thermal performance terms

Several relatively new thermal performance terms are defined below:

Center-of-Cavity R-value: R-value at a point in the wall's cross-section containing the most insulation.

Clear-wall R-value: R-value of the wall area containing only insulation and necessary framing materials (i.e., for a clear section with no fenestrations, corners, or connections between other envelope elements such as roofs, foundations, and other walls).

Interface details: A set of common structural connections between the exterior wall and other envelope components, such as wall/wall (corners), wall /roof, wall/floor, window header, window sill, door jam, door header, and window jamb, that make up a representative residential whole-wall elevation.

Whole-wall R-value: R-value for the whole opaque wall including the thermal performance of not only the "clear wall" area, but also all typical envelope interface details (e.g., wall/wall (corners), wall /roof, wall/floor, wall/door, and wall/window connections).

The most commonly used calculation procedures for conventional residential wood-frame construction tends to over estimate the actual field thermal performance of many of today's popular housing designs, which feature large fenestration areas and floor plans with lots of exterior wall corners. This leads to the need for a thermal performance indicator to represent the whole wood-frame wall including thermal shorts created at wall interfaces with other envelope components. For this procedure to gain popular acceptance it must be accurate yet simple enough to be understood by home buyers and builders, and permit thermal performance comparisons of alternative wall systems to wood frame walls.

Currently, in the typical thermal evaluation of wood-frame wall systems, the wood framing effect (percentage reduction of clear wall area R-value to that estimated at the center-of-cavity) is handled by conducting a simple parallel-path calculation for the cavity and stud area. The area ratio between framing and cavity is almost always suggested by an authoritative source, such as the latest ASHRAE Handbook--Fundamentals (ASHRAE 1993a), or a building energy code compliance document that references an older version of the ASHRAE Handbook. Then the resulting whole-wall thermal transmittance is compared to the desired value prescribed by either an enforced building energy code, volunteer home energy rating program, or standard. Sometimes only the center-of-cavity insulation material R-value is used for comparison to alternatives. With today's residential buildings increasingly constructed with materials such as metal, stress skin-insulated core panels, and novel composites, a more accurate rating is necessary. Opaque envelopes can no longer be compared by frequently misleading "center-of-cavity" insulation material or clear wall R-values. The development of more accurate, consumer-understandable wall labels will spur greater market acceptance of energy-efficient envelope systems.

The benefit of advanced systems with only a few thermal shorts compared to a conventional wood-frame systems will be clearly discernible by comparing whole-wall thermal performance ratings. The large market share currently held by dimensional wood-frame systems, in part, reflects the misleading and inflated thermal performance ratings currently assigned them. The effect of extensive thermal shorts on performance is not accurately reflected in commonly used simplified energy calculations that are the current bases for consumer wall thermal comparisons. Consequently, the market place does not currently account for the thermal shorts that exist in building walls. This results in the consumer not realizing the full energy cost savings anticipated by complying with energy code formulas and standards or even meeting the requirements of home energy rating systems. In addition, several building trends suggest that unless more careful consideration is given to the whole-wall thermal performance, even more energy-savings opportunities will unintentionally lost. With the improvement in window efficiency, the potential exists for residential structures to have more windows. When more windows are installed in a building, the additional framing that is needed produces a higher overall thermal transmittance of the opaque wall. With metal-frame construction gaining popularity in residential construction, the thermal shorts potentially resulting from the relatively higher thermal conductivity of metal compared to wood can mean much more severe heat loss than is accounted for by traditional simplified calculations.

Why are the effects of interface details important? First of all, they are needed to properly baseline the thermal performance of common residential wood-framing systems and to more comprehensively evaluate alternatives. Second, their inclusion creates incentives for alternative wall system manufacturers to focus on the whole wall, including the critical connections to other parts of the building, not just the "clear wall."

Interface details make a difference. The consequences of poorly selected connections between envelope components are severe. Taking into account the interface details can have an impact as much as 50% of the overall wall area. For some conventional wall systems, the whole-wall R-value can be as much as 40% less than what is measured for the clear wall section. With the increasing use of alternatives to dimensional lumber-based systems, (such as metal-frame and masonry systems for residential construction), this procedure highlights the importance of using interface details that minimize thermal shorts. Local heat loss through some wall interface details may be double that estimated by simplified design calculation procedures that focus only on the clear wall. Poor interface details also may 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 propagation of molds and mildews, which can lead to poor indoor air quality.

