Masonry Magazine March 1975 Page. 21
Another restriction for cavity walls is with reference to height above the walls' support. For 10" thick cavity walls the code requires that the height not exceed 25 feet above the support. This may be increased to 35 feet if the wall is thicker than 10 inches. For buildings taller than this it is required to provide intermediate supports so as not to exceed allowable height above the support.
Code requirements for tying wythes of a cavity wall together are spelled out in detail: "The facing and backing of cavity walls shall be bonded with 3/16-inch diameter steel rods or metal ties for not more than each 4 1/2 square feet of wall area. Ties in alternate courses shall be staggered, the maximum vertical distance between ties shall not exceed 18 inches, and the maximum horizontal distance shall not exceed 36 inches. Rods bent to rectangular shape shall be used with hollow masonry units laid with the cells vertical; in other walls the ends of ties shall be bent to 90-degree angles to provide hooks not less than 2 inches long. Additional bonding ties shall be provided at all openings, spaced not more than 3 feet apart around the perimeter and within 12 inches of the opening. Ties shall be of corrosion-resistant metal, or shall be coated with a corrosion-resistant metal, or other approved protective coating."
Vapor Barriers
In the early development of cavity walls it was recommended that a vapor barrier be installed to prevent condensation and frost from forming within the cavity. However, long-time performance of many cavity walls containing no vapor barrier has indicated no harmful effects of condensation in the walls. In a properly functioning cavity wall condensation would be diverted to the exterior through weep holes, similar to water which may have penetrated the exterior wythe. Under conditions where high humidity is maintained in a building where outside air temperature is very low for a long period of time it would be prudent to provide a vapor barrier or otherwise design the wall section if it is desired to prevent condensation from occuring.
TABLE 2. Comparison of sound transmission class for composite and cavity walls.
| Wall Surface Treatment | Composite Wall* Wall Weight Ibs./sq./ft. | STC dB | Cavity Wall* Wall Weight Ibs./sq./ft. | STC dB |
|---|---|---|---|---|
| None | 58 | 51 | 56 | 54 |
| ½" plaster, one side | 61 | 53 | 59 | 57 |
| ½" gyp board, one side | 60 | 56 | 58 | 59 |
*Wall construction outer wythe 4" dense concrete brick; inner wythe 4" hollow lightweight concrete block.
TABLE 3. Allowable compressive stresses for cavity walls.
| Construction | Allowable compressive stresses gross cross-sectional area of wall minus area of cavity between wythes Type M Mortar | Type S Mortar | Type N Mortar |
|---|---|---|---|
| Cavity Walls* Solid Units: 2500 psi plus | 140 psi | 130 psi | 110 psi |
| 1500 to 2500 psi | 100 psi | 90 psi | 80 psi |
| Hollow Units | 70 psi | 60 psi | 55 psi |
*The allowable compressive stresses for cavity walls are based upon the assumption that the floor loads bear upon but 1 of the 2 wythes. When cavity walls are loaded concentrically, the allowable stresses may be in- creased by 25 percent.
Construction
The advantages of a cavity wall in resistance to rain penetration can be easily lost if the wall is not built properly. Since the exterior wythe of a cavity wall is usually not coated or waterproofed, it may be anticipated that under certain conditions some water will penetrate through to the cavity. This water must be drained back out to the exterior before it can accumulate in the cavity and migrate to the interior of the wall. Satisfactory performance of the wall in this regard depends upon proper installation of flashing, weep holes, and good workmanship.
Flashing
Good quality, permanent type flashing material should be used at the bottom of the cavity and also over openings in the wall. The flashing is placed beneath the outer part of the wall and is shaped so that it turns up behind the outer wythe and is carried up into a mortar joint of the inner wythe. Frequently the flashing must be spliced because of long lengths of wall. Special care must be taken to insure that splices are thoroughly sealed to prevent water from entering at these locations.
Weep Holes
Weep holes, used to drain the cavity, are usually placed in a vertical mortar joint of the outer wythe immediately above the flashing and located from 32