by Paul Nutcher, CSI, CDT, AIA Allied
Architects designing with concrete balconies, cantilevered beams, roof penetrations, parapets, canopies, spandrel glass, and other ornamental architectural features are often limited in executing these design elements because they can create thermal bridges that extend beyond the insulation systems within the building envelope. This can cause condensation buildup in exterior systems and significant loss of energy performance for the whole building. Both issues can hamper efforts at designing and constructing buildings to meet energy codes or buildings registered under one of the voluntary green building rating systems. Fluid-applied insulating coatings can help.
While much attention is paid to roof and wall assemblies and their R-values (or lower heat flow, U-values), meeting code-mandated requirements and the benchmarks in voluntary green building rating programs such as Leadership in Energy and Environmental Design (LEED) or Green Globes will challenge project teams. They will need to find every possible thermal bridge during pre-design to improve the assembly’s energy efficiency.
Thermal bridging can have a significant impact on a project from a cost and compliance standpoint. Heat flows determine the building’s heating and cooling system capacity, long-term energy costs, compliance with energy codes, and voluntary energy performance code-overlay programs. How building assembly materials are arranged in the envelope determines surface temperatures, moisture development, long-term durability, and the potential for mold growth within outward-facing building systems.
Much has been done in recent decades to address inaccuracies in HVAC sizing because previous generations of energy modeling did not take into account the interface details, which can have an impact of as much as 50 percent on the overall wall area. According to Oak Ridge National Laboratories (ORNL) experts Jan Kosny and Jeffrey E. Christian, more accurate thermal performance measurements will have to include typical envelope details such as wall/wall (i.e. corners), wall/roof, wall/floor, wall/door, and wall/window connections. This is in addition to measuring the thermal performance of the ‘clear wall’ area, insulation layers, and the structural elements.
By the mid-1990s, ORNL research led to ‘whole-wall’ R-value estimations, compared with simplified ‘center-of-cavity’ and ‘clear-wall’ R-values. However, much of that work dealt with residential single-family dwellings. Over the last few decades, there has been a natural progression in research on commercial mid-rise and high-rise construction.
Studies in thermal bridging
Before delving into the new class of fluid-applied insulating coatings, it is important to understand how heat is transferred within a building envelope. American Society of Heating, Refrigerating, and Air-conditioning Engineers’ (ASHRAE’s) Research Project (RP) 1365, Thermal Performance of Building Envelope Details for Mid- and High-rise Buildings, initiated a catalog of thermal performance data for 40 common building details for mid-rise and high-rise construction. Published in 2011, its goals were to:
- calculate thermal performance data for common building envelope details for mid-rise and high-rise construction;
- develop procedures and a catalog that allowed designers quick and straightforward access to information; and
- provide information to answer the fundamental questions of how overall geometry and materials affect the overall thermal performance.
Engineers at Morrison Hershfield employed heat-transfer software, and calibrated and validated models against measured and analytical solutions. International Organization for Standardization (ISO) standards for glazing were used, as were 29 guarded hot box test measurements. They assessed 40 construction details common to construction methods in North America. While there was some focus on glazing, the highest priority was on the details with thermal bridges in three dimensions.