An ICF foundation is a reinforced concrete wall poured inside rigid foam forms that stay in place, and the 2021 IRC covers it under Sections R404.1.3 and R608 with a 4 inch minimum thickness for flat walls and 6 inch minimum cores for grid walls. The foam must be covered inside and out.
An insulated concrete form (ICF) foundation is a basement, crawl space or stem wall made by stacking hollow foam units, placing steel bars in them and filling them with concrete. The code calls these stay-in-place concrete forms. The 2021 International Residential Code (IRC) does not give ICF its own chapter, so the rules are spread across foundation walls (Section R404.1.3), exterior concrete walls (Section R608) and wall covering (Section R702.3.4). The adopted code edition and local amendments govern, and every ICF system also has manufacturer instructions and tables that the building department will expect you to follow.
Key facts
- Under 2021 IRC Section R608.3.1, a flat concrete wall system has a minimum nominal thickness of 4 inches.
- Under 2021 IRC Sections R608.3.2 and R608.3.3, waffle-grid and screen-grid systems need a minimum nominal thickness of 6 inches for the horizontal and vertical concrete cores.
- Under 2021 IRC Section R404.1.3.3.1, concrete is at least 2,500 psi at 28 days in Seismic Design Categories A, B and C, and 3,000 psi in D0, D1 and D2.
- Under 2021 IRC Section R404.1.3.3.4, concrete placed in stay-in-place forms must have a slump greater than 6 inches and be consolidated by internal vibration (Section R404.1.3.3.5).
- Under 2021 IRC Section R404.1.3.3.7.4, steel in concrete cast in stay-in-place forms has at least 3/4 inch of cover.
- Under 2021 IRC Section R403.1.6, wood sill plates are anchored with at least 1/2 inch diameter bolts at not more than 6 feet on center, extending at least 7 inches into the concrete.
What are the ICF wall types in the IRC?
The IRC recognizes three concrete wall shapes, and an ICF system must match one of them: flat, waffle-grid or screen-grid (2021 IRC Section R608.3).
A flat wall is solid concrete of even thickness. Waffle-grid and screen-grid walls are grids of concrete cores, joined by a thin web in waffle-grid and open between cores in screen-grid. Other shapes must be designed under ACI 318 (Section R404.1.3.1).
2021 IRC Table R608.3, dimensional requirements for concrete walls. Weights are based on concrete at 150 pcf and exclude finishes. Core shape footnotes apply; read the full table for alternative core shapes.
| Wall type and nominal thickness | Maximum wall weight (psf) | Min. vertical core width (in.) | Max. spacing of vertical cores (in.) | Max. spacing of horizontal cores (in.) | Min. web thickness (in.) |
|---|---|---|---|---|---|
| 4 in. flat | 50 | NA | NA | NA | NA |
| 6 in. flat | 75 | NA | NA | NA | NA |
| 8 in. flat | 100 | NA | NA | NA | NA |
| 10 in. flat | 125 | NA | NA | NA | NA |
| 6 in. waffle-grid | 56 | 8 | 12 | 16 | 2 |
| 8 in. waffle-grid | 76 | 8 | 12 | 16 | 2 |
| 6 in. screen-grid | 53 | 6.25 | 12 | 12 | NA |
Flat wall thickness is a nominal value: the as-built thickness may be no more than 1/2 inch under or 1/4 inch over it (Table R608.3, footnote c).
How thick should an ICF basement wall be?
The concrete in an ICF basement wall must be at least as thick as the wall above it and must satisfy the foundation wall tables for its height, backfill and soil (2021 IRC Sections R404.1.5.2 and R404.1.3.2).
The 4 inch flat minimum in Section R608.3.1 is a floor, not a design answer. Tables R404.1.2(1) through R404.1.2(8) depend on wall height, unbalanced backfill and soil class. They are large and not reproduced here, so read the row for your wall in the adopted code. Walls with corbels or projections for veneer support fall outside the tables (Section R404.1.5.2).
Section R404.1.3.2 requires concrete foundation walls to be laterally supported at the top and bottom, and Section R404.1.7 bars backfill until the wall has enough strength and is anchored to the floor above or braced.
What reinforcement does an ICF foundation wall need?
Reinforcement comes from Section R404.1.3.2: horizontal steel from Table R404.1.2(1), and vertical steel from Tables R404.1.2(2) through R404.1.2(8) or, for flat basement walls retaining 4 feet or more of unbalanced backfill, optionally Table R404.1.2(9).
Three rules apply regardless of the table:
- Steel grade. Grade 40 (40,000 psi) is the minimum in Seismic Design Categories A, B and C, and Grade 60 in D0, D1 and D2 (Section R404.1.3.3.7.1).
- Six-inch walls. Where the vertical steel tables show no reinforcement required, 6 inch nominal walls formed with stay-in-place forms still get No. 4 bars at 48 inches on center (table footnotes).
- Openings. Vertical bars interrupted by a window or door opening are replaced by additional bars of the same size within 12 inches of each side (Section R404.1.3.3.7.3).
