How the two differ in kind
Both build a wall of concrete or concrete masonry. ICF stacks hollow lightweight forms dry, takes reinforcement, and is filled with concrete that cures inside them — and the forms are not stripped, because they are the insulation. Blockwork lays units in mortar course by course, grouting cores where reinforcement requires it.
The thermal argument is the clearest one and it is structural rather than incremental. In an ICF wall the concrete core sits ENTIRELY INSIDE insulation on both faces, so there is no path through the structure at all — no mortar joints, no webs, no block material bridging from inside to outside. In a block wall the mortar joints conduct far better than the blocks, and in a hollow block the webs conduct better than the cores, so heat crosses the wall through a grid of paths the block's own figure does not describe.
What ICF introduces is the pour, and it deserves respect rather than nerves. The forms are formwork: fresh concrete behaves as a fluid and exerts hydrostatic pressure until it stiffens under its own weight, so the controlled variable is the RATE OF RISE. Fill too fast, or in lifts that are too deep, and the pressure exceeds what the forms and their bracing can take — and an ICF blowout is a wall full of concrete arriving on a site in a way that is expensive, messy and difficult to recover from.
The factors that actually differ
Show
| Insulated concrete formwork | Concrete blockwork | |
|---|---|---|
| Thermal bridging through the structure | None. The core is inside insulation on both faces, with no path across it. | Continuous. Mortar joints and webs conduct better than the block, forming a grid the block's declared figure does not include. |
| What the failure during construction is | A blowout. The forms are temporary works and the pour is governed by rate of rise, bracing and lift depth. | Incremental and self-limiting. A course laid badly is a course rebuilt. |
| Speed | Fast to stack, then a pour day that has to go right. Whole storeys go up quickly with a small crew. | Steady and predictable, and it scales with gang size rather than with plant. |
| Trade and familiarity | A different skill set, and the pour is where experience matters most. A first-time crew on a tall lift is the risk case. | Universal. Every locality has bricklayers and everyone knows what a block wall is. |
| Services | Chased into the foam with a hot knife, quickly and anywhere — one of its genuine practical pleasures. | Chased into the block, within permitted depths and positions, and a horizontal chase in a loadbearing wall is a structural decision. |
| Fixings | Into embedded webs at known centres, or long fixings into the concrete. Anything heavy needs the web positions, which is a planning item. | Ordinary masonry fixings anywhere, which is a real day-to-day convenience. |
| Fire and the insulation | The foam requires a protective lining — a thermal barrier — on both faces; it is not left exposed. Below-ground faces also need termite consideration where that applies. | Non-combustible, and that is one of its enduring advantages. |
| Acoustics and mass | Excellent, because the core is solid concrete with a decoupling layer on each side. | Good where dense and solid; a hollow block wall is considerably worse than its weight suggests unless the cores are filled. |
| Reinforcement and inspection | A continuous cage of vertical and horizontal bar sits in the cavity, tied before the pour and visible until the concrete arrives. | Bar goes in grouted cells, and those cells need clean-outs at the base so the cell can be inspected and cleared before grouting. |
| Tolerance and the finished face | As flat and plumb as the bracing was. Alignment is set before the pour and cannot be corrected after it. | As flat and plumb as the bricklayer. Correctable course by course, and a poor course is visible while it can still be lifted. |
Which one, and when
Choose insulated concrete formwork when…
- The thermal target is demanding and eliminating bridging through the structure is worth real money.
- The programme rewards speed and a small crew can put up a lot of wall quickly.
- The wall is below ground or partly so, where a monolithic reinforced core with insulation on both faces is a strong arrangement.
- There is an experienced crew, or a first pour is planned conservatively with short lifts and thorough bracing.
Choose concrete blockwork when…
- The local trade is blockwork and there is no ICF experience to draw on.
- The build is incremental, weather-interrupted or done in stages where a single pour day does not fit.
- Fixings will be made all over the wall and web positions would be a constant constraint.
- Non-combustible construction is a requirement rather than a preference.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- What actually happens if an ICF wall blows out?
- A form ruptures under pressure and concrete escapes, usually at a low level where the pressure is greatest, and it does so quickly. The immediate problem is that the pour has to stop, the escape has to be contained, and the concrete already in the wall begins to stiffen while the repair is made — so a cold joint is being created whether or not anyone wants one. The recovery is to clean out, rebuild the damaged forms, re-brace and re-pour onto a prepared joint. It is expensive in materials, in programme and in the quality of the finished wall. Prevention is entirely in the controllables: short lifts, a limited rate of rise, thorough bracing and alignment before the pour, and somebody watching the forms throughout rather than watching the pump.
