Timber frame, block or ICF: choose a build method

Choose the wall system with your designer before fixing the drawings, then get complete quotes for the same house. Timber frame, cavity block and insulating concrete formwork (ICF) can all meet the Building Regulations; the choice changes the design, schedule, workmanship and evidence you need.

All three can meet the nearly zero energy building requirements, usually shortened to nZEB. The material name does not guarantee compliance or tell you the finished cost. Compare the actual wall specification, the people who will build it and the checks before work is covered.

Compare the work involved

Method	What to check
Timber frame	Factory-made wall, floor and roof panels are erected on site. Check the manufacturer’s approval, delivery protection, connections, moisture and fire details.
Cavity block	Two layers of concrete block form a wall, with a gap (the cavity) between them and insulation in or around it. The blocks are laid row by row. Check site workmanship, thermal bridges and airtightness.
ICF	Interlocking insulation blocks are filled with concrete on site. Check the particular system’s certification, approved installer and pour inspections.

Timber frame combines factory quality control with site erection and connections. Keep panels dry and close the shell against weather promptly. Cavity block relies on site work: mortar, plaster and other wet materials need drying time, and bad weather can delay the work. ICF also depends on site workmanship, with temperature limits for concrete pours. Compare those requirements with the people and conditions on your site rather than ranking the methods by shell speed alone.

Timber frame

Standard and manufacturers

Ask the supplier for its National Standards Authority of Ireland (NSAI) registration and confirm it on the register. Manufacturers are assessed for compliance with I.S. 440, including factory production control. Registration gives you evidence to check alongside the project specification.

I.S. 440 was published in June 2009. The edition I.S. 440:2009+A1:2014 covers platform timber frame (each storey’s floor supports the timber walls above): materials, design, manufacture, construction details, site work and quality control. It is referenced in the Technical Guidance Documents (the government’s published guidance on meeting the Building Regulations), Part A on structure and Part B on fire safety.

Before work starts, record who is responsible for planning, structural design, site erection and inspection. The standard also sets out fire details: walls separating homes, fire stopping (seals around gaps) and cavity barriers (barriers in concealed spaces). Ask who will inspect those details while they are visible and where the records will be kept.

Schedule and site protection

Ask the supplier for dates for design approval, factory production, delivery, erection and closing the shell against weather. Compare that with your builder’s foundation and roofing schedule. An erection duration alone leaves out important work and dependencies. See how long it takes to build a house for a way to plan the full sequence.

NSAI’s timber certification assessment covers structural-design procedures, material specifications, construction details, site-erection information and documented quality procedures. Factory approval does not replace project design or careful site erection. Ask who checks delivery condition, storage, moisture protection, connections and fire details before they are concealed. Agree how defects or deviations will be recorded and corrected.

Do not infer insurance terms from the material alone. Give the broker the full construction specification and ask for written terms before choosing a package. Ask whether the proposed insulation, cladding or construction sequence changes the terms, and keep the acceptance with your policy documents.

Cavity block

Cavity block is established construction practice covered by the Technical Guidance Documents. It still needs a structural specification, suitable products and evidence that the completed work complies. Ask your designer to specify the blocks, wall ties (connectors between the two layers of block), insulation, cavity, openings and junctions for your site rather than accepting “standard block construction” as a complete specification.

Site workmanship matters for cavity masonry, as it does for the other systems. In a block wall, thermal bridging (heat escaping through junctions) and airtightness at openings are determined by how carefully the builder detailed and sealed them. The designer must specify the details needed for the new house’s energy targets. A blower-door test measures air leakage; it is one part of the compliance evidence, alongside the design and other checks. Ask your designer how the airtightness and junction details will be checked before plaster goes on, because fixing a leaky block house later is harder than getting it right during the build.

Ask each builder to include weather delays and drying time in their schedule before comparing the three methods.

Insulating concrete formwork

How it is certified

For ICF, check the evidence for the particular system rather than assuming that approval of one product covers every ICF wall. For example, the Agrément certificate for one manufacturer’s system (Amvic, certificate 07/0277, 2025 issue) gives NSAI’s opinion that it complies with the Building Regulations 1997 to 2023. That certificate covers load-bearing and non-load-bearing walls, up to five or six storeys depending on the building use. These are that system’s certified limits, not permission to use any ICF product at those heights. For a house, the certificate refers to a design guide for houses up to three storeys, and structural design by a chartered engineer for anything beyond that guide.

What the certificate asks of you

That example certificate requires (details differ for other systems):

• Site construction by approved installers following the manufacturer’s manual.
• Concrete placed only when the air temperature is between 5°C and 30°C.
• The external render (the coating on the outside walls) must be an NSAI-certified system applied by a registered installer, and fire barriers must be confirmed in place before rendering.
• Seal openings through walls designed to resist fire using the specified fire-stopping system.

Ask your builder whether they are approved for your chosen system and who will inspect the pours. Use its own certificate and manual to agree the checks; another system may have different conditions.

Thermal performance

A U-value measures heat loss through a building element; a lower value means less heat passes through it. The same example certificate gives calculated U-values for its wall options. The 300 mm wall with a 150 mm concrete core is 0.20 W/m²K with graphite-enhanced expanded polystyrene (EPS) insulation or 0.22 with standard white EPS, and the 370 mm wall with a 200 mm core is 0.17 or 0.19. These are one manufacturer’s certified wall figures, not a whole-house energy result. Meeting nZEB requirements depends on the walls, roof, floor, windows, airtightness, heating and renewable energy systems together. Ask your energy assessor to use the proposed wall build-up in the house calculation, alongside the roof, floor, windows and services.

Choosing: questions to ask

1. Who designs the structure? Your architect or engineer should confirm the method suits your site, spans and ground conditions before the planning drawings are fixed, as changing method later can mean redrawing.
2. What proof of quality do I get? For timber frame: the supplier’s NSAI registration and I.S. 440 documentation. For ICF: the current Agrément certificate and the installer’s approval. For block: the specification and the airtightness test result.
3. How will fire details be inspected? Ask for photos or an inspection of cavity barriers and fire stopping before they are covered, whichever method you choose.
4. What is the schedule? Ask each builder for their own timeline, from foundations to weathertight, and what delays them.
5. Can my builder do it? Ask whether the builder has finished homes using your chosen method and whether limited experience adds costs to their quote. Compare like for like, using the same drawings and specification. See choosing a builder and contracts.
6. What does my insurer say? Give your broker the method, then see self-build insurance.
7. What does my lender or certifier need? Agree the inspection and certification route with your professionals early. Some owners building a single dwelling can opt out of using an assigned certifier (the professional who signs off compliance), but the Building Regulations still apply and your lender may still want evidence. See assigned certification.

What you should not rely on

• A single “cost per square metre” claim for one method. See the cost guide: compare fully specified quotes for the same house and scope, including delivery, erection, site work and any exclusions.
• Market-share headlines. They describe housing schemes, not your site.
• Marketing U-values. Ask for the certified figure and the whole-house energy calculation.