Webs of waffle-grid walls get no reinforcement, including stirrups (Section R608.5.4.6).
What are the form and concrete requirements?
Stay-in-place forms must be strong enough to hold the concrete during the pour and be positioned and secured before it starts (2021 IRC Section R404.1.3.3.6).
Form ties must resist the fluid pressure of fresh concrete, and flat ICF forms must conform to ASTM E2634 (Section R404.1.3.3.6.1, item 5). Concrete goes in at a slump above 6 inches and is vibrated internally, unless an approved self-consolidating mix with slump of 8 inches or more is used (Section R404.1.3.3.5).
How is the foam protected?
Rigid foam forms need a thermal barrier on the inside and an approved covering on the outside (2021 IRC Sections R404.1.3.3.6.1, R316.4 and R702.3.4).
2021 IRC requirements for foam in stay-in-place forms.
| Location | Requirement | Section |
|---|---|---|
| Interior face, habitable space | Thermal barrier of at least 1/2 inch gypsum wallboard, 23/32 inch wood structural panel, or an NFPA 275 material | R316.4, R702.3.4 |
| Exterior face | Approved exterior wall covering protecting from sunlight and physical damage | R404.1.3.3.6.1 |
| Foam surface burning | Foam plastic in ICF meets Section R316.3 | R404.1.3.3.6.1 |
| Very heavy termite zones | Foam below grade on foundation walls permitted per Section R318.4 | R404.1.3.3.6.1 |
| Below-grade waterproofing | No organic-solvent-based products on expanded polystyrene forms | R406.2 |
Adhesives for finishes must be compatible with the foam (Section R702.3.4).
What are the height and size limits for ICF walls?
Section R608 applies to buildings up to two stories above grade, 35 feet mean roof height, 60 feet in plan, with floor clear spans to 32 feet and roof clear spans to 40 feet (2021 IRC Section R608.2).
Wind is capped at 160 mph Exposure B, 136 mph Exposure C and 125 mph Exposure D, and the buildings are detached one- and two-family dwellings and townhouses in Seismic Design Category A or B, plus detached dwellings in C. Outside those limits, design goes to PCA 100 or ACI 318. Section R301.3 caps ICF walls at 11 feet 7 inches story height and 10 feet unsupported wall height per story.
How does wood framing connect to an ICF foundation?
Wood framing connects to an ICF wall either on top, through a sill plate with anchor bolts, or on the side, through a face-mounted ledger (2021 IRC Sections R403.1.6 and R608.9).
Sill plate on top of the wall
Section R403.1.6 requires wood sill plates to be anchored with at least 1/2 inch diameter bolts, spaced not more than 6 feet on center, extending at least 7 inches into the concrete and located in the middle third of the plate width. Each plate section needs at least two bolts, one within 12 inches of each end and not closer than seven bolt diameters. The sill plate article covers spacing, washers and treatment, and the stem wall article shows the same detail on a conventional wall. How the bolts pass through the top of the foam is a manufacturer detail.
Floor ledger on the side of the wall
The prescriptive details for wood floors framed to the side or the top of a concrete wall are Figures R608.9(1) through R608.9(4), each with a table of permitted wall thickness, bolt spacing and wind speed (Section R608.9.2). Those tables are not reproduced here: read the one that matches your framing direction. Rules that apply to all of them (Section R608.9.1):
- Anchor bolts have heads, or are threaded rods with a hex or square nut on the embedded end. J-bolts and L-bolts are not used with these details.
- Holes in the foam for ledger bolts are at least 4 inches in diameter for forms 1-1/2 inches thick or less, plus 1 inch in diameter for each 1/2 inch of added form thickness (Section R608.9.1.2).
- The ledger touches the concrete at each bolt. A vapor retarder or other material 1/16 inch thick or less may sit between them.
- Wood framing members have a specific gravity of 0.42 or greater, and parts of the connection not shown follow Section R502.
Proprietary connectors checked against PCA 100 or an engineered design are the other options. For the rim detail, see rim joist.
ICF compared with other foundation types
Footing sizes come from the Section R403.1 tables and are covered in the stem wall article. For the other wall systems, see concrete block foundation, monolithic slab and permanent wood foundation.
Showing an ICF foundation on a framing plan
On a floor framing plan, an ICF wall is the outer support line, labeled with the wall type, concrete thickness, the sill or ledger connection and the manufacturer's detail. The floor framing plan guide covers callouts and span arrows.
Summary
An ICF foundation is a reinforced concrete wall in stay-in-place foam forms. The 2021 IRC requires flat walls at least 4 inches thick or grid cores at least 6 inches thick (Section R608.3), steel from Tables R404.1.2(1) through R404.1.2(8), foam covered inside and out, and sills anchored with 1/2 inch bolts at not more than 6 feet on center (Section R403.1.6).
Framing Plan draws joist layouts, beams and openings and exports PNG, PDF and DXF. It does not check foundations or any other code requirement: the editor works in millimeters and its sample profile uses demo span figures and metric member sizes, so treat its output as a preliminary drawing aid. To draw the floor framing that bears on your ICF walls, open the framing studio.