What is it?
Thermal insulation fitted within the gap (the cavity) between the outer and inner leaves of a cavity wall, forming the main thermal barrier of the wall.
- 1Brick (external leaf)
- 2Wall Ties
- 3Cavity Insulation
- 4Blockwork (inner leaf)
- 5DPC
What does it do?
It's the primary layer preventing heat escaping through the external wall, sitting within the cavity between the outer weathering leaf (typically brick) and the inner structural leaf (typically blockwork). Its performance is expressed as a U-value for the wall as a whole, and the U-value required for every layer of the wall is a legal minimum set out in the Approved Documents.
Because it sits within the cavity, it needs to be held in place against the inner leaf (usually on plastic clips fitted to the wall ties) so it doesn't slump or leave gaps over time.
Where is it used?
Within the cavity of every cavity wall, running continuously between the two masonry leaves except where it's interrupted (and detailed around) by openings, lintels, and cavity closers.
Types
Cavity insulation is generally either a fibre batt or slab (mineral wool, fitted snugly between wall ties) or a rigid foam board (higher performance per unit thickness, often used with a residual clear cavity left for drainage). The choice affects both the thickness needed to hit a target U-value and how much of the cavity width is left as a clear drainage gap.
Typical sizes
Insulation batts typically come in thicknesses from 50–150 mm, and can be combined in layers to build up to greater thicknesses. UK cavity widths are typically 75–150 mm overall, most of which is now commonly filled with insulation, leaving a much narrower residual cavity (sometimes as little as 10–25 mm) purely for drainage than older, wider-cavity walls did.
How to specify
Insulation thickness and type are selected to achieve the wall's target U-value, calculated against the current Building Regulations (Part L in England). The required thickness depends on the insulation's own performance (its thermal conductivity, or lambda value) as well as the wall construction around it.
Other considerations
Insulation needs to remain continuous and in place over time. Gaps, slumped batts, and poorly detailed lintels or reveals can create cold spots, increasing the risk of condensation on the inside face of the wall. Some residual clear cavity is usually still needed even with a fully-filled system, to let any water that does get in drain away rather than bridge across to the inner leaf.
Interacting elements
- Wall ties: hold the insulation in place within the cavity
- Cavity closure: provides a solid edge to the cavity, onto which the internal finishes can be completed
- Lintel: the cavity insulation must be detailed around the lintel, since a common lintel type incorporates rigid insulation to keep the thermal line unbroken