![]() If you are using a performance based design method or beyond-code strategy in your design, energy modeling is a requirement. Thermal bridging is now being given much more consideration. Nonetheless, the importance of air leakage and air barriers to control moisture and heat loss has long been established in building codes and standards like Passive House. While the contribution of heat loss by thermal bridging can be found through thermal modeling, air leakage is more difficult to define in a model since it lacks thermal conductivity properties. Minimizing these two helps to a) prevent moist air from entering the thermal envelope and b) prevent material surfaces in the thermal envelope from cooling below the dew point – both of which can cause condensation. Heat transfer due to air leakage is by convection while heat transfer due to thermal bridging is typically by conduction. ![]() Other than the gains and losses associated with fenestration and mass, the two major contributors to overall enclosure energy loss are air leakage and thermal bridging. Building enclosure design is directly related to this energy consumption. Of the 9.40 quadrillion BTU consumed by commercial buildings in 2018, space heating consumed the most energy (20%). Adding more and more insulation to a wall or roof to overcome the effects of heat loss due to a thermal bridge has proven ineffective and inefficient. Until recently, thermal bridging had been ignored even though it can reduce a wall’s R value by nearly 50%. Thermal bridging and its part in heat loss was a relatively new topic six or seven years ago. This realization has resulted in the code requirement for exterior continuous insulation or “C.I.” * 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. 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 NECB (National Energy Code of Canada for Buildings) does address thermal bridging as it relates to building energy performance. However, thermal bridging will not be addressed specifically in the 2019 version of 90.1. Therefore, in 2018 ASHRAE issued addendum av for public review to specifically address thermal bridging and thermal bridging solutions. However, this approach uses area weighted averaging and assumes that heat flows in parallel heat paths. The 2016 version of 90.1 does require thermal modeling of uninsulated assemblies (balconies, parapets, etc.) to determine their contribution to building envelope heat loss. * 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 R values and U factor methods given in ASHRAE appendix A and IECC chapter 5 for typical construction assemblies, do not include the effects of many sources of thermal bridging.
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