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Hard-to-treat cavity walls

As many as 5.8 million properties in the UK have cavity walls that are classed as ‘hard-to-treat’. These are also sometimes known as ‘hard-to-fill’. This means that they cannot be insulated using the same methods and products as a standard cavity wall.

How is a hard-to-treat cavity wall different to a standard cavity wall?

Hard-to-treat cavity walls usually have one of the following construction characteristics:

  • A cavity that is less than 50mm wide.
  • A prefabricated concrete construction with a cavity.
  • A metal frame construction with a cavity.
  • A stone cavity. Many older properties have uneven cavities in walls constructed of a natural stone outer leaf and a block or brick inner leaf.
  • A timber-framed uninsulated studwork cavity. These properties have a masonry cavity, which must not be filled.
  • Cavities that have already been partially filled.
  • The property is simply too high for standard cavity treatment (i.e. it is three or more storeys tall).
  • The property has features like a conservatory that can create difficulties in terms of access (this can increase costs).

Some properties may also be deemed hard-to-treat because they are exposed to severe wind-driven rain. Properties located near the sea or on top of hillsides can fall into this category. Also, if there is a fault like significant cracking in the outer leaf of the wall this will need to be remedied before the cavity can be filled. In some cases, exposed walls can be clad to prevent water ingress, but this may be a costly option and it may be cheaper to have solid wall insulation installed instead.

How can a hard-to-treat cavity be filled?

Standard cavity walls can be filled using a number of different materials including mineral fibre and bonded beads. However, the standard practice for insulating a hard-to-treat cavity is to drill holes in the outer leaf of brickwork and inject expanding foam into the wall cavity. The stages are outlined below:
  1. The internal wall is checked for holes into the cavity. Any holes are filled to ensure there is no leakage into the property when the insulation is installed.
  2. Holes are drilled in a specific pattern and spray foam is injected in a staged process to ensure the whole cavity is filled from the bottom to the top.
  3. The contractor will often use a camera to ensure that the whole cavity is filled equally and that no gaps are left in the insulation.
  4. Upon completion, the installer will check all flues, vents, and pipes to ensure that all appliances are operating correctly and that nothing has been blocked by the new insulation.
  5. Finally, a mortar is used to fill the holes in the external brickwork.

Frequently asked questions

How can I tell if I have cavity walls?

There are three simple things you can check. Firstly, the age of your house. If it was constructed before 1932 it is unlikely to have cavity walls. If it dates from 1932-1982 it probably does, and after 1982 almost certainly. Secondly, check the pattern of the brickwork. If you see only the long side of the brick ('stretcher bond' in brickie parlance) the property is likely to have cavity walls. If you see a long-short-long-short pattern, this suggests a solid wall with no cavity. Finally, measure the thickness of your wall. If it is more than 30cm (11.5 inches) and constructed of brick it is likely to have a cavity, although some stone walls can be considerably thicker than this.


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Can all cavity walls be insulated?

Most cavity walls can be insulated, but there are a few exceptions including those where the masonry or brick work of the property is in poor condition, the wall-ties are dirty, the cavity is less than 50mm wide or if the walls are exposed to persistent driving rain (e.g. coastal properties)


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What materials are used for cavity wall insulation?

CWI comes in three main types. Expanding foam, mineral fibre and the increasingly widely used poly-bead or carbon bead which tend to be the most energy efficient.


Need more help?

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Next question

View all frequently asked questions

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