Building with insulated concrete forms: how it works, and how to compare cost with blocks and plaster
Insulated concrete forms (ICF) are permanent formwork made of insulating material, assembled on site like building blocks, with rebar placed inside and concrete poured in. After the pour the formwork is not stripped: it stays in the wall as a layer of insulation on both sides, and the wall itself is a load-bearing reinforced concrete wall. I have experience in quantity takeoff for building with permanent insulating formwork, and in comparing it with block-and-plaster walls.
How it works, step by step
- Foundations or a floor slab with starter bars at the wall positions.
- Courses of forms are assembled, with ties between the inner and the outer face.
- Horizontal bars between the courses and vertical bars according to the structural engineer's drawing.
- Frames for windows and doors, before the pour.
- Pouring in lifts, with supervision of how the cavity fills.
- Internal and external finishes on the insulation layer, usually on mesh or on boards.
During the pour it is important to pace the height of each lift, to brace the forms around openings and to make sure the concrete fills the cavity to the top. A void inside a wall like this cannot be seen from outside, so supervising the pour is part of the method, not an extra.
The section in short, for illustration only: outside → finish | insulation 6 cm | concrete 15 cm | insulation 6 cm | finish → inside. The thicknesses vary with the product and the design.
Quantity comparison: a block wall against a wall in insulated concrete forms
The table shows what goes into each m² of finished external wall with each method. The numbers are for illustration; the real quantity comes from the drawings and the specification.
| Component | Blocks and plaster | Insulating forms and concrete |
|---|---|---|
| Wall structure | 1 m² of 20 cm block and mortar | 1 m² of form |
| Concrete | A proportional share in columns and tie beams | 0.15 m³ for a 15 cm core; with 4% waste: 0.156 m³ |
| Rebar | In columns and tie beams only | About 12 kg, assuming 80 kg per m³ of core |
| Thermal insulation | A separate layer, if the requirement calls for it | Included in the form |
| Internal finish | 1.5 cm plaster: 0.02 m³ with waste | Finish on mesh or plasterboard, per the specification |
| External finish | External render or cladding, 1 m² | Finish on mesh or cladding, 1 m² |
| Openings | Lintel and sill per the detail | An opening frame, installed before the pour |
For 100 m² of external walls it looks like this: with the forms, 100 m² of form, 15.6 m³ of core concrete and about 1.2 tonnes of rebar. With blocks, 100 m² of block, 2 m³ of internal plaster, an external finish and an insulation layer if required, plus columns and tie beams. How 0.02 m³ of plaster per m² and a concrete waste of 4% are calculated is shown on the construction inputs page.
What gets shorter and what does not
With this method three trades merge into one stage: masonry, insulation and the substrate for the finish. Each trade that comes off the list is one stage fewer in the construction programme and less coordination between crews. On the other hand, the rebar and the pour need care, a finish on insulation needs mesh, and chasing for building services is done in the insulation, not in the concrete. Every month cut from the programme is a month less of site overheads (preliminaries), as explained on the site overheads page. How much time is actually cut depends on the crew, and general figures are not worth relying on.
How to compare cost without misleading
A single form costs more than a single block, so comparing the two prices alone will always show the block as cheaper. The right comparison is finished wall against finished wall:
Cost of a finished wall = structure plus insulation plus internal finish plus external finish plus a proportional share of openings, columns and tie beams plus lifting and pouring.
The calculation is done for both methods, from the same drawings and with the same insulation requirement. Then the effect of time is added. Only then is there an answer for a given project.
Example: a house with 180 m² of net external walls. With the forms: 180 m² of form, 28.1 m³ of concrete (180 times 0.156), about 2.2 tonnes of rebar (180 times 12 kg), 180 m² of internal finish and 180 m² of external finish. With blocks: 180 m² of block, 3.6 m³ of internal plaster (180 times 0.02), 180 m² of external finish, insulation as required, and columns and tie beams as designed. The unit price of each line goes into the formula, and that gives a real comparison.
In the bill of quantities (BOQ), you can measure a complete wall system per m², describing the form, the core thickness and the finishes, or split it: form per m², concrete per m³, rebar per tonne and finishes per m². Choose one way and state it explicitly, so that all the bidders price the same thing.
The rule: fix the positions of the openings and the building services before the pour. In blockwork you move a window with a hammer; in a reinforced concrete wall it takes saw-cutting, and sometimes the structural engineer's approval.
What to check before choosing insulated concrete forms
- A crew with experience in the method, including the pour and supervising how the cavity fills.
- Fire requirements for the finish and for exposed insulation, checked with the safety consultant and the building standards.
- Product approval and the connection details to the floor and to the slab above, checked with the structural engineer.
- Reinforcement planned in advance for hanging cabinets and air-conditioning units.
- Waterproofing details at the base of the wall and at the transition to basement walls.
With insulated concrete forms, the quantity takeoff covers the form, the concrete and the reinforcement as one package. A quantity surveyor who compares them with conventional building has to add the insulation and the plaster to the other side, otherwise the comparison is not fair.
Frequently asked questions
Is a wall built with insulated concrete forms a concrete wall in every respect?
The core is reinforced concrete to the structural engineer's design. The insulation is formwork and an insulation layer, not a load-bearing part.
What happens when a window has to be moved after the pour?
The concrete has to be saw-cut, strengthened as the structural engineer directs, and the insulation and the finish repaired. That is why openings are fixed early.
How is it different from light steel framing?
In light steel framing the frame is thin steel profiles; here the frame is concrete. More on the light steel framing page, and on other industrialised methods on the industrialised construction page.
Who does the quantity takeoff for a method like this?
A quantity surveyor for construction projects who prepares both options from the same drawings and compares finished wall with finished wall.
Have a project? Tell me briefly what the project is and what already exists. When the project is a good fit, I usually reply with focused questions and a written quote.
Contact me for a quote* Created with AI for marketing, for illustration only. Not reviewed by Yaakov Avital and not professional advice. Every project must be checked on its own by a professional quantity surveyor and an engineer.