Insulated Concrete Formwork (ICF) is a modern construction method that uses interlocking expanded polystyrene (EPS) blocks as permanent formwork for poured concrete walls. The result is a highly insulated, airtight, structurally robust wall in a single build operation. Crown Architecture & Structural Engineering Ltd has experience designing ICF structures for both new build homes and house extensions across the UK. This guide explains what ICF is, how it performs, and whether it is right for your project.

What is ICF Construction?

ICF uses lightweight EPS foam blocks or panels that are assembled like large interlocking bricks to form the wall shape. Steel reinforcement (rebar) is placed within the cavity, and concrete is then pumped or poured in to fill the void. The EPS formwork remains in place permanently, forming integral insulation on both the inner and outer faces of the concrete core.

The result is a composite wall comprising:

• Inner EPS layer (typically 75–100mm) — acts as thermal insulation and backing for internal finishes
• Structural concrete core (typically 150–200mm) — provides structural integrity
• Outer EPS layer (typically 75–100mm) — acts as thermal insulation and backing for external render or cladding

Total wall thickness is typically 300–400mm, giving U-values of 0.15–0.20 W/m²K or better.

Advantages of ICF Construction

Thermal Performance

ICF walls typically achieve U-values well below the Building Regulations Part L minimum of 0.18 W/m²K for walls in new dwellings. The continuous EPS insulation wrapping the concrete eliminates thermal bridging at wall ties and mortar joints, which is a significant weakness in traditional masonry cavity walls.

Airtightness

The monolithic concrete core and continuous insulation jacket creates a naturally airtight envelope. ICF buildings routinely achieve air permeability below 2 m³/h/m² at 50Pa, compared to the 10 m³/h/m² Building Regulations maximum. Excellent airtightness reduces heating demand and is a prerequisite for Passivhaus and near-zero-energy performance.

Acoustic Performance

Mass concrete provides significantly better sound insulation than lightweight timber or steel frame construction. ICF walls typically achieve 50–55 dB Rw, well above Part E requirements for party walls (45 dB) and giving noticeably quieter interiors.

Structural Robustness

Reinforced concrete ICF walls have excellent resistance to impact, wind uplift, and seismic events. They are also highly resistant to fire — the concrete core provides inherent fire resistance without additional intumescent treatment.

Speed of Construction

An experienced ICF crew can assemble, reinforce, and pour a wall significantly faster than an equivalent masonry wall. The blocks are lightweight and can be cut with a handsaw, reducing waste and plant requirements on site.

Longevity

Concrete structures correctly designed and built have design lives well in excess of 100 years. The EPS insulation is chemically inert and does not degrade, rot, or attract vermin.

Disadvantages and Considerations

Cost

ICF typically costs 5–15% more than traditional masonry for the wall element alone. However, this premium is often offset by lower heating bills, reduced foundation loads (ICF is lighter than solid masonry), and faster programme. Total project costs are broadly comparable for thermally equivalent constructions.

Specialist Knowledge Required

ICF requires experienced contractors and careful structural engineering design, particularly around concrete mix design, pour rates, and reinforcement detailing. It is not a system that can be handed to any general builder without training.

External Finish

The EPS outer face requires a protective render, rainscreen cladding, or brick slip system to provide weather protection and an acceptable external appearance. This is an additional cost and programme element, though the range of finishes available is wide.

Planning Considerations

ICF is a structural concrete system that must comply with Building Regulations Part A (structure) and Part L (energy). There is no planning reason why ICF cannot be used for extensions or new builds, provided the external appearance meets local planning requirements. ICF walls are thicker than masonry equivalents, which can affect internal floor areas; this should be considered at the design stage.

