Thermal Bridge: A component or a system of components in a building envelope through which heat is transferred at a substantially higher rate than through the surrounding envelope area.
Thermal bridging through structural framing members in the building envelope can be a major contributor to heat loss/gain. Studies funded by ASHRAE, the California Energy Commission, and Building Science Corporation found that the average framing factor in a US residential home is on the order of 25%(1,2). As a result, if cavity insulations are utilized alone, approximately 25% of the structure (not including doors & wondows) is under insulated. Research conducted by Oak Ridge National Laboratories(1) revealed that thermal bridging accounts for approximately 10 â€“ 15% of residential energy usage in the United States, which is typically not included in building load analyses used in the sizing of critical mechanical equipment.
Further studies by ORNL(3) indicated that the R-value of what we think is an R-19 fiberglass wall system can be reduced to as little as R-14 when proper framing factor and interface details are considered in a wall assembly test such as the ASTM C1363.
(1) â€œHow the Same Wall Can Have Several Different R-Values: Relations Between Amount of Framing and Overall Thermal Performance in Wood and Steel-Framed Wallsâ€, Jan Kosny, David Yarbrough, Phillop Childs, Syed Azam Mohiuddin
(2) John Straube and Jonathan Smegal â€œBuilding America Special Research Project: High -R Walls Case Study Analysis Reserch Reportâ€ -0903, Building Science Corporation, March 11, 2009
(3)Energy Desing Update, vol. 19, no 9, September 1999 â€œHow Thermal Shorts and Insulation Flaws Can Degrade an â€œR-19â€³ Stud Wall to a Measly â€œR-11â€³