External masonry walls

6.1

6.1.7Thermal insulation

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

thermal insulation materials
installation
insulation materials
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.

Thermal Insulation materials

Thermal Insulation materials, either full, or partial fill that will be placed in the cavity between the outer and inner leaf of masonry should be:

the subject of current certification for use in this application from an appropriate third party technical approvals body

assessed in accordance with Technical Requirement.

Thermal Insulation materials may:

either be built in during, or retrofitted after the construction process

fully or partially fill the cavity.

Materials shall be considered as full fill cavity insulation where:

they are designed to be in full contact with both leaves of masonry

described as full fill in their technical approval document although a narrow residual cavity is retained between the outer leaf and the outer face of the insulation.

Partial fill cavity insulation materials shall be installed on the cavity face of the inner leaf and the appropriate residual cavity to the outer leaf as specified in ‘Construction Type’ is maintained.

All retro-fill insulation systems, including, blown mineral wool expanded polystyrene beads and UF foam 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.

Installation

Workmanship should be maintained when installing insulation to minimise the risk of damp penetration or condensation to the on the inner leaf. Gaps in the insulation layer can provide routes for moisture ingress from the outer leaf to the inner leaf and also create localised cold spots where condensation can form. Gaps between the board and the inner leaf can result in thermal bypass (this is exacerbated by gaps at board junctions). Insulation material should be:

close butted with no gaps

in full contact with the inner leaf

taped at the joints where it is formed of rigid boards with non-compressible edges¹

fully engaged with the adjacent board edges where a ship lap or other interlocking edge detail is provided

installed in accordance with the manufacturer’s recommendations.

Notes

1 Where the insulation has a low e facing the tape face shall have the same low e value

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 flush from the inside of the outer leaf

excess mortar should be struck flush from the cavity side of the inner 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.

Rigid insulation boards should:

be stored flat without bearers, otherwise they may distort, making them difficult to fix against the wall

be rejected where warped.

Rigid insulation should not be continuous across the end of a separating wall or floor. A flexible (mineral wool) cavity closer should be used in line with these elements, and the rigid butted up to these. The cavity closers should also be installed at the ends of separating walls where blown fill in used in the external cavity.

Insulation materials

Before installing retro-fill insulation materials the home should be in a condition ready to receive the insulation, this will be defined in each systems instruction documents, but would normally require that:

the cavity wall is inspected by the installing team

the roof is in place, the tops of the walls protected from the rain and window, door and other openings in the cavity should be closed

areas such as below stepped dpc’s and gas membranes and behind cavity trays that cannot be accessed during the retro-fill process are insulated with appropriate built in materials.

Construction type

The following are recommendations and guidance according to construction type:

Partial fill cavity insulation

Where partial cavity insulation is installed:

it should only be fixed against the cavity face of the inner leaf

a minimum 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 fill 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

recessed mortar joints should only be used in sheltered areas

fairfaced masonry includes clay, concete 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 used

painted finishes on bricks or render are not acceptable where they are likely to cause damage (including frost damage or sulphate attack).

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

Notes

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 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.

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 Man	In accordance with the guidance in this chapter.
Scotland	In accordance with the guidance in this chapter.
England and Wales	In accordance with the guidance in this chapter.

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.