Although attaching insulation to cured concrete walls is common, it is increasingly popular to cast foam into the concrete in the forming process.
Two common methods are used for this. The first is to cast the insulation onto one face of the concrete. The second is to position foam sheets along the middle of the wall section and cast concrete on either side.
With either method, a 2-inch-thick layer of expanded polystyrene (EPS) or extruded polystyrene (XPS) is standard in warm climates. In cold climates, as much as 4 inches often is used.
In the first method, the foam may go on either face of the wall. Sheet foam is cut in the length and width to match the dimensions of the form panels. The foam also is notched along the edges to fit around the snap ties. One piece is inserted inside each form section as the formwork is assembled. In some systems, plastic connectors fit over the snap ties, with flanges along the edge to hold the foam after the forms are removed. It also is possible to use no connectors and rely on the natural adhesion of the foam to the concrete. In this case, the foam must be EPS or something else with a rough or porous skin.
With foam pieces in place continuously along one face of the formwork, the concrete is placed. As it cures, it grips the connectors or the foam. When the forms are removed, the foam remains securely attached. Electricians may rout chases and cutouts in the foam to house cable and electrical boxes.
The outer connector flanges also provide a convenient surface for attaching fasteners. When no connectors are used, special plastic anchors may be preinstalled to provide attachment points where needed. Then wallboard, siding, or fixtures may be fastened to the wall on the foam side.
In the second method, the foam sections are drilled at regular points. Some systems include foam with predrilled holes, usually every 12 inches-on-center horizontally and vertically. A composite plastic pin is inserted into each hole so that it fits snugly and it protrudes from each face about 1½ inches.
The pinned foam sections go inside the formwork. They are positioned usually along the centerline of the form cavity. However, design may call for the foam to be offset to one face. This will create a thicker, structural concrete section on one side and a thinner concrete layer on the other that protects the foam.
The pour must proceed so that concrete head pressure on both sides of the foam is maintained. This is achieved by filling each cavity at the same rate. When the forms are stripped, the pins connect the two concrete layers mechanically. Each side of the wall has a concrete face.
Under either method, most other details of wall assembly are conventional. This includes the installation of rebar and the method of placing and consolidating the concrete.
Pieter VanderWerf and Daniel Kokonowski work for Building Works Inc. (www.buildingworks.com), a consulting firm that helps companies introduce new construction products. Wall-Ties and Forms Inc. and Composite Technologies Corp. provided information.