The energy efficiency of construction materials is determined by how they handle heat, how the heat is transferred through those materials and how well the materials hold or store the heat. Since heat always moves from hotter to colder, during the summer when outside temperatures are higher than the temperature inside a building, the heat is transferred through the walls from outside to inside. Conversely, during the winter, the heat is transferred from the inside of the building to the outside. The effectiveness of construction materials – and how much energy consumption they reduce – depends upon how quickly heat transfers through the materials (how well they hold the heat). Energy performance is measured by the stated R-value of the building material; when combined with other materials such as drywall, particle board, or stucco, the adjusted measure is known as mass-enhanced R-value.
The R-values of traditional stud wall construction using either wood or metal studs with insulated wall cavities can vary depending upon the size, makeup and spacing of the studs. Studs conduct more heat than insulation does since they extend through the wall and have a small R-value. To achieve any degree of efficiency with stud wall construction, it is necessary to cover the framework with a structural insulated panel of plywood or oriented strand board filled with expanded polystyrene (EPS) or other foam material, which results in an R-value of 5.0 per inch of thickness. Traditional block masonry construction is one of the more energy efficient construction methods as compared to stud wall construction. A standard, 8″ hollow block has an R-value of 1.75, which is not high enough to be considered energy efficient. However, the R-value can be enhanced by adding foam insulation outside or inside the blocks (typically the hollow block cores are filled with beads or injected foam made from polystyrene or a similar foam). More energy efficient still is precast concrete wall construction, which is built from high strength concrete with reinforcing fibers and steel. The concrete is made more energy efficient by building insulation into it, which gives it an R-rating in the neighborhood of 12.5.
The most energy efficient construction methods are those which combine concrete with insulating foam in an integrated way. The state-of-the-art method is called Insulated Concrete Form (ICF) construction, which is a reinforced concrete technique far more energy efficient and quicker to erect than traditional block masonry or precast concrete walls. In ICF construction, hollow rectangular EPS foam forms are stacked and lined with steel rebar, then filled with concrete. When the concrete sets, the EPS foam which bonds to the concrete in the process becomes an inner and outer layer of insulation. The forms interlock like a children’s Lego set and can be quickly assembled. The blocks are not manufactured to a building’s exact specifications and size. The blocks have to be cut to some degree to exact length and to make openings for windows and doors; but this is easily done by hand. Because the inside foam – concrete – outside foam construction makes for a triple insulation barrier, ICF’s are the most efficient building materials with R-40 or higher energy ratings.