ICF — What it is and How it Works
Insulated Concrete Forms (or ICFs) are forms or molds that have built-in insulation for accepting reinforced concrete. These large, hollow blocks are stacked right off of the truck and filled with reinforcing bar and concrete. The end result is a high-performing wall that is structurally sound, insulated, strapped, has a vapor barrier and is ready to accept final exterior and interior finishes.
Insulating concrete forms systems can vary in their design. “Flat” systems yield a continuous thickness of concrete, like a conventionally poured wall. The wall produced by “grid” systems has a waffle pattern where the concrete is thicker at some points than others. “Post and beam” systems have just that – discrete horizontal and vertical columns of concrete that are completely encapsulated in foam insulation. Whatever their differences, all major ICF systems are engineer-designed, code-accepted, and field-proven.
The two insulating faces are separated by some type of connector or web. Large preassembled blocks stack quickly on site. Panels or planks ship more compactly, but must be assembled into formwork on the job. Foam is most often expanded polystyrene (EPS). It can be extruded polystyrene (XPS), which is stronger, but also more costly. A few products are made with recycled foam or wood fiber in a nod to green construction. The salvaged material is formed into blocks with cement, making units ideal for direct application of plaster finishes.
The ties that interconnect the two layers of insulated forming material can be plastic, metal, or additional projections of the insulation. There are advantages to each type of material, but one current trend incorporates hinges into the ties that allow pre assembled forms to fold flat for easy, less costly shipping.
The joints between individual forms can feature interlocking teeth or a tongue and groove configuration molded into the forming material, or simple butt jointed seams. Many manufacturers have developed units with universal interlocks that allow the forms to stack whether the form is flipped one way or the other. These “reversible” forms save time during placement and prevent improper alignment. Special units for corners, floors, and roof assemblies round out the product lines and improve the engineering of the system and energy efficiency of the final construction.
Block sizes are typically on the order of 16 inches high by 48 inches long. The cavities are commonly six or eight inches wide but can be larger or smaller as needed. The foam faces are also capable of being varied, but 1-7/8- to 2-3/4-inch thickness is a usual range. So an 8-inch cavity with a two-inch foam face on either side would lead to a 12-inch formed wall. More recently some systems have developed the capability of offering thicker layers of foam to enhance performance.
Installation of insulating concrete form systems is similar to masonry construction. Builders usually start at the corners and place a layer at a time to build up the wall. Some units, particularly those that form a “waffle” or post–and–beam concrete wall profile must be glued together or taped at the joints during assembly. Most systems today feature uniform cavities that improve flowability of the concrete, reduce the need for adhesives during stacking, resulting in flat concrete walls of consistent thickness.
Once the forms are in place and braced and required reinforcement installed, concrete is pumped into the forms. Even with the bracing, forms need to be filled at an appropriate rate based on formwork manufacturer recommendation to prevent misalignment and blowouts. Product advancements and improved construction techniques have greatly reduced the potential for form failure. It seldom occurs when manufacturer recommendations are followed. Reinforcement in both directions maintains the wall strength. Openings for doors and windows require bucks to surround the opening, contain the fresh concrete during placement, and provide suitable material for fastening window or door frames.
Block-outs are needed when bearing pockets are required for floor or roof items. Insulating concrete form systems are compatible with concrete floors, and wood or steel floor joists. In smaller buildings, ledger assemblies for floor framing attachment mounted to the side of the formwork are common. In larger buildings or those for commercial uses, steel weld plates or bolt plates can be pre installed within the formwork so they become embedded in the fresh concrete.
Finishes are usually attached via the flat ends of metal or plastic ties embedded in the forming material. Finishes can alternately be furred out with furring strips. Almost any type of finish can be used with these systems. Wallboard remains the most common interior finish and is the most typical means of meeting the code requirement for a 15-minute fire barrier over plastic foams surrounding living spaces. Exteriors are much more varied and depend on customer preference. Cement plasters are applied over ICFs in a manner similar to other sheathed systems.
After finishes are applied inside and out, typical final wall thickness is greater than one foot. This means that the depth of window and door surrounds have to be wider than what is used for traditional frame construction, with resulting deep window sills—a nice feature for homeowners or other building occupants.