A cavity wall or hollow wall consists of two separate walls called leaves-or skins with a cavity gap in between. The cavity wall construction is an effective method of damp prevention in which main wall of a building is shielded by an outer skin wall, leaving a cavity between the two.
The cavity walls are often constructed for giving better insulation to the building. It also prevents the dampness to enter and acts as sound insulation. The cavity extends vertically all along the height of the wall, except at the openings, where it discontinued. The cavity should terminate near coping in case of flat roofs with parapet walls and upto or near eaves level in case of sloping roof. In the foundations, the cavity should start near the ground level or 150 mm below the D.P.C level.
Construction of Cavity Wall
As you can see in the picture the construction of cavity wall includes 2 walls i.e 2 leaves. outer leaf and inner leaf. The thickness of both the walls is usually same, but in some cases the thickness of inner leaf may be greater than the outer leaf, totally depending upon the structural requirement.
Generally 2 slender half brick skins are not as strong as one brick thick solid wall. Therefore to overcome this advantage cavity walls are interconnected by one another by means of wall ties. These wall ties are usually made up of steel.
It is mandatory to place 5 wall ties per square meter construction of cavity wall. Maximum horizontal spacing of the wall ties is restricted to 900 mm and vertical spacing is restricted to 450 mm.
The width of cavity or the open space between the 2 walls should should range in between 40 mm – 100 mm.
During the construction of cavity walls, the openings made for doors and windows should be sealed with damp-proof course, so that there is no entry of moisture through it.
Cavity Wall Insulation
In order to meet building regulation cavity walls which are built of 100 mm facing bricks and 100 mm blocks has to be well insulated between the two walls. This simple method can increase the thermal insulation qualities of the building.
Generally cavity walls insulation is adopted to reduce heat loss through cavity wall. It is generally done in cold areas where the climate is cold during most of the months all around. It prevents the convention and keeps the house warm by making sure less heat is lost through the wall. It is done by insulating the cavity walls by filling the open space between the 2 walls by insulation materials such as polyurethane, glass fiber wool, rock wool panels etc.
For better understanding you can watch this video :
The insulation of cavity walls is not only done for heat loss but also to make the structure sound proof or to reduce noise transmission. This is done by same method as mentioned above but the insulation material used is different. Here cellulose insulation material is used. It is low conductive material used to make the building sound proof. This type of insulation is generally adopted by the structures such as theaters, auditoriums etc.
Advantages of Cavity Walls
- The main advantage of the cavity walls is that the moisture cannot enter from outer leaf to inner leaf.
- The cavity between the 2 walls is filled with the layer of air which are non-conductor of heat due to which there is less transmission of outside heat.
- Cavity walls could be the best choice where winters are at the peak.
- The construction of cavity walls is cheaper than the solid walls.
- They are fire resistant.
- They act as good heat and sound insulation walls.
Disadvantages of Cavity Wall
- If the cavity walls are filled with insulation, there is a high risk of moisture entering form outer leaf to inner leaf, whether whichever material is used for insulating.
- Unlike other wall construction, cavity wall construction requires skilled labours.
- Unskilled labours will result in the faulty construction of the wall.
- Insulation thickness is restricted due to the width of the cavity.
- Mortar dropping on the wall ties of the cavity walls leads to penetrating dampness.