Icf Insulated Concrete Forms

Insulating concrete forms (ICFs) are rigid plastic foam forms that are filled with reinforced concrete to create structural walls. They hold concrete in place during curing and remain in place afterwards to provide thermal insulation. ICFs are used to make structural concrete walls, and can be used to make either foundation or above-grade walls. The forms are typically made from pure foam-plastic insulation but may also be made from a composite of cement and foam insulation or a composite of cement and processed wood. The foam is typically either expanded polystyrene (EPS) or extruded polystyrene (XPS) and occasionally polyurethane. Forms come in three basic form-types: blocks, planks, and panels. Blocks are molded, hollow foam blocks that are stacked, much like Legos ™;. Plank and panel types use flat sheets (typically) of foam held together with plastic or metal ties, with panels using larger sheets of foam, similar to metal or plywood formwork. The resulting shape of the concrete, explained in more detail later, will be one of several shapes: flat, waffle- or screen-grid, or post-and-beam.

ICF Types

There are many ICF wall types. Products are differentiated based on the type of form and the shape of the concrete. Products are further differentiated by how forms attach to each other, how finishes are attached to the wall, insulating values, foam types and other features. Introductory information on the most basic products types follows.

Form Types

As mentioned above, ICFs come in one of three basic form types which are differentiated by the size of the form units and the way they connect to one another.

Panel systems are the largest units, available in sizes from approximately 1’-3" x 8’-9" up to 4’ x 12’ resembling traditional plywood forms in size and shape. Panel systems allow a large section of wall area to be erected in one step, but may require more cutting in the field. The panels have flat sides and are connected to one another with metal or plastic ties. They can be shipped flat.

Plank systems consist of long, narrow planks of foam held together at a constant distance apart by metal or plastic ties. Planks may have notched, cut, or drilled edges that the ties fit into. Plank-shaped forms range in height from 8 to 12 inches and are either 4 or 8 feet long. Plank systems differ from block systems in that they can be shipped flat, either because the ties can bend or because the ties are inserted as the wall is constructed.

Block systems resemble hollowed-out concrete masonry units (CMU) in size and shape, although the dimensions may vary from the typical CMU. Block systems include units ranging from standard concrete block size (8-inches high x 16-inches long) to a much larger 16-inches high x 48-inches long. Their edges interlock without separate fasteners, using a rabbeted edge, tongue-and-groove configuration, mortise and tenon-type configuration, or similar. Blocks arrive on-site, ready to stack with their ties, made of the form material itself, metal, or plastic imbedded in the form.

Concrete Shape Types

ICFs are further differentiated based on the shape of the concrete once poured into the forms. Four distinct cavity shapes are possible: flat, waffle-grid, screen-grid, and post and beam. Figure
1 shows several shape types.

Flat ICF Wall Systems have a solid concrete wall of constant thickness, just like a conventional poured wall formed with plywood or metal forms. They typically have a nominal concrete thickness of 4, 6, 8, 10 or 12 inches (actual thickness of the concrete can range ½ inch plus or minus the nominal thickness).

Waffle-Grid ICF Wall Systems have a solid concrete wall of varying thickness and, as the name implies, look like a breakfast waffle. These systems have a nominal concrete thickness of 6 or 8 inches for horizontal and vertical concrete cores. Maximum spacing of vertical cores is typically 12 inches on center and maximum spacing of horizontal cores is typically 16 inches on center. The webs in between the cores usually have a minimum thickness of 2 inches.

Screen-Grid ICF Wall Systems have a perforated concrete wall of varying thickness, similar to the waffle-type systems but with solid form material (foam, foam-cement composite, etc.) between the horizontal and vertical members instead of concrete. These systems have a nominal concrete thickness of 6 inches for the horizontal and vertical that creates a concrete screen instead of a concrete waffleconcrete members. Maximum spacing of vertical cores and horizontal cores is defined as 12-inches on center in the
Post-and-Beam ICF Wall Systems are similar to the screen-grid systems in that vertical members (columns) and horizontal members (beams) are formed. However, the spacing between them is wider, up to four feet for columns and between four and eight feet for beams.

