Cost Guide: Walls and Insulation

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.

They include options for different external claddings, including brick and render, as well as a comparison with using timber frame as the inner leaf (rather than blockwork).

The prices include: builder’s profit margin, labour, materials, finishes, insulation and internal plastering. The costs below do not include: windows and doors.

Cost breakdown:

Self build projects qualify for zero-rated VAT on labour and materials (the latter reclaimed on completion), but renovators and extenders will usually need to pay the standard 20% rate on both aspects

Alternative Construction Methods

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.

Differences Between Thermal and Sound Insulation

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.

How Much does Insulation Cost?

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).

You can check live insulation prices online with suppliers such as Materials Market, Insulation Superstore and builder’s merchants.

As a rough guide, wool insulations can start from a few pounds per m2, with prices rising to around £20 per m2 for the best performing rigid board options in minimal 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 was updated on 27th April 2020 with the latest cost guidance, including prices from the Build It Estimating Service.