblies by adding insulation to offset the U-value of the glazing system.

It is important that stakeholders understand that each project is unique in terms of addressing requirements outlined in the IECC, whether following the prescriptive design or the performance design. IECC requirements should be discussed during the SD/DD phases of the project as they will often affect the glazing systems and also the opaque wall makeups of the project.

As experts in building enclosure consulting and commissioning (BECx), Pie can assist in outlining these requirements and can also provide guidance on system or materials selections and wall makeups to meet the IECC requirements.