Fastening Options. Some forms come with built-in fastening surfaces for attaching drywall, trim or other finishes. Others require attachment of finishes through the insulating form to the concrete itself or to furring strips.

Corner Details. There are several different types of ICF corners. If pre-formed corners are not available, two standard forms can be miter-cut and glued together to form the corner piece. Each ICF manufacturer has specific recommendations for the corner assembly of their product.

Special Options. Various manufacturers may offer one or more of the following specialty features/options: brick ledge blocks, lintel blocks, hinge corner blocks, foam stops, rebar hangers. See Insulating Concrete Forms for Residential Design and Construction for a comprehensive discussion of products by manufacturer.

Foam Types

ICF forms may be made from pure foams or cement composites. Pure foams can be expanded polystyrene (EPS), extruded polystyrene (XPS), polyurethane or polyisocyanurate. Most systems use EPS. XPS is available only in flat board shapes and is, therefore, not used in any block-type forms or other shaped, molded forms such as for the grid systems. Polyisocyanurate is rarely used. Cement composites may be a combination of foam and cement (always EPS) or wood and cement. Foams are rated on their thermal properties, density, strength, and resistance to wind and moisture. Foam density can have a significant impact on the physical characteristics of the foam, especially thermal insulation (R-value), strength, and moisture retention.

EPS and XPS are both made from polystyrene but the manufacturing process is different. EPS, the type of foam used for disposable coffee cups, begins as small plastic "beads" that are expanded and then fused together. XPS begins as a molten material that is pressed out of a form in a continuous process to form sheets. Polyurethanes are made from a mixture of two
ingredients: an isocyanate, a polyol, and a blowing agent. Cement-foam composites are a mixture
of Portland cement and EPS beads.

EPS is resistant to air infiltration, moderately strong, and usually the least expensive foam. EPS foams have R-values that range from 4.0 to 4.2 per inch when dry, based on respective densities of 1.35 to 1.8 pounds per cubic foot1 for Type II and Type IX foam, respectively. Type II foam, with an R-value of 4 per inch, is most commonly used in ICFs. EPS can be molded to form blocks or panels for grid or post-and-beam systems or cut into sheets for flat panel systems. It is not as resistant to moisture as XPS. Long-term exposure to moist, below-ground conditions in freezing climates will degrade foam R-value. For this reason EPS, especially, should have moisture protection when used below grade as its R-value may be reduced to 2.4 per inch under
more severe conditions. According to the Building Foundation Design Handbook, EPS in ground contact is best suited for application in well-drained situations. Table 2 displays R-values for
foam exposed to these conditions. Manufacturers literature should be consulted for verification. The insulating value of ICFs in above-grade walls should not be expected to diminish. Consult
ASTM Manual MNL 18, Moisture Control in Buildings-Chapter 4: Effects of Moisture on the
Thermal Performance of Insulating Materials, for more information on this subject.

XPS foam (Type IV, V, VI, VII) has an R-value of 5 per inch when dry (4.5 per inch with long- term exposure to moist, below-ground conditions in freezing climates), regardless of density. Like EPS, XPS is resistant to air infiltration, but stronger. It is ordinarily available in sheet form only and is more expensive than EPS.

 
 Table 2: R-values for foam exposed to moist, below-ground conditions in cold climates from the
Design Guide for Frost Protected Shallow Foundations.   
Foam Type R-Value (per inch)   
Type II EPS 2.4   
Type IX EPS 3.2   
Types IV, V, VI, and VII XPS 4.5 
Cement composites have an R-value of 3 per inch when dry, with a density of 21 pounds per cubic foot. All the composite form types contain cement and, therefore, tend to be stronger than any of the foam-only form types. They are also heavier and are more difficult to cut, but potentially more durable than foam forms.

Choosing a Product

There are some significant differences among types and brands of ICFs. Therefore, builders should carefully consider all options and associated advantages and disadvantages before committing to building with ICFs or using any particular type or brand of ICF. Particular advantages and disadvantages of ICFs versus other common wall types are discussed in detail in the  Benefits and  Limitations sections.