Concrete conversations

Concrete continued
In my last column, I talked about the insulating concrete form (ICF) wall, the most popular type of concrete wall for homes in North America. This wall system places rigid insulation (usually expanded polystyrene, or EPS) on both sides of a structural concrete core. The EPS panels serve as the concrete form during pouring and curing, and also provide for a high, whole-wall R-value. This “all-in-one” simplicity explains their popularity.

Locally built
Bob Scardina, of R.A. Scardina Builders in Chico (www.scardinabuilders.com), has built several ICF homes and structures in the Bay Area and currently has two infill projects in the Avenues. One project (on Ninth Avenue) is aimed at moderate-income, first-time home buyers (1,500 square-foot homes) and the other (on 11th Avenue) will have larger two-story homes. Both projects will utilize 6-inch ARXX forms (www.arxx.com), which produce walls with an overall width of 11.5 inches and finished-wall R-value of R-22.

Super strength
Another type of insulating concrete wall system uses removable metal forms. Rigid plastic foam insulation is placed between the forms and held in place with a grid of non-conductive ties. Steel rebar is generally added and then concrete is poured on either side of the insulation. Once the concrete has cured, the forms are removed and can be re-used with minimal maintenance. The Thermomass form system (www.thermomass.com) is an example of this type of wall.

While not as quick to erect or adaptable as ICFs, this system has a few advantages: 1) the forms are much stronger and greater heights can be formed in a single pour; 2) the insulation is better protected against damage; and 3) the cooling thermal mass effect is better utilized by having the concrete next to the living space.

A typical installation has at least 3 inches of extruded polystyrene (XPS) insulation, sandwiched by 4-6 inches of concrete on the interior and 3-4 inches of concrete on the exterior. A 3-inch thickness of XPS produces a whole-wall R-value of R-16 with significant thermal mass properties. The R-value can be increased by 5 per additional inch of XPS.

Lots of labor
Concrete masonry unit (CMU) walls have been used in the South for many years. They usually consist of stacked and mortared concrete “cinder” blocks (8-by-8-by-16-inch is common). The cavities inside the block allow the addition of grouted rebar to compensate for the lack of tensile strength.

Ungrouted cavities can be insulated, but normally foam insulation panels are attached to one surface to provide the needed R-value. While the materials are inexpensive, construction of CMU walls is more labor-intensive than most other concrete wall systems. Check out the Masonry Advisory Council’s Web site, www.maconline.org, to learn more about the many types of CMU walls and finishes.

Modern manufacturing
A more innovative concrete block product is autoclaved aerated concrete (AAC), a closed-cell aerated concrete product that provides both structure and thermal insulation. A gas-liberating chemical reaction during processing allows the concrete to expand to about five times its original volume.

The result is a lightweight, insulating, fireproof product that can be cut, drilled, nailed, screwed, routed and milled with common tools. E-Crete (www.e-crete.com), a leading AAC manufacturer, produces blocks with thicknesses of 4 to 16 inches (R-4 to R-16). Like CMUs, these blocks are set with mortar and reinforced with steel rebar.

Surfaces can be finished with conventional materials such as stucco, siding and drywall. Additional insulation material can be added to the building wall; however, the AAC block thickness is usually chosen to provide the required wall R-value.

More next time …
Further discussion on types and variations of concrete walls—plus relative costs and performance—will continue in my next column.