Build Systems: Insulated Concrete Formwork (ICF)

The ready-mix concrete is delivered on a truck and poured in lifts, with the crew gradually moving around the building. This minimises pressure on the ties and bracing, thus reducing the chance of Grand Designs-style blow-outs.

The concrete must be carefully directed and vibrated to prevent voids forming. Particular attention to detail is needed around structural openings and areas such as gables to ensure a full fill, for maximum strength and airtightness.

Is ICF energy efficient?

Once the pour is complete, the ICF remains in place to provide a robust layer of thermal insulation – one that will last for the lifetime of the building.

The formwork bestows fantastic U-values (a measure of heat loss) ranging from around 0.30 W/m2K right down to around 0.11 W/m2K. So you can enjoy a highly efficient home that easily surpasses Building Regulations standards – meaning lower bills and less energy in use.

ICF structures also offer very good airtightness as standard, as the concrete pour and insulating formwork combine to create a highly sealed house shell. This helps to reduce heat loss even more.

Different thicknesses of ICF block, with varying insulation depths, are available to help you achieve your target thermal performance (including Passivhaus standards).

Another benefit, and a big plus in this age of environmental awareness, is that the system only produces a tiny amount of waste in construction. Any offcuts of polystyrene can be re-used elsewhere, on or off the build, and you can order the ready-mixed concrete to exact volume requirements.

10 reasons to build with icf

Because ICF is such a heavyweight formwork, it also provides fantastic acoustic properties. “ICF homes are incredibly quiet”, says Chris Stride, chair of the Insulating Concrete Formwork Association (ICFA)

“Heating and cooling bills will be minimal, and because you have an air quality system, they never feel stale. It’s almost like they cuddle you!”

How long does it take to build with ICF?

ICF is fast to build with and requires less in the way of traditional building skills when compared to brick-and-block and even timber frame projects.

In most cases, the formwork for a four-room ground floor can be erected in just a couple of days. Your contractor would then spend a few days bracing the walls and any structural openings.

The concrete pour for a whole storey can usually be completed in less than a day.

Only a small team of semi skilled (to around NVQ level 2) operatives is needed to put up the basic structure – which will save money on time and labour.

Most ICF firms in the UK run training courses to bring you up to the right level of competency to help construct the house yourself.

And, as the insulation and structure are all completed in a single process, the homeowner inevitably saves time and expense on this, too.

Design and finish 

While it is easy to spot the construction system used on many homes, it is nearly impossible to tell an ICF house from a masonry or timber frame project. This is because the system is ultra flexible design-wise.

The blocks can be used for creating curved walls, arches and irregular angles – you can build in any style, to virtually any dimension.

It’s easy to construct bay windows, large openings for sliding doors and features such as cantilevers, as they can be integrated into the building envelope and internal walls, eliminating the need to specify specialist products.

The polystyrene blocks or sheets give a uniform and smooth surface that is ready to take most forms of internal and external finish.

Externally you can clad ICF with a myriad of materials. Many ICF homeowners opt for some sort of coloured flexible render, as it works really well and can be applied directly onto the polystyrene.

You can also choose more conventional finishes such as brick slip, stone facade or weatherboarding depending on your design.

Strong and easy to waterproof, ICF is very popular for basements on new build homes, too. In fact, in many cases it’s cheaper for basement builds than other solutions.

Read more: Design Guide to Building with ICF

What to bear in mind

Although ICF sounds like simplicity itself, there are a few things to bear in mind if you choose the system.

The pour stage is critical – the ready-mix concrete has to be of the right consistency to flow to all parts of the formwork.

The ICF system must also be braced to exacting measurements, or else walls could distort. The concrete has to be poured in slowly and steadily to avoid any risk of the forms bursting open.

Obviously mistakes do happen, but an experienced ICF contractor will be able to resolve any problems.

Adapting the house in the future – adding doors and windows, or moving their position – can be fairly difficult, although not impossible using specialist tools.

What does it cost to build with ICF?

As a general guide, an ICF home building project is likely to come in at around 5% more than a typical timber frame home.

As we’ve seen, however, in the right hands insulated concrete formwork can be an extremely quick and efficient system. That can net you significant savings on labour costs.

You can save even more on your build costs if you decide to do some of the work DIY on your ICF project. But remember that, if something goes wrong, it will be up to you to spend the time and money putting it right.

To get an idea of what your home building project might cost, check out Build It’s interactive Build Cost Calculator.