Insulated concrete forms (ICFs) are made from a variety of materials, including rigid plastic foam, composites of wood chips with cement, and recycled polystyrene with cement. But they all work in basically the same way: Units are assembled to form walls, reinforced with steel, and filled with concrete. Forms are later covered with finish materials—stucco or conventional siding on the outside; drywall or plaster on the inside.
ICFs are most often used to build foundation walls, but they also can be used above grade, and some brands are marketed with this in mind. Building with ICFs is more expensive than conventional wood-framed walls, but manufacturers list a number of advantages, including lower rates of air infiltration, high strength, excellent sound deadening and wind resistance, and high thermal mass. EPS Industry Alliance, a trade group for the ICF industry, says using ICFs add between 0.5% and 4% to construction costs, when the house is built by experienced contractors. Higher costs are somewhat offset by the ability to use smaller heating and cooling equipment. Relative costs are affected by a number of variables, including local labor rates, the design of the house, and the type of ICF the builder or designer has chosen.
ICFs come as blocks, panels, or planks. The shape of interior cavities varies, too, so the concrete might be a flat wall of uniform thickness; a waffle grid; or a “screen grid” layout, with both horizontal and vertical columns of concrete. Crews accustomed to conventional wood-frame construction should expect a learning curve when they start building with ICFs. Detailing for windows and doors, stacking walls so they’re plumb and straight, bracing walls adequately for placing concrete, consolidating the concrete to eliminate voids, and running interior wiring all are a little different and take time and practice to master. Durisol, a Canadian ICF manufacturer, recommends builders take a one-day training session, and also offers on-site training.
Houses made with ICFs don’t have as much potential for air leakage as conventional wood-framed houses, and may use less energy for heating and cooling, depending on where you build. In one side-by-side test in Knoxville, Tennessee, the Oak Ridge National Laboratory found that an ICF house used 7.5% less energy than the otherwise identical wood-framed house.
Cement-polystyrene composites. Rastra is a well-known brand of this type, with a composition of 15% cement and 85% recycled expanded polystyrene, a composite the manufacturer calls Thastyron. Blocks, which can be cut with ordinary woodworking tools, are up to 10 feet long, 15 or 30 inches high, and available in thicknesses of 8 1/2, 10, 12, and 14 inches. A 10-inch-thick by 10-foot-long panel 15 inches high weighs 158 pounds—light enough to be handled without a mechanical lift or crane.
Because it’s relatively soft, Rastra panels also can be shaped to make curves or gently rounded shapes. According to the company’s website, building with Rastra adds about $5 per square foot to construction costs over a conventional home built with wood.
Wood chip-cement composites. Durisol and Faswall are two brands of ICFs made with cement-bonded wood fiber. Among the advantages cited by Durisol are no plastics, no volatile organic compounds, no off-gassing, and the capacity to absorb water vapor, which helps maintain healthy indoor humidity levels. Durisol walls won’t burn or melt and have a four-hour minimum fire rating.
Faswall blocks are made from 85% mineralized wood chips and 15% cement. Reported R-values for 12-inch Faswall blocks range from R-21 with a mineral wool insert to R-26 with polyisocyanurate insulation. Durisol blocks come in several thicknesses. With an insert of mineral fiber insulation, the company claims R-14 for a 10-inch wall, up to 21 for a 12-inch wall, and up to 28 for a 14-inch wall. The company says the block material has an R-value of 1.75 per inch, as measured by the National Research Council of Canada. A mineral fiber insulation insert adds another R-4.2 per inch. With these known values, whole-wall R-values can be estimated.