Which kind of wall insulation should be installed depends on the building’s structure and the materials from which it is manufactured?

If a building was constructed since the 1990s, its walls are probably already insulated to some extent. This can be checked with the relevant local authority’s building control department or by a registered installer who can perform a borescope inspection. If the building has a steel, timber, or prefabricated concrete frame a specialist insulation installer will need to provide advice.

There are two main types of wall insulation - cavity wall and solid wall. Both are considered below.  The first step is to ascertain which type is applicable to the building.

A cavity wall is a construct of two walls with a gap between them.

The first step is for a survey to check how they can be insulated. There are some circumstances in which standard cavity wall insulation will be unsuitable. These include:

  • If the cavity is less than 50mm wide or congested by rubble.
  • If the masonry or brickwork is in poor condition.
  • If the walls are exposed to driving rain.
  • If the building is at risk of flooding.
  • If the building has damp patches on the internal walls. (1)

Typically, cavity walls can be insulated by a registered installer injecting insulating material through small holes in the outside wall, usually either mineral wool or polystyrene beads. The holes are then refilled. In more complex situations, the building may be suitable for polyurethane foam cavity wall insulation instead. This is a more expensive but more effective insulator. (1)

If a building has a party wall (that is, sharing a wall with an adjacent building), it may still be possible to have cavity wall insulation installed. Agreement may also be needed from adjoining neighbours, but this depends on the precise circumstances of the building. A barrier within the cavity (called a cavity cover or barrier) may need inserting to prevent new insulation affecting the neighbouring property. (1)

A wall is usually solid (a single wall) if it is:

  • Pre-1920s.
  • Constructed of stone.
  • Constructed of brick and under 260mm thick. (2)

Solid walls can be insulated from the inside (internal wall insulation) or the outside (external wall insulation).

Solid wall insulation is more costly than standard cavity wall insulation but will also save more money. Contact the relevant local authority to check if permissions are required for solid wall insulation, such as a building warrant or Listed Building Consent. This is especially important if the building is in a Conservation or World Heritage Area. (3)

Internal wall insulation can be either rigid insulation boards or a stud wall. Rigid insulation boards use plastic foam between 60 and 100mm thick, depending on the type of plastic. Stud wall insulation uses mineral wool at least 120mm thick, so takes up more space than rigid boards. The main advantage of stud wall insulation is that it is strong enough to attach heavy fittings such as radiators and wash basins to. (4)

External wall insulation can be either cladding or render. Cladding can be finished with timber panels, shingles, stone, tiles, clay tiles, aluminium panels, or brick. Render is less expensive and is composed of sand and cement over a wire or fibre mesh. External insulation also improves weatherproofing and reduces internal condensation.3 It may be necessary to obtain planning permission prior to installation if the insulation alters the buildings appearance.

(1) Energy Saving Trust, Reducing Home Heat Loss, Cavity Wall Insulation. Available from: https://energysavingtrust.org.uk/advice/cavity-wall-insulation/ [Accessed 25th January 2021].

(2) Energy Saving Trust, Reducing Home Heat Loss, Solid Wall Insulation. Available from: https://energysavingtrust.org.uk/advice/solid-wall-insulation/ [Accessed 25th January 2021].

(3) Energy Saving Trust, External Wall Insulation. Available from: https://energysavingtrust.org.uk/sites/default/files/reports/Solid%20wall%20-%20external%20wall%20insulation.pdf [Accessed 25th January 2021].

(4) Energy Saving Trust, Internal Wall Insulation. Available from: https://energysavingtrust.org.uk/sites/default/files/reports/Solid%20wall%20-%20internal%20wall%20insulation_0.pdf [Accessed 25th January 2021].