CAVITY WALL

 

Cavity wall consists of two walls with a 5 cm. to 8 cm. cavity between them. The outer wall also known as outer leaf, consists of a 10 cm. thick wall and the inner wall is sufficiently thick and strong to carry the imposed load safely. The minimum thickness of the inner wall is restricted to 10 cm. the provision of a continuous cavity in the wall efficiently prevents the transmission of dampness to the inner wall.

ADVANTAGES

The advantages of cavity walls may be summarized as below:

1.As there is no intimate contact between the two leaves except at the wall ties, there is no possibility of the moisture travelling from the outer leaf to the inner.

2. The layer of air in the cavity being non-conductor of heat reduces the transmission of heat from the external face to the internal one and as such cavity walls are best suitable for a tropical country like India. Tests have revealed that cavity walls have 25% greater insulating value than solid walls.

3. They have good sound insulation property.

4. They are economical.

POSITION OF CAVITY

The cavity should start near ground level and terminate near eaves level in case of sloping roof or near coping in case of flat roof with parapet wall. The cavity should preferably start 15 cm. below the damp proof course level. This has the advantages of draining any condensed moisture below the level, of damp proof course. Similarly, to prevent infiltration of moisture, the bottom of cavity should lie at least 15 cm. above the outside ground level. The damp proof course for the two leaves is laid separately, although at the same level. This is necessary to continue the cavity below damp proof course. The cavity is kept fully ventilated by providing air bricks in the external wall immediately above damp proof coarse. The air bricks should be kept 90 cm. apart. Similarly, air bricks are provided near the top of the cavity.

WALL TIES

The two walls are tied together with metallic or terra-cotta ties to obtain structural stability. They are spaced 90 cm. apart horizontally and 30 to 45 cm. apart vertically in staggered positions.

CONSTRUCTIONAL DETAILS

The constructional treatment given to the cavity wall at the door or window openings requires special considerations.

Since the object of cavity is to separate the two leaves, a vertical damp proof coarse is essential to be provided when sealing the cavity at window and door reveals, as the openings can provide an easy access to the dampness to pass into the cavity. At the sill level it is always advantageous to keep the back of the sill flush with the back face of the external wall. In case of wooden sills, a horizontal damp proof course is introduced under the sill. The damp proof course used should be of flexible material.

At the top of openings of doors and windows, stone or brick arch or concrete lintel is provided to carry the loads from structure above the opening. In order that the cavity may be continuous, the two walls should have separate lintels or arches. In such a case, an inclined strip of lead or any other flexible damp proof course membrane is provided so as to bridge over the cavity. The damp proof course should extend lengthwise beyond the frame for a distance of about 15 cm. on either side. This treatment helps in draining any condensed moisture beyond the sides of the frame.

In case of flat roof, the cavity is continuous for the full height of parapet wall and is sealed by the coping. In case of inclined roofs, cavity is closed at eaves level. It is important to take special precautions so that no mortar should fall inside the cavity during the process of construction. For this purpose, a wooden batten is supported over the wall ties and the construction of brick work is carried out to the height where next row of all wall ties have to be provided. There after the wooden batten is lifted up, cleaned of mortar droppings, and replaced over next row of wall ties. By this method, the cavity remains clean of mortar droppings.

If the cavity is to be continued to the coping level, the layer of damp proof course should be laid continuous covering the full thickness of wall immediately below coping. In case, it is desired to stop the cavity at the mid height of the parapet wall, damp proof course layer is laid to the full thickness of the wall at level where the cavity joins the solid wall.

Exposure of the two faces of a solid parapet wall may develop cracks due to temperature variations or on account of bad work manship which may invite dampness. By constructing two separate wall thickness, this danger can be successfully overcome. Each wall thickness is exposed to the weather on one side and to the drying out condition on the other. In places of heavy rainfall, when the damp storage capacity of the material used in the outer wall gets exceeded, the rainwater finds its way into the cavity and gets harmlessly drained off at suitable points. Thus the cavity system of construction is equally useful for parapet walls.

PRECAUTIONS

·      As far as possible there should be no intimate contact between the two leaves of the cavity wall.

·      The half brick thick leaf of the cavity wall should be constructed with stretcher bond.

·      The damp proof course layer should be laid separately for the two leaves.

·      The horizontal damp proof course layer should be laid at least 15 cm. above the bottom of the cavity.

·      The openings should be specially treated as described before.

·      Ties used should be rust proof and should preferably be protected by galvanising.