A Guide to Building With Cavities
A cavity wall consists of two walls or ‘skins’, and are separated by a space which today is usually filled with insulation. The two walls are generally made up of an outer skin of facing brick or block that will be rendered, and an inner skin of block which will be faced internally with plasterboard and finished with plaster. The outer skin will absorb moisture and rainwater so the cavity wall is designed to allow this water to drain back out of the wall through weep holes at the base of the wall and through others placed above window and door frames. A cavity wall would normally begin life below ground level and the cavity would be filled with a lean mix of concrete to resist any lateral (side) pressure crushing the cavity and to prevent it from filling with water.
History of Cavity Walls
Cavity walls were first seen in the 19th century but did not gain widespread use until the 1920’s, when they gradually increased in width over the years until they became enshrined in building laws in the 1990’s. Besides the necessity to build a wall incorporating a cavity, building regulations also stipulate a requirement for rigid insulation to be placed inside the cavity, and the thickness of this insulation along with the size of the cavity has altered along with improvements to the regulations. The prime consideration in placing insulation in the cavities is to reduce the thermal output of the walls and thus reduce heating costs. As much as 35% of heat from a property can be lost through the walls alone. Houses built before regulations were in place can be retro filled with insulation, and there are several materials that can be used to accomplish this.
Tips for Cavity Walls
Another requirement when constructing a cavity wall is to place wall ties at set intervals. These are inserted to ensure that the walls remain stable. Before cavity walls the thickness of the wall was determined by the height required, whereas cavity walls will only have two skins which are built separately so higher degrees of engineering are required to provide the structural stability needed. The outer wall will be built with a ‘half brick’ thick outer skin, which means that the bricks are laid end to end to form a single skin wall. Wall ties have also changed in shape and specification over the years and now come in various guises depending on the structural requirements of the cavity.
Care also needs to be taken when constructing a cavity wall so that excessive amounts of mortar do not fall between the cavities. Should the mortar settle on the wall ties or the insulation it is possible for ‘cold bridging’ to happen. This can allow damp to travel from the outer skin to the inner one, which will cause damage to the plaster work along with damp patches to the inner walls and possibly rotting to any timbers work in contact with the walls, such as floor joists, skirting boards etc.
So why does a cavity work better than a solid wall? Simply put, it is because the distance between the particles in the air is greater than in a solid, thus making it harder for cold or damp air to travel. This also means it has the added benefit of improved sound proofing.
Building a Cavity Wall Guidelines
To build a cavity wall successfully the following guidelines should be followed:
- The wall leafs should be tied together with wall ties
- The wall ties must be level or should slope down slightly toward the outer wall with the drip point on the wall tie positioned in the centre of the wall
- Use the correctly specified ties for the thickness of the cavity
- Keep the cavity clear of mortar droppings
- Make sure the DPC membranes are placed correctly at the base of the walls and also above the lintels in the form of cavity trays
- Place weep holes above window and door frames to allow any water or moisture to escape