Insulated concrete formwork (ICF): a complete guide

What ICF is

ICF walls are built from hollow blocks or panels, usually made of expanded or extruded polystyrene or another insulating material. They are stacked like building blocks and reinforced with steel bars. Concrete is then poured into the hollow core. The insulating blocks stay in place permanently, so the finished wall is a reinforced concrete core with insulation on both faces.

How it goes together

1. Foundations are built to receive the wall, with starter bars where the core will be reinforced.
2. Blocks are stacked in courses, with interlocking edges, and cut to fit around openings.
3. Reinforcement is placed in the cores, as designed by the structural engineer.
4. Bracing holds the walls straight and plumb while the concrete is placed.
5. Concrete is poured in stages, with care to avoid voids, and compacted.
6. Finishes are fixed to the faces: render or cladding outside, plasterboard inside, using the fixing strips built into the blocks.

ICF walls are built from hollow blocks or panels, usually made of expanded or extruded polystyrene or another insulating material.

What it does well

• Insulation. The continuous insulation can give very low heat loss, and fewer thermal bridges than many other walls when detailed well.
• Airtightness. A solid, poured core helps to build an airtight envelope.
• Strength. The reinforced concrete core can carry loads and resist wind, and can suit places where strong walls are needed.
• Thermal mass. The concrete can help smooth temperature swings, if it is exposed to the inside.
• Speed and simplicity. The system can be built with a small team once they know it.
• Sound. A dense core can help reduce noise.

Limits and cautions

• Cost. ICF can cost more than some alternatives, and the comparison depends on the project and the market.
• Skills. The pour must be done properly; poor compaction or misplaced blocks cause defects. Training and supervision matter.
• Design discipline. Openings, services and changes are best planned before the pour, because altering a poured wall is hard.
• Moisture. Concrete is wet when poured and needs time to dry.
• Fire and finishes. Polystyrene is combustible, and the wall must be designed and finished to meet fire regulations. Check requirements for external surfaces and any cladding.
• Carbon. Cement has a high carbon footprint. The carbon of the whole wall should be compared fairly with other options. See Embodied carbon in plain language.
• Reuse at end of life. Separating concrete, steel and foam is difficult.

Is it right for your project?

ICF suits some projects well and others less. It is worth considering when high insulation, airtightness and strength matter, and when the team has experience with it. For a comparison, see ICF vs. cavity wall vs. timber frame.

What to ask your designer and supplier

• What certificates and test data support the product?
• How does it meet fire, structural and energy rules in my country?
• Who has experience of building it, and who will supervise the pour?
• How will openings, services and finishes be dealt with?
• What is the whole-life cost and carbon compared with other options?

In short

ICF is a stay-in-place insulating formwork filled with reinforced concrete. It is strong, airtight and well insulated, but it needs careful design and skilled work, and it is not right for every project.

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