Cavity Wall: From Ancient Rome to Modern Britain
The humble cavity wall is loved by builders and developers across the country. It has been used in the construction of over three quarters of Britain’s homes. So how did this construction method come to dominate the British housing stock?
The Early History
The basic principles of cavity wall construction – two leaves separated by a void – have been around since ancient times, with examples found in both Greek and Roman architecture. Like many other technologies however, the approach was essentially lost in time and it wasn’t until the early 19th Century that British architects and builders began to experiment with it in earnest.
The advantages of the cavity walled approach over the typical solid walled construction of the time were obvious. The outer wall acts as a barrier against the elements and works in tandem with the inner leaf to support the structure. The void, in turn, helps to prevent the passage of damp to the inner wall. However it wasn’t until the inter-war years that this approach really began to flourish. By the 1940s it had become the predominant method of domestic construction in the UK.
Insulating the Cavity
Thermal performance requirements for dwellings were first introduced in the 1965 Building Regulations, with external walls expected to achieve a U-value of 1.7 W/m2.K. The oil crisis of 1973, which saw the average barrel price rise by almost 400% in a single year. This prompted further tightening and by 1975 exposed walls were required to achieve a U-value of 1 W/m2.K.
Cavity insulation was a natural solution for builders and architects faced with this new requirement. By fitting insulation within the existing void it was possible to conceal the insulation layer. This also minimised any increases in the thickness of the external walls.
In this period, foam was typically used to partially or fully-fill the cavity but as successive building regulations have further tightened U-value targets. It has been necessary to find insulation solutions which boast ever greater thermal performance.
In terms of compliance, this target will vary depending on which home nation the dwelling is being constructed in. 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 Kingspan Insulation recommends 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|
Table 1: 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 allows wall constructions to be kept slim, furthermore 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. These are such as structural insulated panels, start to challenge its position. If it is to remain relevant in the long-term, developers therefore must help it to evolve by embracing new, more efficient technologies.