Adrian Pargeter of Kingspan Insulation describes how ‘lower lambda’ insulation can be the architect’s friend, as more efficiency means less volume is needed to achieve regulations compliance.
Building Regulations have recently become more demanding when it comes to insulating properties to achieve better U-values. If a building design relies on a conventional cavity wall construction, tighter regulations could leave designers with a potential space problem.
The most recent changes to the regulations (Approved Documents L1A and L1B of Building Regulations 2013 in England and 2014 in Wales, and Section 6 2015 of the Building Standards in Scotland), significantly raised the levels of insulation required for a building to reach compliance. As a result, the overall thickness of the external walls, and consequently building footprint, could also increase considerably.
If you are not able to increase the footprint size, or simply don’t want to, then the only other alternative is to reduce the size of the internal space. This can lead to unsatisfactory compromises on the dimensions and layout of rooms, making them feel cramped and unwelcoming.
Above and beyond these considerations, there is a practical problem. Namely that most contractors simply do not like traditional masonry cavity wall constructions to be much wider than 300 mm. Larger cavities have other knock-on effects, such as the need for longer wall ties and bigger foundations, all of which can add to the time and cost of construction.
Slimming down
A typical cavity wall construction would be a 100 mm medium density concrete block inner leaf with a 100 mm brick outer leaf. That leaves just 100 mm void for the insulation and cavity. So how can designers meet the regulatory requirements and stay within those parameters? The first thing to look at is the thermal conductivity of the insulation to be used, i.e. its lambda value. The lower it is, the more efficient the insulation, so the thinner it needs to be to be to exceed the required U-value.
The thicknesses required for commonly used insulation materials to achieve the same level of thermal performance vary considerably. To achieve a tested R-value of 2.857, mineral fibre required 110 mm (for a lambda value of 0.038 W/m.K), whereas PIR required 65 mm (0.022 W/m.K), phenolic required 60 mm (0.020 W/m.K), and advanced phenolic 55 mm (0.018 W/m.K).
Phenolic insulation cavity boards are widely used in UK construction. They have a fibre-free core, and are designed to be unaffected by air infiltration and to be easy to handle, and offer excellent levels of thermal efficiency. Recent product innovations in advanced phenolic cavity boards have meant that reduced insulation thicknesses can now be achieved that deliver a desired U-value for the entire cavity wall construction.
Boards can be combined with foil facings which are well-suited to partial fill applications. Reflecting radiated heat back into a property, they can significantly increase the thermal resistance of the cavity and reduce the thickness of insulation required.
However, facings still require a 50 mm clear cavity to be maintained, so a further development has been water-tight polypropylene fleece facings to protect against moisture ingress, allowing the air gap to be reduced to just 10 mm. The gap between the board and the outer facing can be simply maintained using a retaining clip which fastens to the wall ties, and a self-adhesive breather tape should be used to create a continuous water-tight facing.
Combining such facings with advanced insulation materials can significantly reduce whole wall construction thicknesses. Take the example of a traditional cavity walled construction with a 102.5 mm brick outer leaf and a 100 mm medium density block inner leaf (with a 6.6 per cent thermal bridging factor assumed for mortar joints).
In this example a 90 mm thickness of lower-lambda phenolic insulation, plus a 10 mm air gap, is all that is needed for a whole wall U-value of 0.18 W/m2.K. This construction could be applied to help meet Building Regulations and standards across the UK, including in Scotland where they are most stringent.
Performance, not compromise
In balancing the different demands of design, preferred construction and building performance, compromises sometimes have to be made. Is space more of an issue than energy efficiency? Will performance suffer at the hands of convenience? Straightforward solutions are however available that resolve the dilemmas, and allow designers to continue to employ the popular cavity wall construction approach whilst keeping walls slim and well insulated.
Adrian Pargeter is head of technical and product development Kingspan Insulation.