What Is Cavity Wall? | Functions| Advantages|
What is Cavity Wall?
A cavity wall or hollow wall is one that consists of two separate walls, called leaves or skins, with a cavity or gap in-between. Both leaves of a cavity wall may be of equal thickness if it is a non-load-bearing wall, or the internal leaf may be thicker than the external leaf, to fulfill the structural requirements.
The two portions of the wall shall be connected together by metal pins or bonding bricks at required intervals. Cavity walls are preferably constructed to provide better thermal insulation to the building. It also prevents dampness to enter and acts as sound insulation. Thus they are generally the external walls of the building. The size of the cavity varies from 40 to 100 mm. The inner and outer skins should not be less than 10 cm each (half brick).
Features of Cavity Wall
Fig. 9.1 shows the vertical sections of various types of cavity walls for flat and inclined roofs.
- In the case of a cavity brick wall, each leaf is half brick thick. Such a wall is capable of taking a load of a two-storeyed building of the domestic type. However, if heavier loads are to be supported, the thickness of the inner leaf can be increased in the multiple half brick thickness.
- The cavity should neither be less than 40 mm nor more than 100 mm in width.
- The inner and outer skins are adequately tied together by means of special wall ties placed in suitable arrangements, at the rate of at least five ties to a square meter of wall area. [According to Building Regulations of U.K., the ties must be ‘ placed at distances apart not exceeding 900 mm horizontally and 450 mm vertically] The ties are staggered. Ties must be placed at 300 mm vertical intervals at all angles and doors and window jambs to increase stability.
- Since the cavity in between separates the two leaves of the wall, to prevent entrance of the moisture, it is essential to provide a vertical and flexible damp proof course at window and door openings.
CONSTRUCTION OF CAVITY WALL
Generally, the cavity wall is constructed centrally over the base of concrete, without any footings or foundations. According to I.S. recommendations, the lower portion of the cavity may be filled with lean concrete up to a few centimeters above the existing ground level.
The top of the filling should be sloped, with weep holes at 1 m intervals along with the outer leaf of the wall. The inner leaf may be of common bricks and the outer leaf with any designed kind of facing bricks or it may also be common bricks with a rendered finish. The two leaves should be tie together with wall ties.
Bonds for cavity wall construction should consist of stretcher bond for half brick leaves and any ordinary bond, like English bond or Flemish bond for leaves which are one brick or more in thickness. Where solid walls are joining cavity walls, bonding of former into the latter should conform to the principle.
Stretchers in the solid wall should extend half-brick into the inner leaf of the cavity wall and closers as shall be used for good bonding.
Bricks should be laid very carefully to leave them cavity-free from mortar droppings. Two leaves of the wall should be raised simultaneously and uniformly. The position of wall ties should be predetermined so as to have uniform spacing preferably in centers. The cavity should be made free from rubbish and mortar droppings by means of a timber batten 25 mm thick and width about 12 mm less than the cavity, resting over the ties.
The battens may be lifted by means of wires or rails attached to the battens, as shown in Fig. 1.3. The batten is supported on wall tiles and the brickwork is carried out on either side of the batten, to the height where the next row of wall ties are to be provided. After this, the batten is lifted up, cleaned of mortar droppings, and replaced over the next row of wall ties.
Precautions To Be Taken Durin Cavity Wall Construction
- The contact between the inner and outer ‘leaves should be the least.
- Ties should be strong and rust-proof. They should not permit the transmission of water along with it from the outer face to the inner face
- The damp proof course should be laid separately for both leaves.
- The bottom-most horizontal damp proof course should be laid at least 150 mm above the bottom of the cavity, or above the top of the concrete fill in the cavity.
- The bottom of the cavity should be well-ventilated by the use of air bricks and ducts.
- Weep holes or narrow vertical joints should be left in the first course.
Advantages Of Cavity Wall
Cavity walls have the following advantages over other walls.
- There is no direct contact between the inner and outer leaves of the wall (except at the wall ties). Hence the external moisture (dampness) cannot travel inside the building.
- The cavity between the two leaves is full of air which is a bad conductor of heat. Hence transmission of heat from the external face to the inside the room is very much reduced. Cavity walls shall have about 25% greater insulating value than solid walls.
- Cavity walls also offer good insulation against sound.
- The nuisance of efflorescence is also very much reduced.
- They are cheaper and economical than a regular solid wall
- Loads on foundations are reduced because of lesser thickness.
Disadvantages Of Cavity Wall
- Cavity wall construction is a super delicate job and high skilled masons & laborers are required.
- It requires close and tight supervision.
- If the cavity is filled with some fillers the cost will be higher.
- Vertical damp proofing is also required.
- Another main disadvantage is the cavity portion can be occupied by insects like bees.