External masonry walls

6.1.7Thermal insulation

Thermal insulation shall be adequate and installed correctly. Issues to be taken into account include:

  1. installation
  2. insulation materials
  3. construction type.

The insulation value of the wall must meet the requirements of the relevant Building Regulations. Cold bridging should be avoided. Particular care is needed:

  • at openings
  • between external walls and roofs, internal walls and floors.


Workmanship should be maintained to minimise the risk of damp penetration to the inside of the home. Gaps provide routes for dampness, and condensation can form on the cold spots where insulation is missing. Insulation should be:

  • close butted with no gaps
  • installed in accordance with the manufacturer’s recommendations.

Where cavity insulation is used:

  • mortar joints, including perpends, should be solidly filled with mortar
  • mortar droppings should be removed from wall ties and the edges of insulation materials
  • excess mortar should be struck smooth from the inside of the outer leaf.

The first row of insulation boards or batts should be supported on wall ties:

  • with a minimum of two ties to each board or batt
  • which coincide with horizontal joints in the insulation.

Where wall ties need to be closely spaced, e.g. at reveals, it is acceptable to make a neat cut in the insulation to accept the extra ties.

Insulation boards for partial fill should:

  • be stored flat without bearers, otherwise they may distort, making them difficult to fix against the wall
  • be rejected where warped.

All retro-fill insulation materials, including UF foam, blown mineral fibre and expanded polystyrene beads should be:

  • installed by a member of a surveillance scheme acceptable to NHBC
  • installed by operatives trained by the assessment holder, and approved by the assessment holder and the assessing organisation.

Insulation materials

Insulation should be:

  • UF foam to BS 5617 and installed in accordance with BS 5618, or
  • assessed in accordance with Technical Requirement R3.

Construction type

The following are recommendations and guidance according to construction type:

Partial cavity insulation

Where partial cavity insulation is installed:

  • it should only be fixed against the cavity face of the inner leaf
  • a 50mm clear cavity between the partial cavity insulation and the outer leaf should be maintained
  • wall ties long enough to allow a 50mm embedment in each masonry leaf should be used.

In areas of very severe exposure in England and Wales, a residual cavity of 75mm is required where the outer leaf is fairfaced masonry.

Full cavity insulation

Where the cavity is to be fully filled with insulation:

  • the type of insulation, its thickness and the wall construction should be suitable for the exposure of the home (see Table 2)
  • render on an external leaf of clay bricks (F2,S1 or F1,S1 designation bricks to BS EN 771) is not permitted in areas of severe or very severe exposure to wind-driven rain
  • mortar joints should not be recessed
  • painted finishes on bricks or render are not acceptable where they are likely to cause damage (including frost damage or sulfate attack).

Table 2: Suitable wall constructions for use with full-fill cavity insulation


1 In very severe exposure locations, fairfaced masonry with full cavity insulation is not permitted.
2 Render on an external leaf of clay bricks (F2,S1 or F1,S1 designation bricks to BS EN 771) in severe or very severe exposures is not permitted where the cavity is to be fully filled with insulation.
3 This table covers walls where the external leaf does not exceed 12m in height.
4 The exposure category of the home is determined by its location on the map showing categories of exposure to wind-driven rain.
5 Fairfaced masonry includes clay, calcium silicate and concrete bricks and blocks and dressed natural stone laid in an appropriate mortar preferably with struck, weathered or bucket handle joints. Cavity walls of random rubble or random natural stone should not be fully filled.
6 Recessed mortar joints should not be used.
7 In Scotland, it is not permissible to fill the full width of the cavity with any thermal insulation at the time of construction.
8 In Northern Ireland and the Isle of Man, it is not permissible to fill the cavity with pumped thermal insulants (for example, UF foam) at the time of construction.

The thickness of materials should be as required in the design, and in accordance with Building Regulations.

Guidance for retro-filling cavities:

Northern Ireland and the Isle of ManNot permitted to fill cavities with pumped thermal insulants at the time of construction.
ScotlandNot permitted to fill the cavity fully with any thermal insulants at the time of construction.
England and Wales In accordance with the guidance in this chapter.

Inner leaf of insulated blockwork

Types of blockwork include:

  • lightweight aerated concrete
  • lightweight aggregate blocks
  • voided blocks with insulation infill
  • blocks faced with insulation material.

For insulated blockwork:

  • manufacturers’ recommendations should be followed
  • a clear 50mm wide cavity should be maintained
  • blocks should be capable of supporting concentrated loads
  • the correct type of joist hanger for the type and size of both the block and joist should be used
  • long unbroken lengths of blockwork should be avoided
  • precautions should be taken to reduce risk of shrinkage cracking
  • restrictions on chasing for services when using voided blocks should be noted.

Insulated dry linings

Where an insulated dry lining contains a combustible insulant, to prevent early collapse of the lining in a fire, the plasterboard should be:

  • a minimum of 12.5mm thick
  • mechanically fixed to the masonry inner leaf.

Dual insulation

Where partial cavity insulation is used in addition to an insulated block inner leaf, the composite construction should be assessed in accordance with Technical Requirement R3.