Structural Masonry Systems
Insulated Cavity Wall System

1) General
2) Details
3) Guide Specifications Section

Details identified thus are recommended by The Masonry Institute of Washington to be used as standard tracing details by Northwest architects and designers.

1. General

"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 Sates 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. 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 materials and diverted back to the outside through weep holes.

2. Details

Rain resistance is obtained from two-stage weathertightening or rain screen walls, with an outer wythe functioning as a rain screen and with an air barrier as part of an inner wall.

The pressure from wind forces on the outer wythe is canceled by arranging for equal pressure at the inner side of the facing wall. This is accomplished by providing openings in the exterior wythe to equalize the pressure on both sides of the wythe. The pressure being equalized, water in cracks and pores will not migrate inward or outward. The inner wall must provide the structural capacity to resist the wind pressure, plus by means of insulation an adequate degree of airtightness to keep the equalization system working. If the cavity does not have an opening weep and/or vent holes to the outside, there will be a pressure differential across the outside wythe, and the wall will not perform as a rain screen. By opening the cavity to the outside, air pressures will be equalized on both sides of the outer wythe, making the rain screen effective. Any moisture penetrating the outer wythe will not be in contact with the inside wall and can drain through the cavity to the outside at flashings and weepholes.

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The NW Masonry Guide Table of Contents

Masonry Institute of Washington
Washington State Conference of Mason Contractors