The Role of the Structural Frame in Zero Net Energy Construction

The first step to buildings that make as much energy as they consume? An efficient structural frame.

Heat Transfer in Wall AssemblyAs more and more jurisdictions adopt stricter energy codes, the push is on for even deeper efficiency, with growing calls for zero net energy (ZNE) structures. For example, California will require zero net energy design for new homes by 2020 and new commercial projects by 2030.

In the simplest terms, zero net energy (also called net zero energy and zero energy) buildings seek to balance the amount of energy consumed on an annual basis to the energy produced by on-site renewables such as photovoltaics and wind turbines. The benefits to implementing ZNE include reducing greenhouse gas emissions through lower energy consumption, lowering a building’s operational costs, and creating greater self reliance as building design moves toward getting off the grid.

Practically speaking, implementing ZNE now allows owners and designers to get ahead of the game and ahead of the learning curve. Early adopters will be more competitive, which can lead to more project wins and greater profits.

The Structure’s Role

Many structures built today are like a leaky bucket. The energy needed to heat and cool a building is used inefficiently as a consequence of “conventional” framing techniques. Before thinking about adding renewable energy sources to the building, the energy demand must be minimized. There are many variables that can minimize energy needs, including building size, orientation, air leakage, and enclosure design.

Once mass and orientation have been addressed, consider the building enclosure and how to efficiently design the structure to maximize the airtightness and insulation, while minimizing the framing factor. Heat transfer through the walls is measured in terms of thermal resistance (R) of a wall by the cumulative effects of the different components of the wall assembly. The U-factor is generally the inverse of the R-value and used to evaluate multiple heat flow paths within a single assembly, such as walls.

Advanced framing techniques leave more room for insulation

Among the methods to reduce the thermal conductance of the exterior walls are to increase the depth of the wall cavity, use higher performing wall cavity insulation (higher R-value per inch), reduce the framing factor (the percentage of the total solid exterior wall area occupied by framing members), cover the outside of the building with continuous insulation, or a combination of these strategies.

Incorporating advanced framing techniques can bring the framing factor down to about 16 percent. Advanced framing is a suite of framing techniques that enhance the energy efficiency of the building, while bringing down the cost of construction, and maintaining structural integrity. Techniques include switching to 2x6 studs spaced 24 inches on center to increase cavity insulation space, energy efficient corners and wall intersections, and eliminating double top plates, among others.

To view details and implementation strategies for advanced framing, download APA’s Advanced Framing Construction Guide, Form M400.

With growing calls for energy designs that approach zero net energy, there is both an opportunity and a need for architects, engineers, and builders to understand the advanced framing concepts, as well as other techniques, that improve energy efficiency while preserving structural integrity.

For more on the topic of ZNE and its implementation in California, download The Role of the Structural Engineer in Zero Net Energy Construction by APA Engineered Wood Specialist Karyn Beebe, PE, LEED AP.