Thermal bridging in structural detailing

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    Q:With the recent buzz about the questionable energy performance of many LEED-certified buildings, what can be done in our structural practice to improve the energy efficiency of a facility in a meaningful way?

    Infrared image of a brick façade building shows heat loss at relieving angle locations in the wintertime.

    A: Actually, quite a lot! In looking at energy performance from a structural engineering perspective, one thought is that energy modeling methods in building design may not be accounting for many of the actual building envelope conditions. Frequently, the details of how the building’s structure interacts with the insulated envelope are not even considered in energy modeling. Poor details can be a significant source of building energy loss, which, of course, is not limited to LEED-certified buildings.

    One all-too-common area of energy loss is thermal steel bridging. Think about the thermal path in the same way as you would a load path; with the energy as gravity pushing toward the un-conditioned or un-heated zone. Rather than stiffness, a material’s thermal conductive properties, or the U-values, are significant. For example, carbon steel is about three times as conductive as stainless steel; about 500 times as polyurethane foam bearing blocks; about 1,000 times as fiberglass reinforced plastic (FRP), or about 1,200 times as the expanded polystyrene (EPS) insulation.

    So, where the EPS insulation on an exterior building wall is bridged by a 3/8-inch-thick continuous carbon steel plate at just 12 feet on center, more heat can potentially flow through the steel plate than the entire rest of the wall, depending on the connection path. This means that the wall assembly’s effective R-value could be less than half of a similar wall with no bridging across the insulation. Now, this is overly simplistic, but the point should not be lost: Thermal steel bridging can transfer a lot of heat energy across a building envelope. We are starting to see this in infrared thermal images of buildings that were assumed to be well-insulated (see photo above).

    Seek to break the thermal steel bridge by substituting or inserting less conductive materials in ways that maintain structural integrity. The use of insulated column bearing blocks in refrigerated warehouse space has been done for years. This simple technology can be applied to break heat loss from steel columns to the supporting exposed foundation wall. Or, insert FRP shims between a relieving angle and the structure to reduce the contact area of more conductive materials. Detail thermal breaks in the support of exposed roof edges and eaves. Even simply switching from carbon steel to stainless steel elements, while initially more expensive, can save energy and utility costs.

    What will this get you in terms of LEED? In tackling the most point-laden area of the LEED-rating systems, the Energy and Atmosphere category, advanced practitioners realize that in addition to an efficient mechanical system, a high-performing building envelope is critical to achieving energy efficiency. For this, the structural engineer should be engaged, along with architects and any building envelope consultants. The questionable energy performances that you ask about suggest these points are best not left exclusively to the mechanical engineer.

    In the April issue, we’ll present some case studies and conceptual details. It’s time we re-engineered some of our tried-and-true structural building details to address the fact that many of them are simply poor energy performers.

    Jim D’Aloisio, P.E., SECB, LEED AP, is a principal with Klepper, Hahn & Hyatt of East Syracuse, N.Y., and is on the Advisory Board of the NY Upstate Chapter of the USGBC. He can be reached at jad@khhpc.com. Russ Miller-Johnson, P.E., is with Engineering Ventures, PC, in Burlington, Vt. He is a member of the Vermont Green Building Network. He can be reached at russmj@engineeringventures.com. Both are members of the Structural Engineering Institute’s Sustainability Committee. The committee’s website is www.seinstitute.org/committees/sustainable.cfm