Insulated concrete formwork (ICF) is a proprietary system used to build concrete structures quickly. In some cases, it is replacing conventional lightweight construction. As well as the strength and durability of concrete, ICF has:

• integrated insulation
• high thermal mass
• draught-proofing
• good sound insulation.

What is ICF?

ICF has been in New Zealand since the early 1990s. It is a system where hollow expanded-polystyrene blocks are fitted together like Lego and concrete is poured into the spaces. The polystyrene formwork is left in place.

Reinforcing steel is added as needed before the concrete is poured. This gives a solid concrete wall with polystyrene insulation on both sides.

Both sides of the wall usually need to be plastered or lined in some way to protect the polystyrene and improve appearance. ICF must also be properly clad to keep water out.

ICF uses almost as much concrete as solid concrete construction.

There are several ICF systems available in New Zealand, each offering different features.

Key features of ICF construction

Cost

While the initial construction cost of ICF may be higher or lower than conventional lightweight timber construction, there can be paybacks in terms of increased sound and thermal insulation, improved durability and an aesthetic appeal of solidity.

Ease of construction

ICF construction can be carried out by an experienced builder. It is faster than building with conventional concrete formwork or concrete blocks.

Strength

With ICF construction, the strength is provided by a reinforced solid concrete wall between the layers of expanded polystyrene.

Durability and weathertightness

ICF is as durable as conventional concrete. It can be low maintenance. However, the polystyrene needs to be protected from physical damage and from ultraviolet radiation.

ICF is weathertight so long as it is properly clad.

Earthquake-resistance

ICF construction can be made earthquake resistant if it is properly designed, and has sufficient reinforcing steel in the right places.

Reaction to fire

The concrete core of ICF is as fire resistant as conventional concrete. The polystyrene layers will degrade if exposed directly to flame.

Pest resistance

ICF is as resistant to vermin and other pests as conventional concrete.

Is it compatible with passive design?

ICF is a good thermal insulator - a standard ICF wall will provide enough insulation to meet or exceed Building Code requirements without any additional insulation. ICF also provides good sound insulation.

While ICF’s concrete core is good at storing heat, you can only use this to store warmth in your home if you remove the insulating polystyrene layer on the interior wall.

With the ICF construction system, doors and windows can be placed where needed to make maximum use of the sun’s warmth and breezes for cooling.

Is it right for my site?

Topography and site impact

ICF can be used to build single and multi-storey buildings. As with all forms of heavyweight construction, it is better suited to level sites.

Climate

ICF can be used in all climate zones. Its high insulation properties make it suitable for both cool and hot climates.

More information

From Smarter Homes

• Timber construction
• Light steel frame construction
• Concrete construction
• AAC construction
• Earth construction
• Straw bale construction
• Construction site practice
• Exterior design
• Materials
• Passive heating
• Passive cooling
• Insulation

From ConsumerBuild

• The Building Act
• Leaky buildings; this includes information about building for weathertightness
• Construction

From other sites

You can find more information about ICF at the Cement and Concrete Association of New Zealand website.

The Department of Building and Housing’s website has information about building law and compliance, including the Building Code, and on weathertightness.

All home construction needs a building consent from a building consent authority (usually your local council). ConsumerBuild’s Council finder web page has contact details for New Zealand local authorities.

You can buy New Zealand Standards relating to various construction systems from the Standards New Zealand website.

You can buy BRANZ publications about construction systems from the BRANZ website (click on the link to the BRANZ bookshop).