Energy Codes and Thermal Bridging
For your building or home to be energy code compliant, which design method or path will you choose? Prescriptive? Performance? Or will your project go beyond code using LEED or Passive House standards?
Using the prescriptive path method in ASHRAE 90.1 will result in a building that meets code but does not necessarily minimize energy consumption.
Using the performance path Performance Rating Method, energy cost is used to evaluate building energy performance. The building is designed to have lower energy costs than its prescriptive twin (based on the 2004 version of 90.1). This method is an accurate way of demonstrating energy cost savings using building energy modeling.
The building envelope is part of the ASHRAE 90.1 and IECC model energy codes. Regardless which version of the codes or method you use to achieve code requirements, the impact of thermal bridging on the envelope is largely ignored. To design a truly energy efficient building, you must design to a higher standard.
A building envelope or building enclosure is the physical separator between the exterior and interior environments of a building. The function of the enclosure is to provide resistance to air, water, heat, light, and noise transfer.
Image via Volodymyr Shevchuk
The R values and U factor methods given in ASHRAE 90.1, appendix A and IECC chapter 5 for typical construction assemblies, do not include the effects of many sources of thermal bridging.* What about masonry shelf angles, cladding attachments and wall-to-foundation transitions that interrupt continuous insulation? These thermal bridging anomalies are not considered in current versions of 90.1 or the IECC, yet it has been demonstrated that these interface details and transitions create substantial thermal bridges.
The 2016 version of 90.1 does require thermal modeling of un-insulated assemblies (balconies, parapets, etc.) to determine their contribution to building envelope heat loss. However, this approach uses area weighted averaging and assumes that heat flows in parallel heat paths. Therefore, in 2018 ASHRAE issued addendum av for public review to specifically address thermal bridging and thermal bridging solutions. However, thermal bridging will not be addressed specifically in the 2019 version of 90.1.
The NECB (National Energy Code of Canada for Buildings) does address thermal bridging as it relates to building energy performance. The technical requirements for energy efficiency in building design include lower U values for roof assemblies and prerequisites for reducing the impact of thermal bridges in all other locations in the envelope.
*The impact of thermal bridging due to steel stud framing has been recognized using a “framing factor” resulting in a more accurate R value for interior cavity insulation values. This realization has resulted in the code requirement for exterior continuous insulation or “C.I.”
The thermal envelope can be defined as the layer of the building enclosure that helps provide conditioned space or provides a boundary between conditioned and unconditioned spaces.