Building Concrete Masonry Homes: Design and Construction Issues This is a summary of a report prepared by the NAHB Research Center for the U.S. Department of Housing and Urban Development, the National Concrete Masonry Association, the Portland Cement Association, and the National Association of Home Builders. The full report is available in PDF format. Executive Summary Although many home builders use concrete masonry units (CMU) for foundation walls, home builders attempting more comprehensive use of CMU have many questions about the feasibility and appropriate use of concrete masonry construction. In an effort to address a number of these questions the construction of two single-wythe CMU homes in non-traditional CMU markets, i.e., northern climates, were fully documented. Although these case study homes cannot address all of the issues involved in concrete masonry construction, key results are presented below. The case study homes were in Ohio and Minnesota. Building codes and plan preparation: There are two methods of design used to provide compliance with local building codes, empirical design and engineered design. The empirical design method is often used for single-family dwellings for reasons of simplicity as well as the elimination of engineering costs. The empirical design approach is limited to short buildings (under 35 feet in height) and buildings in low seismic zones and low wind areas. The engineered design approach does introduce additional costs, but can help address special design conditions. The Ohio case study home was empirically designed and the Minnesota case study home employed the engineered design method. Wall thickness: The three model building codes and CABO prescribe two different minimum requirements for the type of block used for single story buildings. While all four codes prescribe 6-inches as the minimum thickness, CABO and UBC prescribe that the 6-inch block must be of solid masonry or 8-inch thick hollow-core block is required. The above-grade masonry walls of the single-story Ohio case study home were built with 8-inch thick hollow-core block. The above-grade masonry walls of the two-story Minnesota home used 6-inch thick hollow-core blocks with the addition of vertical steel reinforcement bars. Lateral support of walls: Lateral support of walls was not a design issue with the two case study homes, nor will it be an issue for many low-rise residential structures. Homes with tall, unsupported exterior walls or high ceilings, e.g., “great rooms” and entry foyers, as well as buildings with 6-inch thick block will require the attention of a design professional. Crack control: The case studies illustrated the uncertainty surrounding the need for bond beams, horizontal joint reinforcement, and control joints. While the concrete masonry industry offers recommendations for controlling cracks in masonry walls, the model building codes do not include prescriptive requirements for crack control. Practice in this area varies on an individual basis. Connections to concrete masonry: The case study homes illustrated several different techniques for detailing building elements such as floor decks, insulation, windows and doors, flashing, and gypsum wallboard. Several photographs and construction drawings from the case studies are provided throughout the body of the report as well as in Appendix A. Energy performance: The level to which the CABO Model Energy Code (MEC) recognizes thermal mass benefits is based on heating degree days and the location of the insulation. The Ohio case study home (in a climate of approximately 5,400 heating degree days) contained insulation in the core of the block, i.e., “integral” insulation as defined in MEC. Therefore some benefit from thermal mass was achieved, and approval was obtained from the local building department. At the Minnesota case study site the number of heating degree days (approximately 8,200) effectively negated the thermal mass effect and no benefit in reduced insulation was realized.
The empirical design approach in the model building codes is somewhat confusing to the novice user, does not include many common details or fastening requirements, and in some cases may not provide the most cost-effective concrete masonry wall assembly. The home building industry would benefit from a “best practices guide” for residential concrete masonry construction which would simplify key structural items and include support for novice masonry builders. |