It has been demonstrated that the whole-wall R-value of residential wall systems can be determined using a computer model (Childs 1993). More than 40 types of building wall systems already have been analyzed by this method (Kosny and Desjarlais 1994; Kosny and Christian 1995a; Kosny 1994). This approach requires a level of expertise in three-dimensional, finite- difference heat transfer modeling that is beyond what normally is available in residential building design and construction offices. Therefore, the preferred approach for making this procedure available is a user-friendly interface to a three-dimensional computer model database that incorporates this methodology for determining a whole-wall R-value for residential buildings. The interface will allow users to define the building envelope in terms familiar to the industry rather than in the more complex three-dimensional analytical models. This database retrieval tool will build upon specific experimental hot-box results, allowing easy modification for particular details and computation of the whole-wall rating for the specific system. The user of this program will see the effect of interface detail improvements and be able to use them in envelope system design-cost optimization. This proposed evaluation procedure is based on not only a computer model, but also a synthesis of experimental measurements and validated computer simulation, significantly strengthening its accuracy and building market acceptance potential.

The first two performance elements involve 1.) testing full-scale walls under steady-state and dynamic hot-box conditions, 2.) three-dimensional finite-difference computer modeling, and 3) thermal analysis of alternative interface details. Hot-box wall tests are used to validate and calibrate two- and three-dimensional computer simulations. A steady-state whole-wall R-value is derived for each system. To account for thermal mass impacts, if any, customized tables and figures are generated to reflect dynamic thermal mass benefits compared to low-mass systems (Christian 1991). This information may be needed to demonstrate compliance to the Council of American Building Officials' Model Energy Code (MEC) (CABO 1995) and ASHRAE/BSR Standard 90.2-1993 (ASHRAE 1993b).

A calculation procedure and ASTM C236 or ASTM C 976 (ASTM 1989) test are proposed as a starting point for a consensus methodology for estimating whole-wall R-value, independent of construction type. A clear wall section, 8 ft by 8 ft (2.4m x 2.4m), is tested in a guarded hot box. Experimental results are compared with two- and, if needed, three-dimensional heat conduction model predictions, based on finite-difference methods. The comparison will lead to a calibrated model. This procedure can be performed on any type of clear wall assembly: metal, masonry, wood, etc. After the model of the test wall is calibrated, simulations are made of the "clear wall" area with insulation and structural elements and eight wall interface details: corner, wall/roof, wall/foundation, window header, window sill, door jamb, door header, and window jamb which make up a representative residential whole-wall elevation. Results from these detailed computer simulations are combined into a single whole-wall steady-state R-value estimation and compared with simplified calculation procedures and results from other wall systems. A reference wall elevation must be adapted to weigh the impacts of each interface detail.

For each wall system for which the whole wall R-value is to be determined, all details commonly used and recommended (outside corner, wall/ floor, wall/ flat ceiling, wall/cathedral ceiling, door jamb, window jamb, window sill, and door header) must be available. The detail descriptions should include drawings, with all physical dimensions, and thermal property data for all material components contained in the details. If critical material component thermal conductivities are not available, it may be desirable to measure individual material conductivities, particularly if the clear wall hot-box data do not agree with the computer-model predictions.

Although not necessary for every wall system, calibration of the model by hot box measurement of a clear wall test section would enhance its credibility. The clear wall comparison of the experimental measurements and the model predictions minimizes the likelihood of systemic modeling errors throughout the wall detail simulations. The procedure requires (1) building a test wall in a hot-box frame; (2) instrumenting the test wall; (3) testing at steady state conditions; (4) preparing a laboratory test data summary report, which includes a comparison to results of an uncalibrated model of the clear wall;. (5) calibrating the model with "clear wall" hot-box results. (key material components with uncertain thermal conductivity may have to be measured if model and experiment do not agree); (6) modeling the eight details (see sample set; Figures 3 through 10) making up a typical residential wall elevation and determine the area of influence of each detail; (7) calculating whole-wall R-value; (8) conducting parametric thermal analysis to improve details and whole-wall R-value; (9) preparing a paper report and an electronic report for advanced wall database.

Eighteen system whole-wall R-values have been estimated by a finite-difference computer model (Childs 1993). For all eighteen of the systems, the procedure described above for calculating whole-wall R-value has been followed. The model used is a generalized three-dimensional heat conduction code to analyze building envelopes. It can solve steady-state and/or transient heat conduction problems in one-, two-, or three-dimensional Cartesian, cylindrical, or spherical coordinates (Childs 1993). Multiple materials and time- and temperature-dependent thermal conductivity, density, and specific heat can be specified. The boundary conditions, which may be surface-to-environment or surface-to-surface, may be specified temperatures or any combination of prescribed heat flux, forced convection, natural convection, and radiation. The boundary condition parameters can be time and/or temperature dependent. The mesh spacing may be variable along each axis. The model solves transient problems by using any one of several finite-difference schemes: Crank-Nicolson implicit procedure, classical implicit procedure, or Levy explicit method.