ICF vs Timber Frame vs Masonry

Factor	ICF	Timber Frame	Masonry Cavity
U-value (typical)	0.15–0.20	0.18–0.25	0.18–0.25
Airtightness	Excellent	Good (with membranes)	Moderate
Sound insulation	Excellent	Moderate	Good
Fire resistance	Excellent	Moderate (requires treatment)	Good
Speed of construction	Fast	Fast	Moderate
Contractor availability	Specialist required	Widely available	Widely available
Cost premium vs masonry	5–15%	Broadly comparable	Baseline

Building Regulations for ICF

ICF construction must comply with the full suite of Building Regulations. Key parts to consider:

• Part A (Structure): Reinforced concrete ICF walls must be structurally designed by a qualified structural engineer. Calculations are required for Building Regulations submission.
• Part B (Fire): Concrete inherently provides fire resistance. Internal EPS must be protected by plasterboard lining. Party walls have specific requirements.
• Part C (Moisture): External EPS must be protected from moisture ingress by render or cladding. Detailing at openings, base of wall, and roof junctions is critical.
• Part L (Energy): ICF typically exceeds Part L minimum requirements with standard system U-values.
• Part E (Acoustics): Relevant for party walls and floors in multi-occupancy buildings. ICF performs well by mass but junction detailing is important.

ICF systems from major suppliers (Nudura, Durisol, Quad-Lock, etc.) typically come with European Technical Assessments (ETAs) that facilitate Building Regulations approval.

ICF for House Extensions

ICF is well suited to single-storey and two-storey rear extensions, particularly where high thermal performance is a priority or where the extension is to Passivhaus or EnerPHit standard. Key design considerations for extension use:

• Junction detailing between new ICF walls and existing masonry is critical for thermal bridging and airtightness
• The thickness of ICF walls should be reflected in floor area calculations for planning purposes
• Foundations for ICF can be shallower than masonry equivalents due to lower wall weight, potentially saving cost
• ICF pairs well with flat roof and green roof extensions where the thermal mass and airtightness of the wall is complemented by equivalent roof performance

ICF Costs UK 2025

Typical ICF construction costs in 2025:

• ICF blocks/panels (supply only): £25–£45/m² of wall area depending on system and wall thickness
• Concrete (supply and pump): £150–£200/m³
• Reinforcement (supply and fix): £40–£80/m² of wall
• ICF contractor labour: £60–£100/m² of wall
• External render or cladding: £50–£150/m² additional
• Structural engineer’s design: £2,000–£5,000 for a typical residential ICF structure

Total wall cost (complete, ready for finishes) is typically £250–£450/m² of wall area, compared to £180–£300/m² for an equivalent masonry cavity wall meeting the same U-value. The premium narrows significantly when the cost of separate insulation to achieve equivalent thermal performance is added to masonry construction.

How Crown Can Help

Crown Architecture & Structural Engineering Ltd provides structural engineering design for ICF projects including reinforcement design, concrete mix specification, and Building Regulations submissions. We work alongside ICF specialists and architects to deliver thermally efficient, structurally robust buildings. Call us on 07443804841 to discuss whether ICF is right for your project.

Frequently Asked Questions

Is ICF suitable for a single-storey extension?

Yes. ICF is used for single-storey extensions, particularly where the client wants high thermal performance or is targeting Passivhaus standard. The wall thickness should be considered at the design stage to ensure adequate internal dimensions.

Does ICF need planning permission?

ICF is a structural system, not an external material, so it does not affect planning permission requirements. Planning permission is governed by the size, height, and location of the proposed works, not by the structural system used to build it.

How does ICF compare to Passivhaus construction?

ICF is a popular choice for Passivhaus and near-Passivhaus projects because its inherent airtightness and high U-values reduce the amount of additional insulation and vapour control layers needed to meet the standard. However, achieving certified Passivhaus status requires careful design of all junctions, windows, doors, and mechanical ventilation systems — not just the wall system.

Can ICF be used below ground?

Yes. ICF is well suited to basement and substructure use. The concrete core is waterproofed by tanking or integral waterproofing admixtures, and the EPS provides thermal insulation on the retained side. This is a significant advantage over traditional basement construction methods.

How long does ICF construction take?

An experienced ICF crew can erect a typical single-storey extension or house shell significantly faster than an equivalent masonry structure. For a 100m² single-storey extension, the shell construction (excluding foundations and roof) might take 2–3 weeks compared to 4–6 weeks for masonry.

Is ICF environmentally friendly?

ICF has a mixed environmental profile. EPS is a petroleum-based product with embedded carbon; however, the whole-life energy and carbon savings from superior thermal performance and longevity often outweigh the embodied carbon premium over masonry. Concrete also has high embodied carbon, though lower-carbon concrete mixes are increasingly available. A full life-cycle assessment should be considered for environmentally focused projects.