Cavity wall insulation can be a great way of improving the thermal performance of a building, making it both warmer and cheaper to heat, but it's not always a suitable solution. We explore the pros and cons. 

What are cavity walls?

Cavity walls are exterior walls with a gap in the middle and is generally found in houses built after 1920 (though there are exceptions to the 'rule'). The outer leaf is usually made of brick with inner layers either constructed from brick or concrete blocks. The gap can be filled with insulation to improve the thermal performance of a building.

How can I tell if a wall is a cavity wall?

There are often clues on whether a structure has a cavity wall to be found by looking at the exterior brickwork. Typically 'long' bricks denote a cavity but a mixture of 'long' and 'short' bricks show a cavity is likely present as the 'short' bricks will go through the wall. If your house is a steel-frame or timber-framed building, or is made from pre-fabricated concrete different rules apply for insulation.

Why insulate external walls?

Energy Saving Trust studies have shown that around one third of the heat lost in an uninsulated home escapes through the exterior walls as heat flows from a warm area to the cold. This is why insulating external walls is an important way of improving energy efficiency. Homes constructed in the last decade will likely include some form of external wall insulation while older properties will often have a cavity wall that can be used to house insulation.

Should all cavity walls be insulated?

Not necessarily. Walls exposed to driving rain are not suitable and could lead to major problems with moisture ingress and retention. To be suitable for insulation, any cavity should be at least 5cm across and the exterior wall must be in good condition. A reputable installer will be able to advise.

How do I insulate cavity walls?

Insulating cavity walls is a job for professionals. Installers should belong to the British Board of Agrement, the National Insulation Association or the Cavity Insulation Guarantee Agency.

How are cavity walls insulated?

Typically small holes are made in exterior walls from the outside with insulation material (in the form of foam or beads) 'blown' into the cavity through the holes which are later made good. An installer will need access to all outside walls and, where walls adjoin a neighbouring property, should fit a barrier to contain the insulation.

How much will it cost to have cavity walls insulated?

Energy Saving Trust figures suggest the following guideline prices for houses in England, Scotland and Wales:
  • Detached house - c. £720
  • Semi-detached house - c.£475
  • Mid-terraced house - c. £370

How much can I save on my heating bills?

The cost of insulation shouldn't take too long to recoup. Assuming a property has gas central heating the Energy Saving Trust reckon that post-fitting you should anticipate savings between £275 (detached) and £105 (mid-terraced house).
 

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