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Insulated Concrete Formwork (ICF)

If you have ever been to a Homebuilding & Renovating Show, you cannot have failed to notice various stands full of polystyrene building blocks. Maybe you have stopped for a chat and picked up a leaflet, or maybe you’ve simply hurried on by, but, to date, not that many of you have been brave enough to actually build your houses using them.

However, during the past couple of years, there seems to have been a sea change taking place and the suppliers all report sharply increased sales. Not just to self-builders, but to hard-nosed developers as well. An appear­ance on Grand Designs, albeit not without its problems, has further projected the whole concept into the limelight. So what are these polystyrene houses all about? Is it now time to seriously consider using them as an alternative to the mainstream choices of blockwork and timber frame?

Insulating Concrete Forms

Of course, these are not really polystyrene houses at all. The polystyrene is used as a mould or form into which you pour ready-mixed concrete and it is this concrete that forms the structure of the house. The idea seems to have originated in Germany after the war when they were looking for new methods to reconstruct the bombed-out housing stock. Concrete walls are traditionally constructed using timber formwork, which is taken down after the concrete has set. Timber formwork is fiddly to make and a lot of it cannot be reused. Using polystyrene in place of timber effectively kills two birds with one stone: it acts as formwork to mould the concrete and is then left in place to provide insulation for the walls. Hence this building system is known generically as Insulated Concrete Formwork, or ICF.

Whilst ICF is a European innovation, its uptake has been most dramatic in North America. Unlike Europe, North America does not have a tradition of masonry work and if you want a heavyweight structure the options are limited. ICF has thus proved popular for basements and in hurricane areas such as Florida. During the past 20 years, ICF manufacturers and builders have sprung up across the USA and Canada, and most of the systems now available in the UK and Ireland hail from across the Atlantic, often set up here under licence.

In Britain, the first ICF houses were built in the 1970s but they have remained something of a curiosity until recently. What seems to have triggered the recent surge in interest in ICF is the demand for ever-greater levels of wall insulation, which is finally causing builders to question the sense in continuing with cavity wall blockwork. The latest changes to the energy-efficiency regulations, which came into effect in early 2006, seem to have accelerated this process further still.

Insulated Concrete Formwork

Advantages over brick and block

In common with most of the new build systems, ICF is fast to build and requires less in the way of traditional building skills. The formwork goes up with remarkable speed and there are far fewer elements than you get with cavity wall work: no lintels; few, if any, wall ties or cavity trays. Fitting doors and windows is at least as straightforward as it is with masonry work and finishing inside and out is essentially the same. In return you get fantastic insulation levels built in: most ICF systems give a wall U-value of around 0.2, well inside the level demanded by Building Regulations. And ICF systems are flexible enough to allow you to build in any style, to virtually any dimension.

Advantages over timber frame

Timber frame in the UK is primarily built in factories. This is both a strength and a weakness. It is a strength because any structure built under factory conditions tends to be accurate and well made, and on-site assembly is very quick; it is a weakness because in ordering a timber frame you are at the mercy of a factory’s production schedule and it may take weeks or even months for the frame to arrive on site. ICF requires two basic components: the polystyrene forms that should be available on most jobs within days, and ready-mix concrete, which is available at a few hours’ notice right across the country. Some ICF systems require special fixtures and fittings and on occasion jobs get held up because of unavailability, but in principle ICF is not only fast to build but also does not involve long lead-in times.

Costings

Perhaps the main factor that turns many potential ICF builders away from the whole idea is their headline cost. They appear to be very expensive when compared with block­work wall costs. Most ICF systems on sale in the UK cost between £25 and £35/m²; add ready-mix concrete at £10/m² plus a few extras and you have a wall cost of well over £40/m² before you have even taken labour into account. In contrast, blockwork can be built for around £20/m² including labour.

But, as so often happens when you come to cost out elements of building work, a more thorough comparison shows the ICF cost model in a very different light. A blockwork wall on its own is only a small part of the overall wall assembly: it needs an insulated cavity and a waterproof outer skin, usually built from bricks, stone or a rendered second skin of blockwork. Also, the joinery openings require steel lintels over them and there is additional work required with wall ties and cavity closers. A truer figure for the cost of a brick and block wall is between £70 and £100/m².

In contrast, the labour costs on ICF are very low: an experienced ICF crew is capable of laying 5m² of wall per hour. Combine this with an external render coat, costing around £25 or £30/m², and you end up with a wall cost of between £80 and £90/m², slap bang in the middle of the cost range for masonry and timber frame walling.

But in return, you get very high energy-efficiency levels built in at no extra cost, good soundproofing, excellent airtightness and less room for poor detailing, as often happens with masonry cavity wall work.

Another plus factor for ICF costing stems from the speed of build, which reduces the preliminary costs of building, the money spent on fencing, plant and scaffolding. Several of the systems offer the possibility of building the walls up entirely from the inside, thus further reducing the need for external scaffolding until much later in the job. ICF no longer looks expensive and as if to emphasise that point, it is now being taken up by commercial developers as well as self-builders.

Insulated Concrete Formwork

Differences

Although ICF tends to be lumped together as if it were one homogeneous product, the reality is that the various systems are very different from one another. They all use polystyrene and they all use concrete but just how they go together varies a lot. Many are based on a moulded block format, such as Beco, Styrobuild and Logix. Others are based around large flat panels that you have to tie together on site: this is how Polarwall, Quadlock and Eurozone work. Others, like Polysteel, use a hybrid system. By and large, the panel systems are cheaper to purchase but require a little more work on site.

