Insulated Concrete Formwork – ICF

What is

Insulated Concrete Formwork

Insulated Concrete Formwork (ICF), is based upon a hollow lightweight block structure that locks together before either concrete is poured into the core of the block, or with some systems insulation is inserted into the core.

The initial concept came about after the second world war, when there was a need to rapidly rebuild cities and towns and this brought about the development of using wood fibre cement blocks filled with concrete. During the 1970’s the development of ICF changed to include for the use of moulded foam-based insulation formwork which was filled with concrete, providing a structurally sound and thermally efficient wall.

Today ICF is seen as a more mainstream building solution, especially amongst the self-build industry, where the nature of the interlocking blocks, almost like a large Lego set, has seen this system grow with individuals who wish undertake the installation themselves or benefit from the speed and technical performance of the product.

Insulated Concrete Formwork ICF

ICF Diagram

What is ICF constructed from?

ICF can be made from moulded extruded or expanded polystyrene or a waste wood fibre bonded with cement. With the latter type of system, an insulation is usually added to the core area for thermal performance.

How strong is ICF to build with?

ICF whilst being constructed from foam insulation or cement wood fibre is actually a concrete building structure.

The blocks are designed to be a formwork to support the concrete pour and the addition of rebar for strengthening where required, means that many of these systems are designed to be earthquake resistance.

Is ICF quick to build with?

The nature of the quick installation of the blocks and the concrete pour means that it is possible to construct a single storey in a matter of only a few days.

The insulating nature of these blocks also assists curing of the concrete and the construction method is less susceptible to inclement weather.

Is ICF a thermally efficient system to build with?

The design of ICF systems, means that very low U-Values can be obtained and it is a very good system to use if you are looking at Passivhaus standards of construction.

Most system designers have blocks with U-Values of 0.20W/m²K or less, with some systems being able to achieve levels below that of 0.10W/m²K

Can ICF be installed by anyone?

ICF is the closest building system that could be truly called self-build, but you will still need to have an understanding of building, and it is certainly recommended that you either attend one of the many training sessions that the system designers provide, or use one of the approved installing companies on their register.

Is ICF an airtight construction?

The nature of ICF construction is that it provides a monolithic concrete core by using interlocking blocks and therefore the structure will be airtight.

Building Regulations standards (Part L1A) on airtightness, the leakage can be no more than 10m³/h/m²@50pa. What this means is that no more than 10 cubic metres of air can escape per hour for every square metre of the envelope surface area, with an internal air pressure of 50 Pascals.

Passivhaus construction requires 1m³/h/m²@50pa, whilst most ICF systems will provide an airtightness level of 5m³/h/m²@50pa or less.

How do you finish the walls of an ICF property?

Internally, dot and dab plasterboard with a plaster skim is the most common solution, whilst externally there are many varied options. Thin-coat render systems can be applied directly to the surface of the blocks, whilst brick and stone slips as well as battened out timber cladding are also an option.

With ICF using blocks, are there any constraints on the design?

ICF is fundamentally designed around rectangular blocks; however, that does not mean that you can only build in a rectangular form.

Standard blocks can be formed to make curves and some systems already have curved blocks in the range.

The inherent strength of the walls, means that cantilevers and vaulted ceilings are also possible, in addition to opening of around 6 metres.

Is ICF fire resistant?

Full system fire tests have demonstrated that ICF walls can provide a fire rating of 2 to 4 hours with no structural damage to the concrete core.

Where a expanded or extruded polystyrene formwork has been used, then these are usually treated with a flame-retardant additive to prevent the foams from burning by themselves.