Cavity Walls: The Future
Back in 2011, the RIBA carried out an extensive survey of modern homes and homebuyers’ opinions towards therm.
The report showed that for people who would consider purchasing a new-build home, energy efficiency was the most important benefit, whilst for 60% of people who would not consider a new-build home, small room sizes was the key draw-back.
These results neatly summarise the dilemma developers currently face. With demand continuing to spiral, housebuilders naturally want to maximise the capacity of each development, however, delivering homes within these tight plots which offer sufficient levels of both living space and fabric performance is a major challenge.
Within this context, it is understandable that the future of cavity walls may come into question. Unlike modern approaches which are designed specifically to deliver high levels of fabric performance, cavity walls have evolved over time. As a result, they typically require significantly thicker constructions to match the fabric performance of methods such as structural insulated panels. Fortunately, new technologies are helping to close this gap.
When it comes to improving the thermal performance of cavity walls, the most obvious place to start is, of course, the insulation layer. Phenolic insulation cavity boards are already widely used within the UK construction industry and recent product developments mean that the top performing phenolic cavity boards now have a lambda value of just 0.018 W/m.K. This reduces the insulation thickness needed to deliver the required thermal performance, as shown in the table below.
|Lambda W/m.K||Type of Insulation||Thickness|
|0.018||Lower Lambda Phenolic||55 mm|
|0.034||Rock Mineral Fibre||100 mm|
Above: Thicknesses required for commonly used insulation materials to achieve the same R-value of 2.857 (thicknesses are rounded up to nearest 5 mm).
Much like existing phenolic cavity boards, the lower lambda cavity insulation boards are available with low emissivity foil facings which significantly increase the thermal resistance of the cavity and are therefore well-suited to partial fill applications.
For those wanting to further maximise the thermal performance of the cavity construction, the boards are also available with a water-tight polypropylene fleece facing. The polypropylene fleece facing helps to protect against moisture ingress, allowing the required air gap to be reduced to just 10 mm. The gap between the board and the outer facing can be simply maintained with the use of a retaining clip which fastens to the wall ties and holds the insulation in place. A self-adhesive breather tape should be used to create a continuous water-tight facing.
In addition, a further way to reduce the width of cavity walled constructions is with the use of aerated blockwork. These products have better thermal conductivities than traditional blockwork. For example, the table below compares two cavity walled constructions, one using medium density blockwork and phenolic insulation, the other combining lower lambda phenolic insulation and aerated blockwork. Both specifications are designed to achieve a U-value of 0.15 W/m2.K. The savings are clear, with the lower-lambda product coming in at 40 mm thinner than the standard phenolic construction.
|Cavity wall with medium density blockwork and phenolic insulation||Cavity wall with aerated blockwork and lower lambda phenolic insulation|
|Brickwork||102.5 mm||102.5 mm|
|Insulation||125 mm||100 mm|
|Residual Cavity||25 mm||10 mm|
|Blockwork||100 mm||100 mm|
|Total||352.5 mm||312.5 mm|
Above: Cavity wall specifications designed to achieve a U-value of 0.15 W/m2.K.
Of course, thermal performance is not the only consideration when designing the building envelope. As homes become more energy efficient, it is essential to pay even closer attention to detailing in order to limit thermal bridging. Wall ties have typically been a key source of bridging within cavity walled constructions, however, new thermally broken wall ties can help to reduce this. These products are typically capped with a material which has a lower thermal conductivity than the metal tie. The application of tape to joints between the insulation boards within the cavity can also help to create more airtight properties, further improving building envelope performance.
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