The accuracy of the modeling was validated using 28 test results of masonry, wood-frame, and metal stud walls (Kosny and Christian 1995b). Considering that the precision of the guarded hot box is reported to be approximately 8% (ASTM C236 [ASTM 1989]), the ability of the model to reproduce the experimental data was found to be within the accuracy of the test method.

The whole wall R-value is a better criteria then the clear-wall and much better than the center-of-cavity R-value methods used to compare most types of wall systems. The value includes the effect of the wall interface details used to connect the wall to other walls, windows, doors, ceilings and foundations.

References

HUD, "Alternatives to Lumber and Plywood in Home Construction," prepared by NAHB Research Center, Upper Marlboro, Maryland, April 1993.

Nisson, Ned, "Research Center Seeking Ways to Fix Thermal Problems in Steel Framing" Energy Design Update, Cutter Information Corp. Vol. 14, No. 3, March 1994.

Dennis, William F., "The Resurgence of Steel", ASTM Standardization News , Volume 23, Number 2, pp 36-41, February 1995.

ASHRAE Handbook of Fundamentals, American Society of Heating and Refrigerating, Air Conditioning Engineers, Atlanta, Georgia, 1993.

Kosny, J. and Desjarlais, A. O., "Influence of Architectural Details on the Overall Thermal Performance of Residential Wall Systems," Journal of Thermal Insulation and Building Envelopes, Vol. 18, July 1994.

Desjarlais, Andr¨¦, et al, "Laboratory Measurements of the Drying Rates of Low-Slope Roofing Systems," Proceedings of the Low-Slope Reroofing Workshop, Oak Ridge National Laboratory CONF 9405206, Sept. 1994.

Childs, K. W., "HEATING 7.2 Manual," Oak Ridge National Laboratory Report ORNL/TM-12262, Feb. 1993.

Kosny, J. and Christian, J. E., "Thermal Evaluation of Several Configurations of Insulation and Structural Materials for Some Metal Stud Walls," Energy and Buildings, Summer 1995 to be published).

Kosny, J., "Wooden Concrete - High Thermal Efficiency Using Waste Wood," Proceedings of the American Council for an Energy-Efficient Economy 1994 Summer Study on Energy Efficiency in Buildings, Berkeley, CA.

Christian, J. E., "Thermal Mass Credits Relating to Building Envelope Energy Standards," American Society of Heating and Refrigerating and Air-conditioning Engineers, Transactions 1991, Vol. 97, Pt. 2.

Model Energy Code, Council of American Building Officials, Falls Church, Virginia, 1995 Edition.

ASHRAE Standard Energy-Efficient Design of New Low-Rise Residential Buildings, American Society of Heating, Refrigeration and Air-Conditioning Engineers, Inc., Atlanta GA. ASHRAE 90.2-1993.

ASTM, 1989 Annual Book of ASTM Standards Section 4 Construction, Volume 04.06 Thermal Insulation; Environmental Acoustics, ASTM, Philadelphia, PA, 1989.

ASTM, 1995 Annual Book of ASTM Standards Section 4 Construction, Volume 04.07 Building Seals and Sealants; Fire Standards; Building Constructions, ASTM, Philadelphia, PA, 1995.

North American Insulation Manufacturers Association, " The Effect of Insulation on Air Infiltration", Roofing/ Siding/ Insulation, Volume 71 No. 9 September 1994.

Kosny, Jan, Christian, Jeffrey E., "Reducing the Uncertainties associated with Using the ASHRAE ZONE Method for R-value Calculations of Metal Frame Walls, ASHRAE Transactions 1995, V. 101, Pt. 2.

AISI (American Iron and Steel Institute) Residential Steel Framing Manual for Architects, Engineers and Builders, Low-Rise Residential Construction Details RG-934 American Iron and Steel Institute, 1101 17th St. N.W. Suite 1300, Washington D.C., June 1993.

USG - United States Gypsum Company "Drywall Steel Framed Systems," System Folder SA-923-1992 Edition, 092501/USG-3.

Hoke, Jr. J. R. "Architectural Graphic Standards," The American Institute of Architects, John Wiley & Sons, ISBN 0-471-81148-3.

DOE, Office of Energy Efficiency and Renewable Energy, "Voluntary Home Energy Rating System Guidelines ', Federal Register, Vol. 60, No. 142 July 25, 1995.

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Building Envelope Research

Oak Ridge National Laboratory

For more information, contact the program manager for Building Envelope Research:

Andre O. Desjarlais

Oak Ridge National Laboratory

P. O. Box 2008, MS 6070

Oak Ridge, TN 37831-6070

Voice (423)574-0022; Fax (423) 574-9338

E-mail desjarlaisa@ornl.gov

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Revised: May 12, 1998