cavity wall.pdf

CAVITY WALL Cavity in a construction? Don’t you find it alarming enough? Actually yes, the cavity in a construction wall can be disastrous indeed, but when it is about having a cavity wall, it’s something special. In fact, it is an architectural splendor that...

CAVITY WALL Cavity in a construction? Don’t you find it alarming enough? Actually yes, the cavity in a construction wall can be disastrous indeed, but when it is about having a cavity wall, it’s something special. In fact, it is an architectural splendor that can add some specific advantage to a building. The use of cavity walls was once considered a trend, especially in mediaeval age, howsoever, modern construction has explored it as a trendsetter for a building. The walls in a cavity wall are called leaves and are generally referred to as the outer leaf and inner leaf. Cavity wall generally comprised of two walls (not necessarily of equal thickness), with a space in between. OR A wall comprising two leaves, each leaf being built of masonry units and separated by a cavity and tied together with metal ties or bonding units is called a cavity wall”. The structure is made in a way that the two leaves behave as one structural unit, and the space between the leaves is either kept as a continuous cavity or it is filled with waterproofing material or a non-load bearing insulating material. Ideally, the thickness of each leaf of a cavity wall shall not be less than 7.5 cm or 75mm. The space or hollow or cavity between the walls has two main advantages. 1. When the outer leaf becomes wet the space prevent water passing from the outer wall to the inner wall and thus the interior of the wall remains dry. 2. The space will not allow heat to be transferred from one wall to the other. This arises from the fact that air is a bad conductor of heat and the cavity acts as a barrier. The cavity therefore is a good insulator and helps to keep the building cool in summer and warn in winter. If the cavity is sealed at the base, eaves, and all the jambs of openings, then the air trapped inside will be static and this will ensure good heat insulation for the building. Advantages of cavity wall The cavity walls are constructed for the following advantages: 01. Damp Prevention Cavity walls 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. 02. 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. 03. Sound Insulation The air in the cavity acts as a cushion for absorbing sound. By building cavity walls, a premise may work as a soundproof zone as a large quantity of external noise gets absorbed within the cavity. Disadvantages of cavity wall 1. They are costly in their construction. 2. They require skilled labour. 3. They require more time in their construction. 4. They weaker in strength. Size of the cavity. The cavity should be large enough to provide suitable insulation, but not so wide as to make it difficult to tie the two walls together – the Building Regulations state that the width of a cavity at any level should not be less than 50mm nor more than 100mm where suitable ties are placed at distances apart not exceeding 900 mm and 450mm vertically. A cavity may not be more than 100mm wide if vertical twist-type ties are used and are placed at distances apart not exceeding 750mm measured horizontally and 450mm measured vertically. Wall ties These are used to tie the walls together, or to stabilize them. They are generally made of non- ferrous metal or stainless steel. All the wall ties have one thing in common, in that way they are designed to trap the passage of water from the outer leaf to the inner leaf. When the cavity walls are being built it is essential that the wall ties be absolutely free from mortar droppings on completion of the walling. You can keep cavities reasonably clean and free from mortar droppings if you continually used cavity battens during the process of raising the work. Damp-prevention in cavity Other places where dampness may penetrate to the inner leaf are at reveals, head of openings and sills. Therefore, a damp-proof membrane should be provided at these points so that the interior will be kept dry as possible. To give added stability to the leaves it is also advisable to seal the cavity at roof level by bridging across the two leaves. General Principles for Constructing Cavity Walls 1. The outer part of the wall should be constructed of good quality bricks. 2. The bricks should be laid in cement mortar (1:4) and sufficiently reinforced so as to provide an impervious construction. 3. The outer part of the wall should not be less than 100mm in thickness. 4. The inner part of the wall must be designed according to the load to be supported by it. 5. The cavity should be of uniform width throughout the length of the height of the wall. 6. The minimum width of the cavity must be 50mm and should not exceed 75mm. 7. The cavity must be free from excess mortar projection. 8. The horizontal dpc should be built in two separate widths. 9. The outer and inner part of the wall should be tied together with suitable bonding ties for the sake of stability. 10. The ties must be rust-proof and should be able to check the penetration of water from the outer face to the inner face. 11. The ties must be spaced not more than 1000mm apart horizontally and three courses apart vertically in staggered form. 12. The outer and inner part of the wall are connected together by specially moulded bond bricks metal ties in the form of cast-iron or wrought iron cramps with ends split up and fish tailed.