- How fast can an ICF wall be poured?
- Slower than the pump can deliver, and that is the discipline. Fresh concrete exerts pressure that is hydrostatic until it stiffens enough to carry itself, so the pressure at the base of a form depends on how much of the wall is still fluid — which depends on the RATE OF RISE. Manufacturers publish a maximum rate and a maximum lift height for their forms, and the usual practice is to fill in lifts of around a metre, working round the building so that each lift has begun to stiffen before the next arrives above it. Anything that keeps concrete fluid longer makes it worse: cold weather, retarders, high slump, and vibration, which re-liquefies concrete that had started to stiffen.
- How do you fix things to an ICF wall?
- Into the embedded webs, which run vertically at known centres and are usually marked on the face of the form — that is the equivalent of finding a stud, and it is why the web spacing is worth knowing before the plasterboard goes on. Light items fix into the webs directly. Anything heavy — a wall-hung boiler, kitchen units, a television bracket, a handrail — is fixed through into the concrete core with an appropriate anchor, which holds very well indeed. The planning consequence is that heavy fixing positions are worth identifying before the lining goes up, because afterwards you are hunting for webs through a finished wall.
- Is the foam a fire risk?
- It is combustible and it is never left exposed, which is how the risk is managed rather than a reason against the system. Codes require a thermal barrier over the insulation on occupied faces — typically a specified thickness of plasterboard — and the requirement applies to both sides, including in a garage or a plant space where a wall might otherwise be left bare. The more common real-world failure is not the finished building but the construction period, when an unfinished ICF structure with exposed foam is on site: hot work, temporary heating and site fires are the hazard, and the site fire plan has to recognise it. Blockwork carries none of this, which is a genuine advantage where fire performance is a governing requirement.
- Does ICF really perform as well as claimed?
- Its structural thermal performance is real and is the strongest part of the case: with the core inside insulation on both faces there is genuinely no bridging through the wall, which is something almost no other construction achieves. Two caveats keep it honest. The declared wall figure is only realised if the junctions are detailed with the same continuity — at the slab edge, the roof line and around openings, where the insulation has to carry through rather than stop. And the claims sometimes made for thermal mass are softer: the concrete core is decoupled from the interior by insulation, so it moderates the structure rather than the room in the way an exposed internal mass would. The wall's steady-state performance stands on its own without that argument.
- Which is cheaper?
- Blockwork on material, ICF on programme, and the honest comparison is the building rather than the wall. ICF stacks fast with a small crew and needs a concrete supply and a pump on the day; blockwork needs a bigger gang for longer and no plant. Against that, ICF delivers insulation, formwork and structure in one operation, so the comparison has to include the insulation and the render or lining that a block wall would need separately to reach the same performance — and once it does, the gap narrows considerably. Where the thermal target is demanding, ICF frequently wins on the whole build-up even where it loses on the wall.
- Can ICF be rendered directly onto the foam?
- Yes, with a render system designed for insulation — a reinforcing mesh bedded in a basecoat, then a finish coat — and not with a conventional cement render intended for a rigid backing. The difference matters because the substrate moves and compresses in ways masonry does not: a brittle render applied thickly over foam cracks at the form joints, and those joints are a regular grid, so the cracking is regular too. The systems that work are thin, reinforced and flexible, and they are specified as a system with a compatible basecoat, mesh and finish rather than assembled from whatever is on the van. The same applies below ground, where the insulation needs a protection board and a drainage layer rather than render.
- How do you know the concrete filled the wall?
- You largely do not, once the pour is over, which is why the controls are all before and during it. Consolidation is the issue: an ICF cavity is narrow, congested with reinforcement, and interrupted by the form webs, so concrete can bridge across a gap and leave a void behind it. The practices that prevent it are a suitable mix with enough workability, placing in short lifts, and consolidating each lift with a small-diameter internal vibrator or by rodding — thoroughly enough to fill, briefly enough not to re-liquefy the lift below and raise the pressure. Tapping the forms and watching the concrete level around window bucks tells you a great deal at the time; nothing tells you afterwards without cutting the foam off.