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| Keywords: | ASHRAE, energy codes and standards, energy conservation, multi-family housing, residential, thermal mass |
ASHRAE Standard 90.2-1993, Energy Efficient Design of New Low-Rise Residential Buildings (ref. 2) contains design requirements for new residential buildings. This includes one and two-family houses, townhouses, apartment buildings and condominiums (three stories or fewer above grade), mobile homes, and manufactured (modular) houses. Less permanent housing, such as hotels, nursing homes, and barracks are not governed by the requirements of the Standard.
Standard 90.2 is the state-of-the-art national energy efficiency standard for low-rise residential buildings. The energy requirements are intended to be an advance in energy efficiency, as well as being economical. For concrete masonry homes, the Standard includes the energy efficiency of thermal mass. Thermal mass describes concrete masonrys ability to store and release heat slowly over time, reducing the need for added insulation.
Traditionally, ASHRAE energy standards have been adopted by model code organizations, and incorporated, either by reference or in total, into model and local energy codes. Although the Standard is not written in code language, ASHRAE has published a codified version of Standard 90.2 (ref. 1).
Standard 90.2 includes requirements for the building envelope, the heating and air-conditioning systems, and water heating systems.
Compliance is achieved through either the prescriptive or the systems analysis methods. The prescriptive criteria contains an independent energy requirement for each building component, such as exterior walls or duct insulation. This method is the easiest to use, but is the most stringent and offers the least design flexibility.
The systems analysis approach bases compliance on achieving an acceptable annual energy cost for the building. This method requires a detailed analysis, but allows for increased design flexibility, as well as accounting for energy efficient features such as passive solar or innovative designs. A full discussion of the systems analysis method is beyond the scope of this TEK.
The Standard presents prescriptive wall insulation requirements as a series of graphs, as shown in Figure 1. One set of graphs covers single family houses, including townhouses, and a second set applies to multi-family structures. The user determines the required wall U-factor based on the heating and cooling requirements for the buildings location. (U-factor is the inverse of R-value -- the steady state resistance to heat flow.)
For example, heating and cooling climate data for Raleigh, North Carolina is: HDD65 = 3538, CDH74 = 11,845, respectively. Using these data, Figure 1 shows that the required maximum U-value for exterior concrete masonry walls in a multi-family building is 0.172 Btu/hr.ft2ºF, which corresponds to an R-value of 5.81 hr.ft2ºF/Btu. For simplicity, the tables in this TEK list wall requirements by city, rather than by climate data. Climate data for cities throughout the United States and Canada are listed in the ASHRAE 90.2 Standard.
The Standard contains similar graphs for different exterior wall constructions, walls adjacent to unconditioned space, floors, ceilings, basement walls, fenestration, and doors.
Two sets of requirements for concrete masonry walls are included in the Standard. The first applies to concrete masonry walls with exterior or integral insulation. (Integral insulation refers to insulated cores of concrete masonry units.) The second case applies to walls with interior insulation, such as furring with batt insulation. To take full advantage of thermal mass, concrete masonry walls should be insulated on the exterior or integrally, to keep the masonry in contact with the interior environment.
The prescriptive building envelope requirements in Standard 90.2 can be modified somewhat using a trade-off procedure outlined in Section 5.9 of the Standard. The procedure allows less stringent requirements for some components, as long as the performance of other component(s) is increased such that there is no net increase in the buildings heating and cooling loads.
The prescriptive requirements for above grade concrete masonry walls of single and multi-family housing are listed in Table 1 for various locations. These required R-values are for the opaque wall only, excluding fenestration and doors. The fenestration area can be as large as 125 ft2 before the trade-off procedure, discussed above, must be used. The Standard contains separate requirements for walls adjacent to unconditioned spaces, such as the wall separating a garage from the interior of a house. Note that the requirements listed in Table 1 and Table 2 are based on Standard 90.2. Local codes will govern the design and should be consulted for local energy requirements.
Similar requirements for below-grade basement walls for single and multi-family housing are listed in Table 2. These requirements apply to the exterior walls of any story where more than 50% of the gross exterior wall area is below grade. For example, if a walkout basement has more than 50% of the wall area above grade, the basement walls are considered above grade walls for the purposes of complying to Standard 90.2. In unheated basements, the basement walls do not require any insulation where the floor above is insulated in accordance with Standard 90.2. The values listed in Table 2 for the opaque wall only. In addition, air films and the effect of the soil are neglected.
Virtually any R-value requirement can be met using one of the many concrete masonry systems available. The information below provides examples of some of the concrete masonry walls that comply with the ASHRAE Standard 90.2 requirements. These are a few of the many options available with concrete masonry construction. For example, concrete block shapes have been developed with reduced web areas, to reduce heat loss through the webs and accommodate insulation inserts.
Concrete masonry R-values vary with factors such as aggregate types, mix design, and moisture content. References 3 and 4, or manufacturer's data, should be consulted to verify compliance.
Above Grade Walls:
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Examples of walls that meet the requirement (refs. 3, 4): |
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Below Grade Basement Walls:
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Examples of walls that meet requirement (ref. 4): |
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1. Energy Code for New Low-Rise Residential Buildings, Codification of ASHRAE 90.2-1993 Energy-Efficient Design of New Low-Rise Residential Buildings. Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.
2. Energy Efficient Design of New Low-Rise Residential Buildings, ASHRAE 90.2-1993. Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 1993.
3. R-Values of Multi-Wythe Concrete Masonry Walls, TEK 6-1A. National Concrete Masonry Association, 1995.
4. R-Values of Single Wythe Concrete Masonry Walls, TEK 6-2A. National Concrete Masonry Association, 1996.
