Masonry Magazine August 1978 Page. 9
Technical Notes
on Brick Construction
Brick Institute of America 1750 Old Meadow Road, McLean, Virginia 22101
BRICK MASONRY CAVITY WALLS
2 (51 mm) MINIMUM CAVITY
METAL TIES
WEEP HOLES
@ 24" (610 mm) O.C.
FLASHING
Typical Brick Cavity Wall
FIG. 1
INTRODUCTION
Brick masonry cavity walls consist of two wythes of masonry separated by an air space, and tied together with corrosion resistant metal ties (see Fig. 1). The exterior masonry wythe is brick or hollow brick, and the interior masonry wythe can be brick, hollow brick, structural clay tile, or hollow or solid concrete masonry units, depending on the required wall properties and features. The cavity of 2 to 42 in. (51 to 114 mm) between the two wythes may be either insulated or left as air space. In either case, parging of the cavity face of either wythe is neither necessary, nor recommended. The interior surface of the cavity wall may be left exposed or finished in a normal way.
Cavity walls, long common in Europe, were first built in the United States over 80 years ago. However, it was not until 1937 that this type of construction gained official acceptance by any building or construction agency in the U.S. Since then, interest in and use of cavity walls in this country has increased rapidly. This has resulted in extensive testing to determine cavity wall properties.
The early use of cavity walls in this country was limited primarily to exterior loadbearing walls, one and two-stories high. In the 1940s, designers of high-rise buildings began to recognize the advantages of cavity walls and use them as curtain and panel walls in structural frame buildings.
21
Revised
Jan/Feb
1977
Today, masonry cavity walls are used extensively throughout the United States in all types of low- and high-rise buildings. The primary reasons for their popularity are: superior rain penetration resistance, excellent thermal capabilities, good sound transmission resistance, and high fire resistance.
This Technical Notes will discuss the properties of cavity walls, and the proper design to achieve these properties. Other issues in this series deal with insulated cavity walls, details and proper construction practices.
PROPERTIES OF CAVITY WALLS
Resistance to Moisture Penetration
One of the major functions of an exterior wall is to resist rain penetration. A masonry cavity wall, properly designed and built, is totally resistant to rain penetration through the wall assembly. The outside wythe may not be totally resistant to moisture penetration, but the overall design of the cavity wall assembly takes this into account. In a cavity wall, any moisture which may pass through the exterior wythe will run down the cavity face of that wythe to the bottom of the cavity where it is diverted to the outside by continuous flashing and weep holes. Due to the decreased moisture problem, the chance of efflorescence may be less than with solid walls. For further discussion of moisture penetration refer to Technical Notes 7 Series.
Thermal Properties
Heat losses and heat gains through masonry walls are minimized by the use of cavity wall construction. The complete isolation of the exterior and interior wythes by the air space allows a large amount of heat to be absorbed and dissipated in the outer wythe and cavity before reaching the inner wythe and the building interior. This ability is further increased by the use of insulation in the cavity. Steady-state U values can range from 0.40 (2.27 W m² K) for an 8 in. (203 mm) uninsulated cavity wall to 0.06 (0.34 W m²K) for a cavity wall with 2-in. (51 mm) polyurethane board insulation. This is further enhanced when considering dynamic conditions and using the "M" factor (see Technical Notes 4B). Thus, considerable energy savings can be realized by proper design, detailing, construction and use of cavity walls. Refer to Technical Notes 4 Series and 21A Revised for U values and further information about insulated cavity walls.