6.1.7 Thermal insulation
Thermal insulation shall be adequate and installed correctly. Issues to be taken into account include:
- thermal insulation materials
- installation
- injected and blown fill insulation
- construction type.
The insulation value of the wall must meet the requirements of the Building Regulations. Cold bridging should be avoided. Particular care is needed:
- at openings
- between external walls and roofs, internal walls and floors
- behind or around components installed in the cavity such as sub-floor vents, inset meter boxes, cavity trays or windposts.
Thermal Insulation materials#
Thermal Insulation materials, either full, partial, or injected and blown fill that will be placed in the cavity between the outer and inner leaf of masonry should be:
- satisfactorily assessed by an appropriate independent technical approvals authority accepted by NHBC as suitable for the proposed use
- 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 injected and blown fill insulation systems, including blown mineral wool, and expanded polystyrene beads should be:
- installed by the certificate holder or their approved installers subject to the certification bodies assessment and surveillance scheme
- 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 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, eg 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.
When installing full fill insulation:
- ensure fibres in the insulation are laid parallel to the wall
- butt joint slabs and do not bend at internal and external corners
- cover all exposed areas of insulation slabs at the end of each day or in driving rain to prevent excessive moisture in the cavity and pressure on freshly laid masonry from expansion of the insulation
- ensure vertical joints are staggered when layering slabs of insulation.
Figure 7: Full fill insulation
Figure 8: Insulation to combined lintel ends
Figure 9: Cavity ties to rigid partial fill insulation
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 insulation butted up to these.
Injected and blown fill insulation#
Injected and blown fill insulation should comply with the relevant standards:
| Material | Standard | Settlement class |
|---|---|---|
| Mineral wood | BS EN 14064-1 ‘Thermal insulation products for buildings. In-situ formed loose-fill mineral wool (MW) products - Specification for the loose-fill products before installation’
BS EN 14064-2 ‘Thermal insulation products for buildings. In-situ formed loose-fill mineral wool (MW) products - Specification for the installed products’ | S1 |
| Expanded polystyrene (EPS) beads | BS EN 16809-1 ‘Thermal insulation products of buildings. In-situ formed products from loose-fill expanded polystyrene (EPS) beads and bonded expanded polystyrene beads - Specification for the bonded and loose-fill products before installation’
BS EN 16809-2 ‘Thermal insulation products of buildings. In-situ formed products from loose-fill expanded polystyrene (EPS) beads and bonded expanded polystyrene beads - Specification for the bonded and loose-fill products after installation’ | N/A |
Injected and blown fill insulation systems should meet the requirements of the relevant standards and hold a satisfactory assessment by an appropriate technical approvals authority acceptable to NHBC confirming suitability for use in a masonry cavity wall and for the exposure rating of the site.
Separating wall and floor constructions and their associated flanking walls should be detailed correctly when using injected and blown fill insulation. Cavity stops should be installed at the ends of separating walls and separating floors. Refer to the Robust Details Handbook for the specification of materials and construction details.
Before installing injected or blown 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 as a minimum would require that:
- the cavity wall is inspected by the installing team
- the roof is in place, tops of the walls are protected from rain
- all edges of cavities at window, door, and other openings are closed with the permanent cavity closers, as per the design.
Areas that cannot be accessed during the fill process by appropriate adaptation of the installation method (eg below gas membranes or low level continuous DPCs, where there is a cavity tray one course above a lintel, or where separate lintels are used for each leaf) should be insulated with appropriate built-in materials.
On completion of the work, the Installer shall provide a declaration of compliance in accordance with the relevant standard for the product.
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 fair faced 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
- fair faced masonry includes clay, 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 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, fair faced masonry with full cavity insulation is not permitted in cavity widths of less than 150mm.
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. Fair faced 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.
Multiple layers of insulation may be used where in accordance with manufacturer’s recommendations and within the scope of satisfactory assessment by an independent technical approvals authority accepted by NHBC.
The thickness of materials should be as required in the design, and in accordance with Building Regulations.
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.
Last updated: 2nd January 2024