What is Cavity Wall?
Cavity walls are those which are constructed in that way that an empty space or cavity is left between the single walls. They are also known as Hollow Wall.
Cavity walls are two walls constructed as a single wall. 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. The empty space or cavity size should be in between 4 to 10 cm.
The internal and external leaves should have 10 cm thickness. These two leaves of cavity wall are interconnected by links or metal ties for a strong bond.
Cavity Wall Construction:-
In general, cavity wall doesn’t require any footings under it, just a strong concrete base is provided on which cavity wall is constructed centrally. Two leaves are constructed like normal masonry, but minimum cavity must be provided in between them.
The cavity may be filled with lean concrete with some slope at top up to few centimetres above ground level as shown below :
Cavity Walls are useful in two ways. Firstly, the cavity prevents the dampness from the outer leaf percolating into the inner leaf. Secondly, they provide excellent insulation from heat and sound.
Weep holes are provided for outer leaf at bottom with an interval of 1 m. Normal bricks are used for inner leaf and facing bricks are used for outer leaf. Different masonry is also used for cavity wall leaves. The leaves are connected by metal ties or wall ties, which are generally made of steel and are rust proof.
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 wall ties usually placed 900 mm apart vertically and 450 mm horizontally in every 6th course staggered.
The wall ties are provided in such a way that they do not carry any moisture from outer leaf to inner leaf.
These wall ties are made from mild steel wires of 3 to 4 mm diameter or MS bars and fabricated to std. shapes. They are dipped in hot tar and sanded or made from galvanized steel to prevent rusting. In very important works, copper may be used.
Wire ties are placed with their twisted end down to allow water that may seep inside, to drip down in the cavity.
The bond to be used for both the leaves, when the thickness is half brick, is the stretcher bond. Where the inner leaves are made thicker for carrying heavy loads. English bond can be used for that part.
To prevent mortar dropping in cavity, wooden battens are provided in the cavity with suitable dimensions. These battens are supported on wall ties and whenever the height of next wall tie location is reached, then the battens are removed using wires or ropes and wall ties are provided.
Important Points to be Observed in Cavity Wall Construction:-
1) The cavity should extend to 15 cm below the damp-proof course level.
2) Below the ground level, the walls are built solid, or preferably the cavity should be filled up to 15 cm below the damp-proof course with fine concrete.
3) Under no circumstances, should the D.P.C be laid to the span of one leaf only. It should cover both leaves of the wall.
4) 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.
5) 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. Some bricks may be temporarily left out at the ground floor level to form openings to permit the bottom of the cavity to be cleared of mortar droppings at the end of each day’s work.
6) In exposed positions, it is desirable to leave a few vertical joints in the outer leaf open, at the
bottom of the cavity to permit water to drain away.
7) A certain amount of ventilation to the cavity is desirable to prevent stagnation of air and excessive humidity. It can be provided by vents, say 150 x 75 mm (6″ x 3″) at intervals, near the base and top of the wall, by leaving a few joints open.
8) The cavity walls should not be built solid at the jambs [the sides of door and window openings] to D.P.C are only permissible in fairly sheltered sites and where the wall surface is rough- cast.
9) 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.
10) The cavity wall should not be built solid below window sills also, and a damp-proof course is desirable at this point also.
11) Damp proof course is provided for two leaves separately. In case of doors and windows, weep holes are provided above the damp proof course.
Advantages of Cavity Walls:-
1) Sound waves travel faster in solid walls as compared to hollow walls. Hence, the cavity walls are also best for sound insulation.
2) Cavity walls give better thermal insulation than solid walls. It is because of the space
provided between two leaves of cavity walls is full of air and reduces heat transmission
into the building from outside.
3) The construction cost of the cavity wall is about 20% less than the construction of solid walls. Hence, economically they are cheaper than solid walls.
4) Moisture content in outer atmosphere is does not allowed to enter because of hollow
space between leaves. So, they also prevent dampness.
5) They also reduce the weights on foundation because of their lesser thickness.
6) Outer Efflorescence is also prevented.
Disadvantages of Cavity Walls:-
1) Highly skilled labour and masons are required for cavity wall construction.
2) Require standard supervision during its construction.
3) A vertical damp proof course is also necessary for it.
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