Copyright Goodheart-Willcox Co., Inc. Chapter 14 Wall Systems 281 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 that passes through the exterior wythe runs down the cavity face of that wythe. At the bottom of the cavity, the moisture is diverted to the outside by continuous flashing and weep holes. Due to the decreased moisture problem, there is less chance of efflorescence than with solid walls. 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 increased by the use of insulation in the cavity. Thus, considerable energy savings can be realized by proper design, detailing, construction, and use of cavity walls. Resistance to transmission of sound is accom- plished with heavy massive walls or discontinuous construction. A cavity wall employs both of these— the weight of the two masonry wythes plus the partial discontinuity of the cavity. In cavity wall construction, an air cavity provides a partial isolation between the two wythes. Sound on one side of the wythe strikes it and causes it to vibrate, but because of the separation and cush- ioning effect of the cavity, plus the massiveness of the wythes, the vibration of the other wythe is greatly reduced. The results of ASTM E119 fire resistance tests show that masonry cavity walls have excellent fire resistance. Fire resistance ratings of cavity walls range from two hours to four hours, depending upon the wall thickness and other factors. Due to their high fire resistance properties, brick walls make excellent fire walls for compartmentation (division into separate sections) in buildings. Compartmentation can halt the spread of fire. Cavity walls that are properly designed, detailed, and constructed can be used in any building requiring loadbearing or nonloadbearing walls in the same manner as other masonry walls. Separation of the wythes and the use of metal ties results in increased flexibility that permits freedom of differ- ential movement between the wythes. This is critical in construction that makes use of combinations of dissimilar materials. Refer to Chapter 10, Laying Brick for cavity wall construction procedures. Insulation Suitable types of insulation materials for cavity walls are rigid boards and granular fills. Each of these, if properly used, produces a thermally effi- cient wall. See Figure 14-9. Expanded or molded polystyrene, expanded poly- urethane, rigid urethane, cellular glass, preformed fiberglass, and perlite board are used as rigid board insulation materials in cavity walls. See Figure 14-10. Rigid board insulation is installed horizontally within the air space against the cavity face of the backup Portland Cement Association Figure 14-8. Flashing should be installed to prevent moisture from entering the inner wall from the cavity. Brick Industry Association Figure 14-9. Rigid board insulation in a cavity wall. Minimum 1" clearance Granular fill or rigid board insulation Weep holes @24" O.C. Flashing Minimum 2" cavity Metal ties