This article was originally published in the March/April 1997 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online March/April 1997
Wall R-Values that Tell It Like It Is
by Jeffrey E. Christian and Jan Kosny
Jeffrey E. Christian is the manager of the DOE Building Envelope Systems and Materials Program at the Oak Ridge National Laboratory, Oak Ridge, Tennessee, and Jan Kosny is a research engineer at the University of Tennessee in Knoxville.
There's a lot more to most walls than meets the eye, and the R-value of a whole wall can be considerably lower than the R-value of the insulation that fills it. At DOE's Buildings Technology Center, scientists have developed a system for measuring whole-wall R-value, and have already tested several types of wall system.
In these common procedures, the user enters a framing factor (ratio of stud area to whole opaque exterior wall area). The framing factor is usually estimated, is seldom verified against actual site construction, and is frequently underestimated (see Is an R-19 Wall Really R-19? HE Mar/Apr '95, p. 5). Framing factors range from 15% to 40% of the opaque exterior wall area, yet lower values are commonly used. Unfortunately, the wall's energy efficiency is usually marketed solely by the misleading clear-wall R-value (Rcw).
Clear-wall R-value accounts for the exterior wall area that contains only insulation and necessary framing materials for a clear section. This means a section with no windows, doors, corners, or connections with roofs and foundations. Even worse is the center-of-cavity R-value, an R-value estimation at the point in the wall containing the most insulation. This converts to a 0% framing factor and does not account for any of the thermal short circuits through the framing.
The consequences of poorly selected connections between envelope components are severe. These interface details can affect more than half of the overall opaque wall area (see Figure 1). For some conventional wall systems, the whole-wall R-value (Rww) is as much as 40% less than the clear-wall value. Poor interface details may also cause excessive moisture condensation and lead to stains and dust markings on the interior finish, which reveal envelope thermal shorts in an unsightly manner. This moist surface area can encourage the growth of molds and mildews, leading to poor indoor air quality.
Metal-framed walls are particularly vulnerable to thermal shorts. Unfortunately, builders often attempt to solve metal wall problems by making thicker walls and adding more insulation in the cavity between the metal studs. In fact, the thicker walls have an even higher percentage difference between clear-wall and whole-wall R-value.
We estimated whole-wall R-values for 18 wall systems, using a computer model. We validated the accuracy of the modeling using the results of 28 experimental tests on masonry, wood frame, and metal stud walls. The model was sufficiently accurate at reproducing the experimental data.
The whole-wall R-values estimated for the 18 wall systems are shown in Table 1 along with the clear-wall R-values. A reference building was used to establish the location and area weighing of all the interface details. The comparison of these two values gives a good overall perspective of the importance of wall interface details for conventional wood, metal, masonry, and several high-performance wall systems.
In general, construction details for the wall systems chosen come from the ASHRAE Handbook and from the respective manufacturers. In the case of the metal frame systems, the details come from the American Iron and Steel Institute and other common sources.
A wall's thermal performance is often simply described at the point of sale as the clear-wall value. The results shown in Table 1 indicate that the whole-wall value could be overstated by up to 26% for these systems. These differences can be even greater with interface details that are easier to construct but that may have more thermal shorts.
Whole-Wall versus Clear-Wall
Interesting comparisons can be made using the data in Table 1 to illustrate the importance of using a whole-wall value to select the most energy-efficient wall system. It could be argued that the difference between the clear wall and whole-wall R-value represents the energy savings potential of adopting the rating procedure proposed in this paper. Most building owners assume that they have the higher clear-wall value, rather than the more realistic whole-wall value.
Knowing whole-wall R-value could affect consumer choices. Systems 5 and 6 in Table 1 show two different high-performance masonry units. If one used the clear-wall data to choose the unit with the highest R-value, one would pick System 5, the low-density concrete multicore insulation unit, because its clear-wall value is 19.2 compared to 15.2 for System 6, expanded polystyrene (EPS) block forms. However, if one used the whole-wall data, one would choose just the opposite, because System 6 has the higher value--15.7 compared to 14.7 for System 5. Also, the whole-wall value of the foam form system is actually higher than the clear-wall value by more than 3%. This illustrates the effect of the high thermal resistance of the interface details.
Systems 7, 8, and 9 are all conventional wood frame systems. Note that the details affect the whole-wall R-value more for 2 x 6 walls than for 2 x 4 walls. The ratio of Rww to Rcw is about 90% for the 2 x 4 walls and 84% for the 2 x 6 wall.
Comparing System 11, the 6-inch stressed-skin panel wall, to System 9, the conventional 2 x 6 wood frame wall, shows that the Rcw for the former (R-24.7) is 51% higher than that for the latter (R-16.4). However, the figures for the Rww are R-21.6 to R-13.7 respectively, an improvement of 58%. As this example shows, advanced systems will generally benefit from a performance criterion that reflects whole-wall rather than clear-wall values.
