CAVITY WALLS
INTRODUCTION
The term «cavity wall» is applied to 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 means of metal ties for structural strength. They are extensively used in European countries, particularly Great Britain, where they have been developed as a means of obtaining protection from penetration of rain through masonry walls.
In recent years in North America many important buildings have been constructed with cavity walls.
METHODOLOGY
In the construction of a cavity wall there are no changes required in basic bricklaying techniques, only modifications of practices commonly used in the construction of any brick masonry wall. The fundamental principle in a cavity wall is that there shall be no bridge of solid material capable of carrying water across the minimum 2-in. (50-mm) cavity space.
WORKMANSHIP:
The importance of the workmanship used in constructing masonry has been stressed by many, sometimes to the point that it may appear that workmanship alone is responsible for the performance of masonry walls, regardless of the wall design, detailing, or the materials used. While this is by no means true, good workmanship is a very important factor in the construction of high performance masonry.
KEEPING THE CAVITY CLEAN:
It is vital that the cavity be kept clean of mortar droppings and other foreign materials. If mortar falls into the cavity, it may form "bridges" for moisture passage, or it may fall to the flashing, blocking the weep holes.
Over the years many methods have been developed and considerable time and discussion have been devoted to the proper method to use in keeping the cavity clean. One method is to take a wooden or metal strip, slightly smaller than the cavity width, and place it in the air space. This strip rests on the wall ties as the wall is built. Wire or rope is attached to the strip. Then, as the brick mason builds the wall, this strip is easily lifted out. Before the next row of ties is placed, any mortar which may have fallen into the cavity is removed (Figure 1).
In addition to the above mentioned methods of cleaning the cavity, the brick mason can use techniques that, if properly applied, should eliminate a considerable amount of mortar falling into the cavity in the first place
After spreading the mortar bed, the brick mason should bevel the cavity edge with the flat of the trowel . When mortar is spread in this manner, very little will be squeezed out of the bed joints into the cavity when the units are laid
The brick units are next rolled into place, keeping most of the mortar on the outside
After the brick mason has placed the unit on the bed joint any mortar fins protruding into the cavity should be flattened over the backs of the unit, not cut off . This prevents the mortar from falling into the cavity and provides a smooth surface, which will not interfere with insulation materials, which may be placed in the cavity.
TOOLING:
Weather tightness and textural effect are the basic considerations of mortar joint finish selection and execution. Properly "striking" or "tooling" the joint helps the mortar and brick units bond together and seal the wall against moisture. Nine common joint finishes are shown in Figure 8 in order of their decreasing weather tightness. Compression of the mortar makes the concave, V, and grapevine joints the most weather tight and acceptable to use. The remaining six joint types are not recommended for exterior use. All holes in the mortar joints should be filled. Joints should be tooled when the mortar is "thumbprint" hard.
WEEP HOLES:
Weep holes must be placed at the base of the cavity and at all other flashing levels. They provide a means of draining away any moisture that may have found its way into the cavity. Weep holes must provide a clear access to the cavity and must be placed directly on the flashing for proper drainage.
Weep holes can be easily created or installed by various methods. In order of effectiveness these are:
Eliminating each second or third head joint.
Inserting oiled rods, rope or pins in the head joint at a maximum of 16-in. (410-mm) and removing before final set of the mortar.
Placing metal or plastic tubing in the head joint at a maximum of 16-in. (410-mm).
Placing sash cord or other suitable wicking material in the head joint at a maximum of 16-in. (410-mm).
TIE PLACEMENT:
In a properly constructed cavity wall, both Wythe of masonry must be adequately and properly tied together. The main concern to the designer is assurance that all of the ties are in place and remain operative, firmly embedded in and bonded to the mortar. To achieve this, the two Wythe of the cavity must be laid with completely filled bed joints, and the ties must be in the correct position so that later disturbance of the wall assembly is unnecessary. There exists extensive data showing that wall ties have excellent tying capacity if they are well embedded in the masonry, and also that the cavity wall is a weak structural member if few working ties must do the job of many
From a performance standpoint, the most important factors for wall ties are:
Being corrosion resistant.
Placing ties at proper spacing. Spacing of ties should be reduced by one half for ties with drips. Crimping of the metal ties to form a drip is not necessary, and will decrease the strength of the tie.
Full bedding of the bed joint and placing the wall tie in the mortar 5/8 in. (16 mm) from either edge of the brick
PROTECTION OF WALLS:
Masonry walls exposed to weather and unprotected during construction can become so saturated with water that they may require weeks, or even months (depending upon climatic conditions), to dry out. This prolonged saturation may cause many of the slightly soluble salts to go into solution, thus raising the possibility of efflorescence. Such conditions may also contribute to the contamination of the masonry with soluble salts from elsewhere in the construction (concrete, concrete block, plaster, trim, etc.).