ICF structural systems
Insulating concrete formwork builds walls from hollow insulation blocks that are filled with reinforced concrete, leaving a solid concrete core wrapped in insulation. We design ICF structures for clients across the UK.
Is ICF just concrete or is the insulation structural?
The concrete core is the structure. The insulation blocks are permanent formwork that stay in place as insulation on both faces, but once the reinforced concrete inside has cured, the wall carries load as a solid reinforced concrete wall, designed to Eurocode 2.
How ICF carries load
Insulating concrete formwork uses hollow blocks or panels of rigid insulation, usually expanded polystyrene, that are stacked dry to form a wall, then filled with ready-mixed concrete and steel reinforcement. The insulation is permanent formwork; it stays in place as insulation on both faces, but it is the reinforced concrete inside that is the structure. Once cured, the wall is a solid reinforced concrete wall, and it carries load exactly as one, in compression from the floors and roof above and in bending and shear where it resists lateral pressure from wind or, below ground, from retained earth.
Because the finished element is reinforced concrete, we design it to Eurocode 2. The concrete carries compression while the steel reinforcement carries tension and controls cracking, and we size the wall thickness, concrete grade and reinforcement to suit the vertical load, the wall's slenderness and any out-of-plane bending. The core thickness varies with the block system, and taller or more heavily loaded walls simply take more reinforcement or a thicker core. Below ground, where ICF is common for basements, the wall is designed as a retaining structure for the soil and water pressure acting on it.
Stability, reinforcement and continuity
An ICF building is stabilised the way any reinforced concrete wall building is: the solid walls act as shear walls, resisting wind in their own plane and carrying it down to the foundations, and the floors act as diaphragms tying the walls together. Because the concrete core is continuous and monolithic once poured, an ICF structure is inherently stiff and stable, which is one of its strengths. We plan the layout of load-bearing and shear walls so wind on every elevation has a clear path to ground and the building does not rely on a wall with too many openings in it.
Reinforcement detailing is where the design is realised. Vertical and horizontal bars are placed in the cavity before the pour, laps and anchorage are set out so the reinforcement is continuous around corners and across floor levels, and starter bars from the foundation tie the wall to the substructure. Lintel reinforcement carries the wall over window and door openings, and the concrete pour is planned in lifts so the fresh concrete does not blow the formwork or leave voids. We coordinate reinforcement, openings and the connection of floors into the wall so the poured structure matches the calculations.
Where ICF suits, and its limits
ICF suits houses, basements, flats and low to medium-rise buildings where strength, thermal performance and airtightness are wanted together, and it is particularly well suited to basements and retaining walls because the same system gives a waterproofable, insulated, structural wall in one operation. The finished wall is heavy and solid, which brings good thermal mass, sound insulation and resistance to fire and impact, and the continuous insulation gives a warm, airtight envelope with little cold bridging. Foundations must carry more load than for a lightweight timber or SIP building, which we allow for.
The limits are weight, ground and buildability. A concrete wall is heavy, so foundations are more substantial and the ground conditions matter more than for a light frame; on poor ground this can mean a raft or piled foundation. The pour is a critical operation, so bracing the formwork, controlling the rate of pour and vibrating the concrete to avoid voids all have to be right, and openings and services are planned before the pour rather than cut afterwards. Below ground, waterproofing has to be integrated with the structural design. We set these requirements out so the wall is built as designed.
What we check.
The points our calculations resolve for a project like this.
- Reinforced concrete wall design to Eurocode 2 for compression, bending and shear
- Wall thickness, concrete grade and reinforcement for load and slenderness
- Shear wall layout and diaphragm action for stability against wind
- Reinforcement continuity, laps, corners and starter bars from the foundation
- Retaining wall design for soil and water pressure on basement walls
- Foundation design for the heavier load, and integration with waterproofing below ground
From enquiry to sign-off.
Enquiry
Send drawings or describe the problem. We confirm the scope, the deliverables and a target timescale.
Information
We agree the survey, drawings or data we need and any site access required.
Engineering
Design, calculation or assessment to the relevant Eurocodes and UK National Annex.
Issue
A clear, defensible report or set of calculations, with assumptions and limitations stated.
Common questions
Is ICF just concrete or is the insulation structural?
The concrete core is the structure. The insulation blocks are permanent formwork that stay in place as insulation on both faces, but once the reinforced concrete inside has cured, the wall carries load as a solid reinforced concrete wall, designed to Eurocode 2.
Why is ICF good for basements?
Because one operation gives an insulated, structural and waterproofable wall that resists soil and water pressure. We design the wall as a retaining structure for the ground and water loads, and coordinate the reinforcement and waterproofing so the basement performs.
Does an ICF house need bigger foundations?
Usually yes, because a concrete wall is heavier than a timber or SIP one. We design the foundation to the actual load and ground conditions, which on poorer ground may mean a raft or piled solution rather than a simple strip.
What can go wrong during the concrete pour?
Blown formwork, voids or misplaced reinforcement if the pour is not controlled. We set out the reinforcement, the pour in lifts and the openings before concreting so the built wall matches the calculations, and the formwork is braced to hold line and level.
Tell us what you are building.
Send the drawings or describe the project. We confirm scope, deliverables and a realistic timescale.
Start a project