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ORNL develops procedure to rate thermal performance of whole walls
OAK RIDGE, Tenn., Nov. 20, 1996 — What's the R-value in your walls? The answer used to be simply the R-value of your wall insulation. For standard wood frame construction, determining R-value, or resistance to heat flow, based solely on wall insulation was not too far off.
But because of the increasing use of metal frame systems and other more conductive materials, such as masonry and concrete, in our walls, R-value should now be determined by studying the whole wall, not just the insulation. How? By using a new wall testing and rating procedure developed at the Department of Energy's (DOE's) Oak Ridge National Laboratory (ORNL).
"We've created a procedure to help users measure the ability of whole walls to hold in heat in the winter and keep out heat in the summer," says Jeff Christian, program manager for Building Thermal Envelope Systems at ORNL's Buildings Technology Center (BTC), a DOE user facility at ORNL.
"At BTC, we test and rate whole walls for users. We can work with traditional wood-frame walls containing traditional materials, ranging from face brick to interior drywall. And we can test metal-frame walls and insulating concrete forms, which are gaining popularity in residential construction."
Previously, rating procedures produced a wall R-value based on measurements at the center of a wall cavity. For example, if a wall's insulation had an R-19 value, the whole wall was rated as R-19. However, this approach fails to take into account increased heat leakages at corners, doors, windows, and studs that offer lower resistance to heat flow than insulation materials. The ORNL approach accounts for these factors.
"Because of the complicated nature of energy flows in buildings, we note a particular need for accurate and comprehensive measures of building energy performance. Limited comparisons such as R-value differences can lead to considerable misrepresentation of benefit. As a result, energy performance measurement research may be the best use of shrinking federal research dollars. The national laboratories can provide the scientific basis necessary to 'level the playing field' without the bias of a particular technology, material, or strategy. ORNL's initiative on whole wall performance ratings is an excellent example of how federal research resources can benefit both industry and the general public," said John M. Talbott, program manager, DOE Office of Building Technology, State and Community Programs.
"Several state energy agencies," Christian says, are thinking about adopting the procedure. "We have already worked with seven firms and currently are working with another four. About 36 companies have expressed interest in becoming BTC users and adding their wall system to the electronic base available on the Internet's World Wide Web."
Users pay $3000 to $15,000 each for testing and detailed computer modeling of their systems. A whole wall's resistance to heat flow is measured using steady-state and dynamic testing in BTC's new Rotatable Guarded Hot Box. The test results are then entered into an ORNL-developed simulation model called HEATING, which predicts the thermal performance of the opaque wall (everything except the windows and doors).
Total wall R-value ratings allow builders and buyers to compare thermal resistances of dissimilar walls. This capability is especially important because of the growing use of alternative wall systems and construction materials such as steel, structural ceramics, left-in-place insulating foam forms and structural insulating panels (wood-foam-wood sandwiches).
"We put the wall performance information in a database on the World Wide Web," Christian says. "Home designers, builders, realtors, and buyers can use it to predict ratings for these wall systems customized for their own housing plans." The Web address is http://www.cad.ornl.gov/kch/demo.html.
Funding for the research is provided by DOE, Office of Building Technology, State and Community Programs and the host of private firms developing alternative exterior wall systems for the future.
ORNL, one of the Department of Energy's multiprogram national research and development facilities, is managed by Lockheed Martin Energy Research