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How to install cavity wall insulation

How much will I save?

Insulating your walls is very important – around a third of the wasted heat in an un-insulated home is lost through the walls. Insulating your cavity can save up to 40% of this wasted heat. Installing cavity wall insulation can save in the region of £145-200 off your heating bills every year, and costs in the region of £100 to £300 with discounts from fuel companies; in some cases, funding may be available (see below). It also saves around 550 kg carbon dioxide emissions every year. The Energy Saving Trust gives more details on costs, savings and payback time. If all the houses with unfilled cavity walls had them filled, the energy saved could heat 1.4 million homes each year.

Cavity wall insulation should be carried out by a specialist contractor who can provide a CIGA (Cavity Insulation Guarantee Agency) certificate guaranteeing the work for 25 years.

What is a cavity wall?

insulated cavityObviously you can only have cavity wall insulation if your house was built with a cavity to fill. A cavity is a gap between two leaves of brick, or a leaf of brick on the outside, and blockwork on the inside. The two leaves are held in place by wall ties. Houses built from the 1930s onwards generally have cavity walls, although many 1930s houses in Stafford were built with solid walls.

 

A cavity wall has all the bricks placed lengthways, (stretcher bond) whereas a solid wall has some bricks placed end on (headers).  It is less easy to tell with rendered houses, but houses built with cavities have thicker walls than solid wall houses, as the cavity is usually about 50mm (2 inches) wide. cavity wall

By 1983 houses were being built with insulated walls, so any house built after this date will not need cavity wall insulation. Many modern houses are timber framed, even if they have a brickwork outer face, and cannot be cavity wall insulated.

How is the insulation installed?

cavity wall injectionThe contractor injects insulating material into the gap between the two skins of brickwork, through holes drilled at regular intervals into the mortar joints. The work causes little or no disruption as the insulation is injected into the cavity from the outside, typically taking two or three hours. The holes are then filled with mortar or render to match the existing. As well as massively decreasing the amount of heat which escapes through the walls, it reduces draughts and will help create a more even temperature in your home, help prevent condensation on the walls and ceilings and can also reduce the amount of heat building up inside your home during hot weather.

Grants and funding 

Funding is available for cavity wall insulation via the ECO (Energy Companies Obligation) scheme. Low-income or vulnerable households in particular should benefit from the ECO grants, so make sure you talk to your energy company about the options. Availability can vary according to your postcode, so as a first step, phone the Energy Saving Advice Service on 0300 123 1234, or if you live in Stafford Borough, Warmer Homes Stafford on 0800 677 1785.

Important Points

  • The contractor will do a pre-installation inspection to check that the walls are suitable. Make sure that your house has cavity walls, and that they have not already been filled, (look for signs of holes in the mortar which have been filled) but if you are not sure, the contractor will check this first.
  • Cavity wall insulation does not increase the risk of dampness, but any damp or structural problems must be cured before the cavity is filled, for example pointing, failed damp proof course, debris in the cavity, failed wall ties, cracking, or condensation caused by poor ventilation.
  • If you live in a semi-detached or terraced house, the contractor will install cavity barriers to prevent the insulation entering the cavities of your neighbours – or get them to have cavity wall insulation at the same time! Generally blocks of flats all need to be insulated at the same time.
  • A conservatory may make insulation difficult on that wall – but ask the contractor for advice.
  • Very exposed walls, or walls in properties over 12 metres high, may not be suitable for insulation.
  • The material used for cavity wall insulation is generally mineral wool, polystyrene beads, polyurethane or urea formaldehyde foam. If an installer is unable to carry out the work because of unusual conditions, it may be worth contacting a contractor who uses one of the other insulating products.
  • Once installed, the contractor should check that that the filling has been done correctly, for example that there are no gaps in the insulation.

Potential problems with faulty retro-fit installation

Inappropriate or faulty installation of cavity wall insulation can lead to a range of problems, including:

  • Movement or shrinkage of insulation within the cavity leading to cold bridging.
  • Tracking of moisture across the cavity where outer brickwork is in poor condition or on a very exposed site; filling of the cavity is not advised in these circumstances.
  • This will lead to: condensation and mould on the internal wall; cold interiors; increased levels of respiratory illness; corrosion of wall ties and premature structural failure; impact on safety of electrical sockets; premature decay of timbers.
Damage to the external brickwork should be repaired before cavity wall insulation is installed. This will reduce any risk of moisture entering the cavity and affecting the insulationBrick face missing.

Where the insulation material is sodden or degraded the only option is extraction. This typically costs around £4000 for a semi-detached house. Hence it is essential that the initial survey and installation are undertaken by a fully qualified contractor and that a 25-year guarantee is provided.

The Building Research Establishment has published a study of Post Installation Performance of Cavity Wall and External Wall Insulation, which highlights the issues.