Masonry Magazine March 1975 Page. 19
NCMA-TEK
An Information series from National Concrete Masonry Association
Concrete Masonry
Cavity Walls
62
Introduction
"Cavity wall" is a type of masonry wall construction in which a continuous air space or cavity is provided inside the wall. A cavity wall therefore is actually two walls separated by an air space but joined by metal ties so the wythes will tend to support each other. They have become a standard type of construction in many European countries, particularly in England where they have been used for a great many years. There is evidence that masonry cavity walls were built in the United States as far back as 70 years ago but their use was very limited during the early decades of this century. In 1937, FHA issued a bulletin permitting the use of cavity walls for exterior enclosing walls of dwellings otherwise eligible for mortgage insurance. Since that time their acceptance has grown substantially. Limited to exterior loadbearing walls in one and two story buildings for many years, the masonry cavity wall today is used for the design and construction of curtain and panel walls as well as loadbearing walls in multi-story buildings.
Properties
Moisture Resistance
The most important property of the cavity wall is its ability to resist rain penetration. Properly designed and constructed cavity walls do not permit rain or moisture to penetrate the inner wythe as may be the case for some solid masonry walls under severe conditions of exposure to prolonged wind-driven rain. Rain which penetrates the outer wythe of a cavity wall runs down the inner surface of the outer wythe where it is collected at the base with flashing material and diverted back to the outside through weep holes.
Thermal Resistance
Another important property of cavity walls is its thermal insulation value. The continuous air space breaks the continuity of the masonry, which may have less resistance to heat flow, and improves its thermal insulating value considerably. A comparison of calculated U-factors is shown in Table 1. In the table, a composite wall, consisting of 4" dense brick outer wythe with 4" hollow concrete block of various densities for the inner wythe, is compared with cavity wall construction of the same units. The only difference is the 2" air space in the cavity wall. It is apparent that the cavity will improve the thermal insulation by 30% to 100% depending upon the density of units used. The table also shows the influence of adding loose-fill insulation in the cavity.
TABLE 1. Comparison of U-Factors for composite walls and cavity walls built with the same masonry units.
| Wall Type | Unit Weight, Ib. per cu, ft., of concrete used for hollow block |
| ----------- | ----------- | ----------- | ----------- | ----------- | ----------- |
| | 60 | 80 | 100 | 120 | 140 |
| Composite* U-Factor | .30 | .45 | .50 | .55 | .66 |
| Cavity* U-Factor | .23 | .25 | .27 | .29 | .33 |
| Cavity*, air space filled with loose fill insulation U-Factor | .14 | .14 | .15 | .15 | .16 |
| *Wall construction - 4" dense, solid units outer wythe; 4" hollow units of various densities inner wythe. | | | | | |
TEK 62 © 1975 National Concrete Masonry Association
Concrete Units