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Knauf Insulation: Technical Column

Best practice for insulating a cavity wall.

Product choice and installation can affect thermal performance, so this month Bradley Hirst, Technical Services Manager from Knauf Insulation, looks at best practice for insulating cavity walls.

Always use a product certified for the application:
In England and Wales, cavity walls must be insulated to achieve the thermal performance outlined in Approved Document L of the Building Regulations, while Approved Document C, which addresses the issue of moisture, states that cavity wall insulation must have third-party certification or European technical approval for the application. In the UK, this means insulation must have a BBA certificate for use in cavity walls, such as Knauf Insulation DriTherm Cavity Slab.

The benefits of a full-fill solution:
There are two options for insulating a cavity wall; full-fill, which completely fills the cavity with Mineral Wool insulation, and partial-fill, which leaves an air gap between the insulation and the internal leaf. Fully filling the cavity can provide thermal benefits over a partial-fill solution because there is no air movement in the cavity.

Mineral Wool has other benefits over insulants typically used for partial-fill solutions. Unlike rigid boards, it adapts to imperfections in the substrate making it easier to install correctly, eliminating the gaps that can reduce thermal, fire safety and acoustic performance. It is also non-combustible, and if the cavity is fully filled, additional fire barriers are not required to maintain fire safety, which reduces costs and speeds up installation.

Use the correct thickness of insulation:
Full-fill insulation solutions must be in intimate contact with both masonry leaves, so always use a Mineral Wool slab that is the same thickness as the cavity (i.e. a 100mm thick slab for a 100mm cavity).

Using a thinner insulation slab won’t fully fill the cavity, so won’t deliver the required thermal performance. Similarly, using a slab that is thicker than the cavity means the insulation is compressed during installation, which also reduces thermal performance. Once installed, the insulation will try to recover its natural thickness and can dislodge masonry when the mortar is wet, forcing the wall out of alignment.

Either way, the installation won’t meet the standards determined by the BBA certification or the Building Regulations.

Protect insulation from the elements:
The British weather does mean there might be downpours during construction. Mineral Wool designed for external applications, such as cavity walls, contains a water repellent additive, but you must cover the cavity during heavy rain or at the end of the day to protect the insulation.

Mineral Wool that has become saturated should not be used. However, if it gets damp during construction, it will recover its full thermal, fire and acoustic performance once it dries out.

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