Systems 12 through 18 are all metal-framed. On average, the whole-wall value for these seven systems is 22% less than the clear-wall value. Metal can be used to build energy-efficient envelopes, but not by using techniques common to wood frame construction. The conventional metal residential systems reflected in Table 1 do not fare as well, compared to the other systems, when the whole-wall value is used as the reference. For example, if one is considering either System 6 (EPS block forms) or System 12 (a 4-inch metal stud wall), the clear-wall R-value is about the same--R-15. However, if the comparison is made using the whole-wall R-value, the EPS block form system has a 45% higher value--R-15.7 compared to R-10.9.
Whole-Wall versus Center-of-Cavity
We also compared whole-wall R-values to center-of-cavity R-values. When a real estate agent or contractor states the R-value of insulation across the cavity to a potential home buyer, the implied whole-wall R-value is often overstated by 27% to 58%. If one compared metal (System 13) and wood (System 7) frames using center-of-cavity R-values, one would conclude that there was no difference, since both have center-of-cavity values of about R-14. However, the whole-wall value of the 2 x 4 wood wall system is 56% better than the whole-wall value for the metal system -- R-9.6 compared to R-6.1.
These comparisons are not meant to imply that one type of construction is always better than another. They are all based on representative details. Whole-wall R-values could change if certain key interface details were changed. The purpose of making these sample comparisons is simply to show the importance of having the whole-wall value available in the marketplace, to guide designers, manufacturers, and buyers to more energy-efficient systems.
Now that a growing wall database and an evaluation procedure are available, the building industry can develop a national whole-wall thermal performance rating label. This would establish in the marketplace a more realistic energy savings indicator for builders and homeowners faced with selecting a wall system for their buildings.
Labels could also help specific systems to gain the acceptance of code officials, building designers, builders, and building energy-rating programs such as Home Energy Rating Systems (HERS) and EPA Energy Star Buildings. The whole-wall R-value procedure has been proposed for adoption in the ASHRAE Standard 90.2, the Council of American Building Officials Model Energy Code, and U.S. Department of Energy's national voluntary guidelines for HERS. Many of the documents that are available to show builders how to comply with applicable codes, standards, and energy efficiency incentive programs would benefit by using the whole-wall R-value comparison procedure.
Ultimately, wall comparisons should include five elements: whole-wall R-value, thermal mass benefits, airtightness, moisture tolerance, and sustainability (see Beyond R-Value). Publication of this article was supported by the U.S. Department of Energy's Office of State and Community Programs, Energy Efficiency and Renewable Energy.
Continuing research is being cofunded by DOE's Office of Buildings Technology and Community Programs and by private industry to add more advanced wall systems to the database, and to address not only thermal shorts, but thermal mass benefits, airtightness, and moisture tolerance. Industry participants so far include American Polysteel, Integrated Building and Construction Solutions (IBACOS), Icynene Incorporated, Society for the Plastics Industry Spray Foam Contractors, Hebel USA L.P., Composite Technologies, Structural Insulated Panel Systems Association, LeRoy Landers Incorporated, Florida Solar Energy Center, American Society of Heating, Refrigerating and Air-Conditioning Engineers and Enermodal.
The database of advanced wall systems is available on the Internet (http://www.cad.ornl.gov/kch/demo.html). For more information, contact Jeffrey E. Christian at Oak Ridge National Laboratory, P. O. Box 2008, MS 6070 Oak Ridge, TN 37831-6070. Tel:(423) 574-4345; Fax:(423)574-9338; E-mail: email@example.com.Further Reading Kosny, J., and A. O. Desjarlais. Influence of Architectural Details on the Overall Thermal Performance of Residential Wall Systems. Journal of Thermal Insulation and Building Envelopes Vol. 18 (July 1994) pp. 53-69.
Kosny, J., and J. E. Christian. Thermal Evaluation of Several Configurations of Insulation and Structural Materials for Some Metal Stud Walls. Energy and Buildings, Summer 1995, pp. 157-163.
Christian, J. E. Thermal Mass Credits Relating to Building Envelope Energy Standards. ASHRAE Transactions 1991, Vol. 97, pt. 2.
Kosny, Jan and Jeffrey E. Christian. Reducing the Uncertainties Associated with Using the ASHRAE ZONE Method for R-Value Calculations of Metal Frame Walls. ASHRAE Transactions 1995, Vol. 101, pt. 2.
Christian, J.E., and J. Kosny. Toward a National Opaque Wall Rating Label. Proceedings from Thermal Performance of the Exterior Envelopes VI conference, December 1995.
Publication of this article was supported by the U.S. Department of Energy's Office of State and Community Programs, Energy Efficiency and Renewable Energy.
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