The variations reflect the piecemeal way ICF has developed, with lots of relatively small producers working through prototypes to develop their ideal format. You’d be right to infer from this that some systems are more suitable for DIY construction than others and as a rule it is the small-format moulded block systems that the inexperienced find easiest.

Follow-on trades

You can clad ICF externally with any material. However, it is true that through-coloured flexible renders tend to work best, as you can apply these directly onto the polystyrene. If you want a brick or stone façade, you have to tie into the structural shell. How you do this varies from system to system: with some there are fixings built in, with others you have to set wall ties through the polystyrene before the pour.

Internally, the walls are routinely finished with plasterboard, stuck on to the polystyrene with sticky dabs. This is very similar to how blockwork homes are finished. Electric cable and pipework can be run within the polystyrene in channels cut with a hot wire cutter.

Internal walls

Internal walls are one area where ICF walls are much more expensive than the industry standard solutions, timber or steel studwork. However, they also produce fantastic acoustic insulation without the need for any additional work. And any ICF wall is also a structural wall, capable of taking the load for precast masonry floors. For these reasons, many self-builders specify ICF walls for at least their downstairs room-dividing walls, as well as their external walling.

3 reasons to use formwork

It is quick: As you might expect with a construction system so brilliant in its simplicity, the structure of houses built using ICF goes up incredibly quickly. There are also far fewer elements than you might get with cavity wall work — no lintels, few (if any) wall ties, and no cavity trays. Taking into account the fact that the structural element of a home’s construction accounts for around 30% of the total, self-builders using ICF can expect to save 1-3 months on their project time.

It is inherently warm: ICF houses come with fantastic insulation levels built in — most systems give a wall U-value of around 0.2, which is well inside the levels demanded by the Building Regulations.

It is flexible: While it could be argued that some homes wear their construction method on their sleeve, it is impossible to tell an ICF home from a house built using masonry or timber frame. ICF homes can be contemporary, traditional; curved or straight.

Insulated Concrete Formwork

ICF vs SIPS

Many self-builders weigh up ICF against Structural Insulated Panels, a new construction form rapidly gaining popularity in the UK. There are some similarities and some differences.

Similarities

  • Both systems use insulation structurally, rather than adding it later to the structure.
  • Both systems could be categorised as being fast, when compared to their mainstream alternatives, and both demand much less in the way of traditional building skills, like masonry work and carpentry. The presence of one experienced supervisor on site is probably all that is required.

Differences

  • ICF is wet: the concrete has to be added on site. SIPs are a completely dry method.
  • ICF is built up on site from pre-formed modular units. In contrast, SIPs tend to be made to order in a factory, just like timber frame.
  • ICF depends on the concrete for its strength: SIPs are inherently strong without the addition of anything on site. SIPs, therefore, give you a narrower wall profile.
  • Whilst both can be used for walls, SIPs can’t be used for basements whilst ICF can’t be used for roofs. Several homes have now been built using both ICF and SIPs.

Tips

Although ICF sells itself on its simplicity, it is far from foolproof. There is in fact a fair amount of knowledge needed to build well with it. With all the systems, the concrete pour is an absolutely critical phase. The ready-mix has to be right in order for the concrete to flow evenly throughout the formwork; the support and bracing on the polystyrene blocks also has to be adequate or there is a risk of the walls distorting or even bursting open at pressure points. Mistakes can happen and, because of the speed involved, they can appear to be quite alarming, but an experienced hand can generally fix any mishaps within a few minutes.

In fact Kevin Oelmann, site manager at a Polysteel site, advises others not to be fearful of ICF walls moving around a bit during a pour. “When we first started working with the Polysteel system, we spent almost as long bracing the walls before a pour as we did building them. Then we had a visit from one of the experienced American contractors who showed us how to pour with minimal bracing and then to get everything level and plumb after the pour was complete. Overnight we halved our build speed and improved the finished result.”

This article is sponsored by Nudura

9 Comments

I am an employee of a builder in western Canada and this system seems like it would be very good for the areas we build in. Our clients are primarily those looking for custom homes and I wonder if these methods would be acceptable to them as well as if they are allowed in the codes here. I will be talking to the owner Monday. This would be awesome for some applications. Especially for basements I would think. The post certainly has me thinking...
We have recentley done a comparative costing on a wall with 0.1 U value. We find Sipcrete is about £65 a sq meter cheaper for a finished product than Beco ICF Blocks. This does not include labour costs, Sipcrete Labour input is half that of Beco Walls.
If you where to build a simple box structure about 5m by 10m would this be a simple and cheap way of doing it?
Hi, Great article, I am amazed at the amount of new and not so new products their are to replace traditional brick and block. One of the things I am interested in though; you mentioned that ICF needs less ties and no lintels. I can understand how you might not need lintels if these blocks straddled the opening, but fixings and supports systems are there for different reasons. In a large area of blockwork the wall ties and masonry fixings prevent undue flex in the wall. The support system I guess isn't really used in everyday home design. What I'd be interested in hearing is how you've determined that there is less need for typical restraint fixings using ICF. Cheers Andy

How thick are walls constructed with ICF and how much space are you likely to lose or how much extra space would you require when building with ICF?

Hi Tony,

This article http://www.homebuilding.co.uk/advice/key-choices/structural/build-system... has the widths (with diagrams) of all the main structural systems (including ICF).

Hope it helps and kind regards,
Samuel Joy (Online Editor)

Thanks for this. The articles is splendid. It answers the question perfectly

No problem, glad it helped.

This article was so interesting, we have been using insulating concrete forms (as well as other similar techniques) for several years now and I had no idea where it had originated from! We have built a couple of houses but never anything as extravagant as some of the examples in this article.

http://doyleconcrete.ie/