Cavity Wall Insulation

Cavity Wall Insulation

Over a quarter of your home heat is lost through the outer walls. Most Irish homes are made up of either single concrete block walls or double leaf known as cavity walls. If you are fortunate enough to have cavity walls then filling these cavities with insulation material is a very good method of insulation.

The cavity wall is made up of two walls with a space in between, known as the cavity; the outer wall is usually made of brick, and the inner layer is usually either brick or solid concrete block. Cavity wall insulation fills that space between the outer and inner walls. This reduces heat loss through the building fabric thus saving energy. It can also help reduce condensation inside the house if this is a problem stemming from your external walls.

It is standard practice when building new houses to insulate this cavity as the wall is being constructed, either with full-fill batts or partial-fill boards.

For cavity walls a minimum of 78mm rigid insulation board, with a thermal conductivity of 0.25W/mK must be used, unless there is additional insulation outside the cavity.

Initially the architect or installer will carry out an assessment of the walls to check their suitability for cavity insulation, particularly for full-fill insulation. This assessment will determine the degree of exposure of the house to wind- driven rain and the construction details of the walls.

When full-fill cavity insulation is being used it is essential that such walls are designed and constructed to incorporate the normal precautions to prevent moisture penetration. Insulation thickness should remain constant and should any change in vertical thickness occur, a horizontal damp-proof cavity tray should separate each thickness change.

It is important to ensure during installation that wall ties are fitted correctly and cleaned, excess mortar is cleaned from the inside faces of the wall, mortar droppings removed from the cavity and cleaned from the exposed edges of the insulation slabs. All insulating materials should be installed in accordance with manufacturers 'instructions and procedures.

In older houses insulation can be injected into this cavity through holes drilled through the outer leaf, by means of a blowing or pumping machine.

Since the work is done from the outside, it causes minimum disturbance inside. The drilled holes, which are typically about 25 mm (1 inch) in diameter and spaced a metre or so apart, are then filled to match the wall appearance as closely as possible. The job typically takes less than a day.

Materials used for cavity insulation include mineral wool (glass or rock), polystyrene beads or granules and cellulose fibre. The thermal properties of these materials do not differ greatly (in technical terms, their thermal conductivities tend to fall within the range 0.025 to 0.04 W/m K).Insulating the cavity with such materials will typically reduce heat loss through the wall to one-third of its original value. Such cavity insulation materials are generally water-repellent and rot-proof and some are non-combustible.

It is most important before deciding to insulate existing cavity walls that you check their suitability for cavity insulation. This assessment will take account of the degree of exposure of the house to wind-driven rain and the construction details and condition of the walls. Any ventilation openings in the wall will also be checked to ensure that these will not be blocked by the insulation. If the walls suffer from rain penetration at times, this problem must be remedied first. In some cases, the walls may be found to be unsuitable for cavity insulation or may require some remedial work beforehand.

The Irish Agrément Board (IAB) provides independent certification of cavity wall insulation systems and maintains a register of approved installers for certified systems. These installers should be able to show you an IAB certificate for the insulation system they propose to use and proof of their reputation as approved installers.

The cost of cavity wall insulation depends on a number of factors, including the width of the cavity, but is typically in the region of €5 to €7 per square metre. For a typical semi- detached house, this gives a total cost of about €550 - €700. With annual fuel savings of €100 to €160, the pay-back period will be in the region of 4 to 7 years.

You may be able to negotiate a lower price if you can persuade some of your neighbours to have their walls insulated at the same time.