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Cavity Wall Insulation materials

cwiAround 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 around £90 of your heating bills every year, and only costs in the region of £150 with discounts from fuel companies. It also saves around 750 kg carbon dioxide emissions every year. If all the houses with unfilled cavity walls had them filled, the energy saved could heat 1.4 million homes each year.

The National Insulation Association represents UK installers and manufacturers of insulation material, and its members follow a Code of Professional Practice. You can find a local installer by using their postcode locator. Cavity wall insulation done by NIA members is covered by a CIGA (Cavity Insulation Guarantee Agency) certificate guaranteeing the work for 25 years. Inspection and certification of insulation products and installers is undertaken by the British Board of Agrément.

Insulating material is injected into the gap between the two skins of brickwork, through holes drilled at regular intervals into the mortar joints.  There are quality control systems in place to ensure that the cavity is filled to the correct density. If necessary, cavities can be inspected, before or after installation, with a borescope. A thermal imaging camera will show any “cold spots” where insulation may not be present.

The main insulants for cavity wall insulation are as follows. All have similar insulation properties:

Blown mineral wool

Either rock wool or glass wool, is inert and inorganic, and should be free of CFCs and HCFCs. Manufactured to BS3533:1981, it has zero ODP (ozone depleting potential) and zero GWP (global warming potential).  It is blown into the cavity starting at the bottom, and it is important to check that filling is even and to the correct density.

Beads and granules

Expanded polystyrene (EPS) beads are now made using pentane as a blowing agent, therefore are free of CFCs and HCFCs.  They are spherical with diameters varying from 2mm to 8mm.They are very free flowing and therefore require fewer injection holes. They are usually coated with adhesive to limit their potential for escape through holes in the inner leaf of the wall, for example through service entry points. Granules, either EPS or polyurethane, being irregular in shape, are less free flowing. There is no fire hazard where the cavity is capped at eaves level.

Urea Formaldehyde foam

UF foam consists of a resin and hardener solution injected with compressed air into the cavity.  After injection, the foam dries and hardens, producing formaldehyde vapour which may enter the dwelling if the inner leaf is not well sealed. Formaldehyde vapour can cause allergic skin reaction, irritation to the eyes or
upper respiratory tract. However, once it disperses there is no ongoing problem, and ventilation of the dwelling quickly removes any traces of formaldehyde.

Polyurethane foam

Polyurethane cavity wall foam consists of thin liquids mixed together and injected into the cavity. The mixture expands in the cavity adhering to both leaves. The thermal insulation value of the foam is very good but it is more expensive than the other products.

Natural fibres or recycled materials are not generally available for cavity wall insulation.

Cavity Wall insulation and dampness

Cavity wall insulation does not cause dampness. Cavity walls were introduced originally to avoid the problems of dampness experienced by older solid walled dwellings. Some people think that filling the cavity (which is in effect bridging the gap between the two leaves of the cavity wall) will lead to a greater risk of dampness
passing from the ‘wet’ outer leaf to the ‘dry’ inner leaf. In fact, this is not the case. The insulation repels moisture. A government sponsored independent study carried out in the mid 1990s found that  filling the cavity with insulation does not cause a greater incidence of damp problems than occur in cavity walls that have not been filled with insulation.

Damp issues

Filled cavities

Unfilled cavities







Rain penetration



Other issues



No problem



However, any house where it is installed must be in good condition and well-maintained, otherwise the insulation may show up areas of penetrating dampness or condensation which may have occurred anyway due to structural problems or the walls not being correctly built. For example problems may occur if any of the following applies:

  • Rubble at the bottom of the cavity or mortar droppings on the wall ties due to careless construction
  • Poor pointing
  • Spalled brickwork
  • Penetrating dampness for example around badly fitting windows
  • Poor ventilation causing condensation
  • Thermal bridges, for example concrete lintels or floors, or where the cavity is closed at reveals or eaves level provide colder surfaces where condensation may form. Internal insulation at these points should solve the problem.
  • Failed or missing damp proof courses – for example early cavity wall construction did not include damp proof courses at window reveals.

In most cases it is possible to remedy the problem before having cavity wall insulation installed.