Some people get so excited at the prospect of building homes out of concrete that they don't realize that there are several different ways to do it. These require different contractors and have different advantages and costs. To be rational, what the project needs to accomplish should be looked at to choose the system that fits best.
Even in the downturn, market share of most types of concrete homes has grown at the expense of frame construction. Total concrete home share passed 15% of new homes in the U.S. and Canada. The recession has thrown things into disarray because Florida, home to the largest concentration of concrete homes, has been hit particularly hard. But expect concrete's share to continue its march upward throughout the recovery.
Surveys show the major attractions of concrete homes are energy efficiency, disaster resistance, indoor comfort, and sound attenuation. All of the major concrete systems generally outperform standard frame construction on all of these.
Yet there are important differences between the systems. Most people are not familiar with the range of options or their relative strengths. This can leave them with higher cost or less of the features they value.
The most widely used systems for building houses in the U.S. and Canada are concrete masonry (or block), insulating concrete forms (ICFs), removable forms, and precast panels.
Block holds about an 80% of the market share in Florida. Masons stack the walls with conventional mortar and place rebar and grout cells as called for by engineering. The most popular exterior finish is some form of stucco applied directly to the block surface, partly because this is highly economical. On the interior, workers fur the walls with nominal 1 inch straps. The following trades treat the strapped surface much as they would a frame wall. They run electrical cable and place insulation between the straps, and fasten wallboard over. In more northern climates, the furring is with 2x2s or 2x4s to provide room for thicker insulation. Sometimes new foam board systems are installed over the surface, providing studding and high insulation.
ICFs are hollow foam blocks stacked in the shape of the exterior walls and filled with reinforced concrete. The foam stays in place to provide insulation. Again, the most popular exteriors are troweled finishes, although straps embedded in the blocks make it practical to use fastened sidings as well. On the interior, electrical chases are cut into the foam and wallboard is screwed to the straps.
Removable forms are conventional handset concrete forms of the type commonly used to build basement walls. They are assembled for the walls of a house, outfitted with rebar, filled with concrete, and stripped to create the structural shell. There are multiple options for insulation. The interior can be furred and outfitted the same way as block walls. The crew also can use various foam systems, sliding foam sheet inside the forms before casting. When the forms come off, the foam is locked into place. These systems make it practical to install a layer of foam on the interior face, the exterior face, on both, or in the middle of the wall (leaving a layer of concrete on the inside and outside). Electrical lines go in chases in the foam or conduit that was pre-installed in the forms. The exterior finish is usually a troweled material.
Precast panels are built at a plant and shipped to the jobsite. There cranes set them in place while workers bolt them together. Most panels for homes are cast with a layer of foam and an attached stud or strap every 16 inches on the interior. Sometimes conduit is pre-installed for electrical lines. In other cases, chases are cut into the foam and wallboard attached to the surface. The exterior may be textured by formliners and painted, or troweled with a stucco product.
Block home construction benefits from an infrastructure like that available in Florida. Kelvin Eder, general contractor for Regal Park Homes, Orlando, says, "Block is more common to the local market here and all the trades know how to do it." The area has inexpensive masons experienced on low-rise walls, abundant block production, and familiar subcontractors and code officials. These streamline the use of block and make it competitive. Some other regions have a similar level of support, while others do not which creates obstacles. The system is fairly flexible in design and at the jobsite. The standard module easily allows for any dimensions in an 8-inch increment. Straight walls and right angles are most efficient. Within these constraints, workers at the site readily can make last-minute modifications.
ICFs are becoming more familiar throughout most of North America. They typically are installed by special ICF crews, with multiple crews available in most places. Because the contractor works with foam, it is relatively easy to create irregular wall features and make last-minute changes of any dimension. However, this flexibility drops sharply once the concrete is cast.
Removable forms crews are available almost everywhere. Most do not commonly build abovegrade walls for homes, however, and these crews may need support. Although forms are most efficient for straight walls and right angles, a competent crew can make any wall length or opening dimension. They also can change the layout at the jobsite within these constraints. Again, after placing concrete, changes become more difficult.
Building with precast panels depends on having a qualified plant within 100 to 200 miles. Beyond that distance transportation costs climb sharply. There are a handful of plants around North America that offer panels for homes. Most plants also supply the crew to erect them at the site. Depending on the precaster, panels can have great flexibility in wall dimensions, features, and angles. However, most changes are costly to make at the site and may require new panels be made.
Wall costs vary widely depending on many different factors. However, experience provides a rough guide to how the systems compare.
Consider an average home located near the center of the U.S. For a single home, wood-frame construction to minimum code would normally be less expensive than any of the concrete systems. The concrete systems tend to cost around 20% to 40% more per square foot of wall area, and increase the total cost of the house around 1% to 5%.
However, this difference can close in many circumstances. Block costs are virtually identical to those of frame in an area with the ideal infrastructure such as Florida.
ICF costs, normally thought of as relatively high, are more competitive in high-end custom homes. These projects often require tricky, unconventional designs, many field changes, and premium features such as high insulation values and noise reduction. These things all drive up the cost of wood, but do less to affect the cost of ICFs. The foam is easily cut and modified, even in the field. The walls have very high R-values and sound reduction, while frame requires costly additions to achieve the same results. In some custom homes, the cost of frame can rise up to approach or exceed that of ICFs.
In a development of many similar, repeated units, removable form walls quickly come down in cost. Most of the cost of removable form construction is in the capital expense of the forms and the labor of transporting and setting them up. According to Jay Barnhardt, owner of J.E. Barnhardt Construction, Kansas City, Mo., "Even in a single-family home this can be pretty fast. But it's even more so with townhouses or condos where you have repeated units. Once the forms are set for one unit, the crew can move them next door and reset for the next one, at little additional cost." Although the cost of frame homes falls with volume, too, the cost drop on removable forms is so steep that it can come close or match frame in developments of as few as 30 units.
The cost of precast panels also drops sharply with repeated homes because once the plant can set up to produce a particular design, the cost of additional output is low. Cost also comes down quickly if the plant is nearby. Some precast plants offer houses at about the same cost as frame in their immediate surrounding area.
In addition, in high-wind areas all concrete systems are more competitive. Frame construction requires extra measures and hardware to meet wind code requirements in vulnerable areas, increasing cost. But concrete walls typically require little or no beefing up. In the affected areas?mostly the coastal Southeast?this factor also closes or eliminates the cost gap.