Cavity Walls: The Present

Insulating a cavity wall with Kingspan Kooltherm

Thermal performance is the fundamental building block of the modern cavity wall.

Whether you’re looking to build a Passivhaus property, or simply meet Building Regulation / Standard requirements, achieving the correct U-value for the external wall is absolutely critical.

In terms of compliance, this target will vary depending on which home nation the dwelling is being constructed in. We’ve already discussed the current domestic regulations for England, Wales and Scotland in previous blogs, however, the table below provides an “at a glance” run-down of the notional dwelling specifications for new dwellings. In addition, it also shows the U-values that we recommend as the best starting point in most cases.


ADL1A 2013 – England

ADL1A 2014 – Wales

Section 6 – Energy (Domestic) – Scotland
Notional Dwelling Specification Recommended Starting Point Notional Dwelling Specification Recommended Starting Point
Roofs (W/m2.K) 0.13 0.11 0.11 0.10
Walls (W/m2.K) 0.18 0.16 0.17 0.15
Floors (W/m2.K) 0.13 0.11 0.15 0.13

Above: Notional and recommended starting point U-values.

There are a wide range of insulation materials and solutions available to help meet these requirements, however, with UK building plots becoming increasingly tight, it makes sense to opt for a top performing material such as phenolic insulation. This will allow wall constructions to be kept slim, maximising internal living space.

Take a typical cavity wall construction with a 102.5 mm brick outer leaf and a medium density blockwork inner leaf. In order to achieve a U-value of 0.17 W/m2.K (meeting the notional requirements in all three regions), a 100 mm thickness of phenolic cavity insulation should be used to partially fill the cavity. This can be raised to the recommended best starting-point U-values by increasing the insulation thickness to 125 mm. In both cases, double drip wall ties and retaining clips should be used to hold the insulation in place.

If house builders prefer to maintain a slimmer cavity, one alternative is to install insulated plasterboard on the internal leaf. For example, if a 62.5 mm thickness of phenolic insulated plasterboard is fitted, a U-value of 0.15 W/m2.K can be achieved with a 60 mm thickness of phenolic cavity board.

A challenging future?

Whilst the cavity wall approach remains the standard choice for most UK developers, the tightening of energy performance requirements has seen new methods of construction, such as structural insulated panels, start to challenge its position. If it is to remain relevant in the long-term, developers must help it to evolve by embracing new, more efficient technologies. In next week’s blog we’ll look at some of these new approaches and what they mean for the future of cavity walls.

Other posts in this series:

Cavity Walls: The Past

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Kingspan Insulation is a market leading manufacturer of optimum, premium and high performance rigid insulation products and insulated systems for building fabric and building services applications.

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