Insulated concrete formwork (ICF) wall
A load-bearing reinforced-concrete wall cast inside polystyrene formwork blocks that stay in place. The blocks interlock dry like toy bricks, are reinforced and filled with concrete: the wall is born already insulated on both faces, without thermal bridges and with great inertia. It is a fast and very energy-efficient system, where the care lies in the pour and in the fire protection of the polystyrene.
Technical section of the system, from inside (left) to outside (right).
A load-bearing reinforced-concrete wall cast inside polystyrene formwork blocks that stay in place. The blocks interlock dry like toy bricks, are reinforced and filled with concrete: the wall is born already insulated on both faces, without thermal bridges and with great inertia. It is a fast and very energy-efficient system, where the care lies in the pour and in the fire protection of the polystyrene.
The ICF wall - Insulated Concrete Forms - turns the usual order upside down. Instead of building the wall and then insulating it, hollow polystyrene blocks are dry-stacked as permanent formwork; reinforcement is placed in their cavity and concrete is poured, which on hardening becomes the load-bearing structure. The forms are not removed: they stay to insulate the wall forever.
The concrete is enclosed between two continuous polystyrene leaves, inner and outer. The insulation is uninterrupted by columns or beams - they are inside the pour - so the wall is free of the thermal bridges that plague traditional walls. The central concrete mass, protected by the insulation, adds thermal inertia: it damps the peaks of heat and cold and stabilises the indoor temperature.
All the soundness of the system depends on the pour. The concrete must be placed fluid but controlled, in limited lifts and well vibrated, so it fills every corner without voids and without pushing too hard on the light forms. Excessive pressure deforms them or bursts them (blow-out): hence they are propped, braced and poured in layers. It is reinforced-concrete work in every sense, with the added fragility of a polystyrene formwork.
Polystyrene is combustible: by law it must always be protected by a covering that delays its contribution to a fire - render outside, plasterboard inside. All finishes are made on those faces. Hung loads (brackets, cabinets) are not fixed to the EPS but anchored to the concrete behind with suitable fixings. With these points taken care of, what remains is a robust, quiet, earthquake-resistant wall with excellent energy performance.
Why it works
Continuous insulation, mass in the middleAn ICF wall is, in section, a concrete sandwich: a structural core of reinforced concrete with a continuous slab of polystyrene on each face, the whole thing cast inside formwork blocks that are never removed. That simple arrangement quietly solves the hardest problem of a masonry or framed wall — the thermal bridge. Because the insulation runs unbroken on both sides, with no beam, column or floor edge piercing it, the heat-loss «leaks» that normally form at those junctions are simply not there, and the temperature falls almost entirely across the two polystyrene layers. Meanwhile the heavy concrete core, kept warm on the inside of the insulation, stores heat and gives the wall a thermal inertia that damps the day-night swings — comfort in summer, stability in winter, and good airborne sound insulation as a bonus. The price of all this is paid at the pour: the concrete has to be placed and vibrated carefully, in lifts, so it fills the forms completely without bursting them, and the combustible polystyrene must always be covered by a fire-protective render or board, with heavy loads fixed through to the concrete behind.
Insulation and speed in one wall
Comparison · insulantsNodal details
Critical junctions · sectionsTwo hollow polystyrene blocks, stacked dry. Their top and bottom edges interlock like a building brick, so a course locates itself on the one below without glue or mortar. Plastic webs cast into the block hold the two EPS faces a fixed distance apart and carry the steel reinforcement in the cavity between them. Concrete poured into that continuous cavity fills it and, on hardening, becomes a monolithic reinforced wall - permanently jacketed in insulation on both faces.
- Outer form (EPS)
- Web tie of the block
- Pour cavity (R.C.)
- Interlock between courses
- Reinforcement
- Inner form (EPS)
Where a floor lands on the wall, the inner polystyrene is interrupted to let the slab bear on the concrete core, and the wall steel runs on past so the structure is continuous. Two things must be looked after at that gap: the outer insulation is kept unbroken across the floor edge so no thermal bridge forms, and the interruption on the inside is fire-stopped and re-insulated, since the exposed polystyrene must never be left as a path for fire.
- ICF wall (core)
- Floor (bearing)
- Continuity of reinforcement
- Fire-protective lining
- Continuous outer insulation
- Form stopped at the floor
Installation controls
Specification · checklist01 · Forms
02 · Reinforcement
03 · Pour
04 · Thermal
05 · Fire & finish
Recurring defects
Diagnostics · siteComponent materials
The network · materialsReference regulations
2 norms- D.P.R. 380/2001Consolidated Building Act (Testo Unico Edilizia)In force
- UNI EN 13501-1:2019Fire classification of construction products and building elements - Part 1: Reaction to fireIn force
Informational links to the regulatory framework. Always verify the current text on the official source.