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Guide to Designing an Insulating Concrete Formwork (ICF) Home

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Energy efficiency and speedy build times are the headline benefits of using insulating concrete formwork – but many architects also favour this system because of its design flexibility, says Emily Brooks
ICF house with steel supports

Although the technology behind it is more than 50 years old, ICF (insulating concrete formwork) is just hitting its stride in the UK. Its key benefits – a quick, all-weather build, and impressive thermal performance – make it a good fit with most self-builders’ preoccupations. But how do you create an ICF home that maximises these advantages and brings some appealing design flair? 

This system is one of the simpler build methods to understand. The insulating formwork that gives the system its name consists of modular hollow blocks, usually made from polystyrene (but some, such as Durisol, from wood cement). These are interlocked on site and reinforced with steel rebar, before concrete is poured into the cavity. The pour is generally done in stages, floor by floor. Once this phase is completed, the building can be finished externally with render or whatever cladding is desired.

The concrete has fantastic structural strength as well as excellent thermal mass – absorbing, storing and releasing heat to moderate internal temperatures. Thanks to its two layers of insulation, an ICF house can achieve Passivhaus standards of efficiency (assuming it is combined with other appropriate measures, such as high-performance glazing).

Nigel Design ICF home

Steel supports were hidden within the Beco Wallform ICF core of this house in the Scottish Highlands by architecture Nigel Design

ICF and design considerations

Although the ICF system consists of modular blocks, that doesn’t mean you’ll end up with a blocky-looking house (unless you want one, of course). Robin Miller from Beco Wallform suggests that using an architect with some familiarity with the construction method is always useful, both because they will know how to maximise thermal performance but also because of their design flair. “You don’t want to end up with a box – or even worse, something that can’t actually be built,” he says.

Robin works regularly with architect Nigel Johnston of Nigel Design, who has created homes using many construction methods. “With ICF you can create anything, from a stone-clad castle to a contemporary house that’s filled with glass,” says Nigel. “It’s extremely adaptable and you don’t need to feel confined by any design constraints.”

Architectural technologist Jake White of Ecotecture says that “the most prominent reason to use it, from a design perspective, is flexibility.”

Ecotecture ICF house

Curly House was designed by Ecotecture and built using Nudura’s ICF system

The practice created Curly House in Sussex, which was built using Nudura’s ICF system. The property has become a celebrated example of the flexibility this construction method offers. It’s the least blocky building that you could imagine, with a crescent-shaped footprint and daring curved glazing.

If you want to build as efficiently as possible, your house should have a design that is aligned with the dimensions of whatever ICF system you’re using (Beco Wallform’s, for example, works on 62.5mm increments). This means less work, and less waste, on site. “If you design to our dimensions, we can keep waste to 1% or less,” says Jean-Marc Bouvier, Nudura’s director of sales and business development.

Creating striking features using ICF

“Architects who have used Nudura love the fact that there are no design restrictions,” says Jean-Marc. But as with any system, you may end up paying a bit more for unusual work.

To create Curly House, for instance, bespoke Nudura blocks of the correct radius were specially made for the project, which is more expensive than using off-the-peg formwork. “The house design started out with lots of different radiuses, but we refined and rationalised it until we had just two – the fewer types of block you are procuring, the better, from a price perspective,” says Jake.

A cost-effective alternative is to use straight blocks cut into wedge-shapes to fit the desired radius, although this will result in a more faceted appearance rather than a smooth curve. But the angled elements could be covered up with an additional layer of carrier board.

Logix system ICF extension

The Logix system was used as part of a hybrid construction for this extension, in tandem with steel frame and swathes of glass. This allowed for a concrete upper floor structure and external balcony, whilst also offsetting the heat loss from the glazing. The 4m tall ground floor walls were cast in a single operation

The concrete core’s structural strength is an advantage when adding features such as brise soleils, which can be bolted straight on.

“For contrast, if you were adding an overhang on a timber frame house, you might need an additional steel frame to provide the primary support, which has the potential to create thermal bridging, as well as some movement and cracking,” says Jake.

Large spans are also possible with ICF, with some structural engineering expertise. “The standard opening is up to 6.3m, but we did a house with 12.8m openings – it’s just a case of adding a bit more rebar and a bit more concrete,” says Jean-Marc.

Nigel Johnston loves ICF for its ability to conceal any additional structural elements. “You can put extra reinforcement in and beef it up where you need to,” he says. “By adding a steel 139mm circular hollow section within the concrete core, the structure will hold up just about anything – one project I worked on was able to support a very heavy grass roof, for example.”

Insulating concrete formwork homes and exterior finishes

  • ICF homes can be clad in any material externally, so the build system lends itself to all styles of home, from country cottage to minimal modernist dwelling.
  • Render is the easiest and cheapest way to finish the outside of an ICF house. A layer of metal or plastic lathing is fixed to the polystyrene outer layer first, and the render goes straight on top.
  • Brick slips can also be adhered to the outer face of the formwork. The preparation is the same for render:a waterproof coating and a layer of metal or plastic lath.
  • Real brick or stone outer leafs can be built up with a cavity back to the ICF inner wall, tied back as with conventional masonry.
  • Timber cladding can be affixed using a batten and rail system, screwed into the concrete core.

On-site and later changes

It’s possible to make alterations to your design once you are on site, but it’s a case of the sooner the better. “Before the concrete is poured, it’s easy to take the blocks apart and put them back together again if you need to move something, such as window and door positions,” says Jean-Marc.

The monolithic structure of an ICF home makes it quite difficult to change once it’s built, however. Jean-Marc suggests that one way to future-proof a design is to have the lintels cast in for any potential windows and doors, so that if you do ever plan to extend, the structural integrity will already be built in.

Robin at Beco Wallform says that “if you wanted to put in a small window, you might not even need to put in a new lintel – there’s a good chance that the concrete would provide sufficient support in itself. You’d need to have a word with an engineer to see what they say.”

Most people don’t use ICF for their internal walls, despite the advantages when it comes to soundproofing. By sticking to standard non load-bearing studwork partitions, future internal layout changes will be straightforward.

Main image: Jim Stephenson – Durisol blocks (made from wood cement, with a concrete core) were used for the ground floor of this build in Shoreham-by-Sea, by ABIR Architects. The walls were clad with gabions, their stones making the building melt into its surroundings.

First published: April 2017

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