When it comes to specifying your external walls – and your ultimate choice of construction system – there’s a wide range of really credible options to choose from.
Each will have core benefits, which generally fluctuate around price, speed of build, performance, eco-friendliness and aesthetics. Getting the mix right for your project often takes lengthy debate and research.
Often referred to as traditional cavity wall construction, this is the most commonly used method of building houses in the UK and we’ll use it as our benchmark here.
Facing bricks are used on the outside skin and then lightweight blocks for the inside, which have both strength and thermal properties. When insulation is added to the wall, either in the cavity or as an extra layer on the inside (or both), it can provide a good overall performance.
How well it functions depends on the type and thickness of both the lightweight blocks and the insulation.
A typical specification that would comply with the LFP of a brick-and-block external wall could be a 100mm facing brick, a 100mm mineral wool fully filled cavity, and a 100mm internal lightweight block with 15mm lightweight plaster.
Overall, and on an average-sized project, costs could be £50 per square metre (m2) for the brickwork, £10 per m2 for the insulation and £20 per m2 for the blocks.
Excluding the plasterwork, windows, cavity closers etc, the built price of masonry walls should be around £80per m2, which should include the wall ties and DPCs.
The table below shows a model cost guide to various walling systems. These benchmark costs (courtesy of Andrian Wild from the Build It Estimating Service) are based on constructing a new house with a net wall area of 117m2.
The prices include: builder’s profit margin, labour, materials, finishes, insulation and internal plastering. The costs below do not include: windows and doors.
When you start to consider some of the other options available it can be quite difficult to make financial comparisons against the benchmark of traditional brick-and-block construction mentioned above.
The common distinction with timber/metal frame, structurally insulated panels (SIPs), insulated concrete formwork (ICF) and thin joint blockwork as alternatives is that they are all substitutes for the inner skin and, with the exception of thin joint, the location of the insulation.
Additionally, for timber/metal frame and SIPs they will also need to be used in conjunction with a lighter weight intermediate floor structure.
Generally speaking these other options will cost you more, as they all add different performance levels. In most instances they will also save a considerable amount of time on site, something that should not be underestimated as a real cost saving.
Insulation is a fundamental element of a building’s design and it serves two purposes: its inclusion in our ground floors and external walls is for thermal reasons, while in our internal walls and intermediate floors it is for acoustic reasons.
To make matters more complicated, insulation with high thermal properties won’t be that good for acoustic performance, and vice-versa. This is because for thermal performance you need pockets of air (like a duvet, for example) and for acoustic performance you need density.
There are minimum standards for the thermal performance and air tightness of our external envelopes. More recently, new rules have been introduced to help regulate standards concerning sound reduction, especially with reference to joined terraced properties or flats.
Your designers will have to ensure that your specifications meet minimum levels of compliance, but this is rarely a problem with self-builders, who tend to try to exceed current standards in order to benefit from the extra performance that it delivers.
Part L1A of the 2010 Building Regulations calls for minimum ‘limiting fabric parameters’ (LFP) for each thermal element. Beware though, because if one part of your specification is on the low side, other areas may have to be upgraded to compensate for it.
This is because building control assesses thermal performance based upon minimising carbon emissions through the design as a whole, and not through minimum compliance of each thermal element.
The sliding scale of thermal insulation pricing starts with mineral wools and moves through to expanded polystyrene, and then on to foil-backed polyurethane (which has now been improved to combat the pollutant chemicals used in its production).
These options can start from a few pounds per m2, ending up at around £20 per m2 for the best performing options/thicknesses.
When it comes to acoustics, look for the denser insulation options – 24kg per m2 for walls and anything up to 100kg per m2 for intermediate floors.
My preference with timber-joisted floors is to use a really dense 30-40mm of acoustic insulation with a 19mm plasterboard plank sandwiched between two layers of flooring-grade chipboard; this combination deals brilliantly with both airborne and impact noise.
It’s a more expensive option for both materials and labour, and has to be thought through as a detail before you start building, but it’s well worth it if noise mitigation is important to you.
Tim Doherty’s article was originally published in 2013. It has been updated with the latest cost guide from the Build It Estimating Service, published in June 2018.