What Is ICF? How Insulated Concrete Form Walls Work

What happens to the formwork after the concrete is poured? With ICF, which stands for insulated concrete forms, it stays in the wall as foam insulation. This guide sets out what an ICF wall is, how it is built and what named makers and government pages say about it.

What Is an ICF Wall Made Of?

An ICF wall is a layer of foam, a concrete core and a second layer of foam. The U.S. Department of Energy’s Building Science Education Solution Center describes hollow rigid foam insulation blocks stacked like bricks to form walls, which are later reinforced with steel rebar and filled with poured concrete (DOE page, accessed 2026-10-21).

Makers describe the parts in their own words. NUDURA calls its forms stay-in-place permanent formwork for structural concrete, made of two expanded polystyrene (EPS) foam panels. Amvic describes two 2.5 in Type II EPS panels connected by polypropylene webs placed 6 in on center.

The forms remain after the concrete cures. NUDURA’s specification says the concrete must be protected by approved interior and exterior finish material, as it states all codes require. To count the blocks and core concrete for your own wall, use the ICF calculator for wall blocks and core concrete.

What Do Two ICF Makers State for Their Blocks?

Each maker states its own figures, and the two products below differ in core, foam and block size. These are the numbers on the NUDURA XR35 data sheet and the Amvic R22 data sheet, accessed 2026-10-21.

NUDURA’s R-35 and Amvic’s R-22 are each that product’s own stated figure. The basis for NUDURA’s figure is not stated on the sheet, so this article does not treat it as foam alone. The two numbers are not compared, ranked or averaged here.

How Are ICF Walls Braced and Poured?

The blocks are braced, filled with concrete in lifts and vibrated. Sources describe the steps with different numbers, so the figures below stay with the document that states them.

The Building America Solution Center, run by Pacific Northwest National Laboratory (PNNL), says to brace the wall forms every 6 ft and to plan to get beyond the 32 in level with the first lift. It says most houses up to a 12 ft wall will need three passes, and that concrete should be vibrated unless a self-consolidating mix is used (PNNL page, accessed 2026-10-21).

Amvic’s installation manual says to pour with a boom pump in layers of about 3 to 4 ft and to use a pencil vibrator. It warns that leaving out the bracing will frequently produce a blowout. NUDURA’s specification recommends a boom pump and internal vibration. Lift height is therefore one maker’s instruction, not a universal rule.

What Concrete and Steel Do the Makers Call For?

Makers name their own concrete and steel requirements, and the numbers differ between documents. Follow the document for your own product and your local rules.

• NUDURA’s specification. Concrete is specified as normal-weight, with maximum aggregate of 1/2 in for cores up to 6 in and 3/4 in for 8 in cores and beyond, and a minimum of 2,900 psi at 28 days. Walls are normally reinforced with deformed steel bars.
• Amvic’s manual. Structures must be designed under the applicable building codes or a licensed professional engineer, and the code takes precedence over the manual.
• Amvic’s US Design Guide. It reproduces the NAHB/PCA Prescriptive Method: flat ICF walls need a minimum concrete thickness of 5.5 in for basement walls and 3.5 in above grade. Maximum slump is 6 in, maximum aggregate is 3/4 in and minimum strength is 2,500 psi at 28 days.

The Prescriptive Method figures here come from Amvic’s reproduction. The original document could not be read for this article. A core thinner than 5.5 in sits below the basement figure, so the maker’s own manual and the local code govern.

What Do Energy Agencies Say About ICF Walls?

The DOE says ICFs have demonstrated a 9% better whole-wall R-value and are 10% more airtight, compared to stick-frame construction in otherwise identical homes. The PNNL page cites the same finding to a 1996 study.

Both pages credit the reinforced concrete with adding thermal mass. PNNL adds that ICF walls have almost no thermal bridging in the wall itself, and that final wall thickness is greater than 1 ft after finishes are applied. Both pages say to install ICFs according to the manufacturer’s specifications to get a continuous air barrier and thermal boundary.

These are comparisons with stick-frame homes in one cited study, not a rating for any one product. For where the wall sits, our guide on how deep footings should be covers the footing. For the masonry alternative, see concrete block sizes.

Worked Example: Blocks and Core Concrete for a 40 by 9 ft Wall

Take the ICF calculator’s default: a 40 ft by 9 ft wall with 30 sq ft of openings, 16 x 48 in blocks, a 6 in core and 5% waste on blocks and on concrete. This matches the dimensions of Amvic’s straight 6 in block. All sums below are arithmetic.

1. Net wall area. 40 x 9 – 30 = 330 sq ft.
2. Block face. 16 x 48 / 144 = 5.333 sq ft.
3. Block count. 330 / 5.333 = 61.9, and 61.9 x 1.05 = 65 blocks.
4. Layout. 108 in / 16 in = 6.75, so 7 courses. 40 ft is 480 in, and 480 / 48 = 10 blocks per course.
5. Core concrete. 330 x (6 / 12) = 165 cu ft, and 165 / 27 = 6.11 cu yd. With 5% waste, 6.11 x 1.05 = 6.42 cu yd.

Cross-check with Amvic’s sheet: 6 x 16 x 48 = 4,608 cu in per block. One cubic yard is 46,656 cu in, so 4,608 / 46,656 = 0.0988 cu yd, which Amvic prints as 0.099 yd3. Then 65 blocks x 0.0988 = 6.42 cu yd, the same figure. Both numbers are gross volumes that do not allow for the webs.

FAQs About ICF Walls

Sources and Further Reading

This guide reports what two government pages and two named makers state, each tied to its own product. Each ICF maker states its own core, foam and pour rules, so follow your block maker’s manual and your engineer. Reviewed for accuracy by Prof. Dr. Khalil Mudassar, PhD. Last updated 2026-10-21.