Cavity Walls

CAVITY WALLS

Cavity walls are those which are constructed in such a way that an empty space or cavity is left between the single wall. They are also known as hollow wall. Cavity walls are two walls constructed as a single wall. A little space is lifted between these two walls for insulation purposes. These two walls having little space between them for insulation purpose are known as leaves of the cavity wall. The outer wall is called an external leaf, and the inner wall is called an internal leaf. These two leaves of cavity wall are interconnected by links or metal ties for a strong bond.

The inner leaf is found to take a greater portion of the imposed load transmitted by floor and roof. Hence, the two leaves of the wall are bonded together with ties (wall ties) usually placed 900 mm apart vertically and 450 mm horizontally in every 6th course staggered. This gives more than two ties per square meter. These wall ties are made from mild steel wires of 3 to 4 mm diameter or MS bars. In very important works, copper may be used. The bond to be used for both the leaves, when the thickness is half brick, is the stretcher bond. The inner leaves are made thicker for carrying heavy loads, hence English bond can be used for that part. Particular care is required at the top and bottom of the walls and around openings in the cavity wall construction.

PURPOSE OF CAVITY WALLS

The use of cavity walls is common in cold countries. However, when used in tropical countries, sufficient precautions must be taken to see that they do not become breeding places for lizards and insects. The major purposes of cavity walls are as follows:

❖ Damp Prevention: Cavity wall reduces the chance of moisture intrusion from outer leaf to the inner leaf and thus helps in keeping inside of the building free from dampness.

❖ Thermal Insulation: The air in the cavity wall acts as a non-conductor of heat and hence minimizes the transmission of heat from the outer leaf to the internal face of the interior leaf. Thus, cavity walls help in maintaining the thermal insulation of a premise.

❖ Sound Insulation: The air in the cavity acts as a cushion for absorbing sound. By building cavity walls, a premise may work as a sound proof zone as a large quantity of external noise gets absorbed within the cavity.

❖ Efflorescence Prevention: As dampness is not allowed to penetrate the cavity, the inner leaf of the cavity, which is always a load bearing wall, is kept free from efflorescence effects.

POINTS TO BE CONSIDERED IN CAVITY WALL CONSTRUCTION

The following are the Points to be considered during cavity wall construction:

❖ The cavity should extend to 15 cm below the damp-proof course level. Damp proof course should be laid to both leaves of the wall.

❖ The upper part of the wall where it ends should also be built solid for two or three courses below the wall plate or roof line, to stiffen the head of the wall and distribute the load over both leaves.

❖ The wall ties must be kept free from mortar droppings by means of a timber batten suspended in the cavity and raised as the work proceeds during its construction.

❖ In exposed positions, a few vertical joints in the outer leaf are left the bottom of the cavity to permit water to drain away.

❖ A certain amount of ventilation to the cavity is desirable to prevent stagnation of air and excessive humidity.

❖The cavity walls should not be built solid at the jambs [the sides of door and window openings] unless a vertical "damp-proof course" is inserted to prevent water driving to the inner face.

❖ A lead, galvanized iron or other suitable material made to form a trough or gutter, may be placed in the cavity above all openings for exposed doors and window to collect water which may drive through the outer leaf.

❖ The cavity wall should not be built solid below window sills also, and a damp-proof course is desirable at this point also.

ADVANTAGES AND DISADVANTAGES OF CAVITY WALLS

Advantages

❖ Cavity walls are best for damp prevention than solid building walls.

❖ They work best as heat insulators; it can decrease heat transmission from the outside environment.

❖ Sound waves travel faster in solid walls as compared to hollow walls. Hence, the cavity walls are also best for sound insulation.

❖ The construction cost of the cavity wall is about 20% less than the construction of solid walls. Hence, they are also economical.

Disadvantages

❖ Highly skilled labor and masons are required for cavity wall construction.

❖ Require standard supervision during its construction.

❖ A vertical damp proof course is also necessary for it.