Strict Building Regulations relating to insulation can mean thick external walls that eat into the available living area. Kingspan Insulation’s Adrian Pargeter explains how to comply without compromising on space.
For many years, the major stumbling block for eager self-builders has been the challenge of finding a suitable plot. However, the introduction of the Self-build and Custom Housebuilding Act 2015 at the start of April should help to change things. The Act requires local authorities to keep a full register of interested self-builders within their catchment and to permission sufficient plots of land to meet that requirement.
Of course, once you have a plot, the key question is how to get the most from it? One of the primary reasons people choose to self-build is to gain a bit more space to stretch out and enjoy, but the home also has to meet the increasingly strict energy requirements in the Building Regulations and Standards. If you’re planning on sticking with a conventional cavity walled construction, this could lead to a potential problem.
Walls closing in
The most recent changes to the regulations governing the energy performance of homes 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 the footprint of your building, could increase considerably. If you aren’t 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 ugly compromises on the dimensions and layout of rooms, making them feel cramped and unwelcoming.
Beyond these considerations, there is an additional problem. 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.
Keeping it slim
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 only a 100 mm void for the insulation and cavity. So what options are available to meet the regulatory requirements and keep within the preferred 100 mm void?
The first thing to look at is the thermal conductivity of the insulation to be used, otherwise known as lambda. The lower the lambda value, the more efficient the insulation is, so the thinner it can be to meet or exceed the required thermal performance, or U-value, for the construction.
For example, the table below shows the thicknesses required for commonly used insulation materials to achieve the same level of thermal performance (R-value).
There is a huge variation, and the solution to the issue of space is a simple one. Phenolic insulation cavity boards are already widely used throughout the UK construction industry; they are easy to handle, have a fibre-free core, are unaffected by air infiltration and offer excellent levels of thermal efficiency. Recent product innovations mean that the advanced phenolic cavity boards now have a lambda value of just 0.018 W/m.K. This latest development further reduces the insulation thickness needed to deliver a desired U-value for the entire cavity wall construction.
Much like existing phenolic cavity boards, these lower-lambda advanced phenolic cavity insulation boards are available with foil facings which are well-suited to partial fill applications. These facings are classed as low emissivity. Put simply, this means their shiny surface helps to reflect radiated heat back into a property. As a result, they can significantly increase the thermal resistance of the cavity and reduce the thickness of insulation required.
However, these facings still require a 50 mm clear cavity to be maintained, so a further development has been the addition of water-tight polypropylene fleece facing. This helps 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 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.
The combination of lower-lambda phenolic insulation boards with this new facing can significantly reduce whole wall construction thicknesses. Take, for example, a traditional cavity walled construction, with a 102.5 mm brick outer leaf and a 100 mm medium density block inner leaf. A 90 mm thickness of
lower-lambda phenolic insulation, together with a 10 mm air gap, is all that is needed to achieve a whole wall U-value of 0.18 W/m².K. This construction could be applied to help meet the Building Regulation/Standard requirements right across the UK, even in Scotland where they are most stringent.
A well insulated approach Any self-build is a project of passion and most owners are looking to create a property which will last them a lifetime. It’s important to keep that in mind when it comes to decisions such as the insulation specification. While it can be tempting to scrimp on the thermal performance to reduce wall thicknesses, ultimately this
decision could lead to much greater costs in the long run as a result of higher energy bills and may even lead to the property being deemed non-compliant.
Lower-lambda rigid phenolic insulation boards offer a straightforward solution. They allow homeowners to stick with the tried and tested masonry cavity wall construction approach while keeping the walls both slim and well insulated. This should help to keep you warm without burning a hole in your wallet.
Adrian Pargeter is head of technical and product development at Kingspan Insulation.