The rise of concrete

Concrete conversations
Concrete and steel are some of the most prevalent structural materials for modern commercial buildings in the United States and the rest of the developed world. The reasons primarily are due to their high structural strength and longevity.

Concrete is designed to last centuries. Other materials, such as wood, just can’t compete in this department. On the other hand, concrete–walled homes are rare in the United States (outside of the desert Southwest), mainly due to cost and the tradition of wood-framing. But this situation is changing due to some new products that integrate insulation with concrete and make residential concrete walls affordable and easier to erect.

Wonder walls
In my last column, I listed a number of features that contribute to an ideal residential wall system. The structural insulated panel (SIP) wall fulfills many of these features and may seem unbeatable. But some of the new insulating concrete wall systems are very competitive and are gaining popularity due to their superior strength and longevity, especially in the face of nature’s extreme “forces” (e.g. earthquakes, wildfire, hurricanes, tornadoes and floods).

Insulating concrete walls also have thermal mass, which can provide additional energy-saving benefits beyond R-value in hot climates. (I will talk about the interesting and often misunderstood thermal-mass effect in a future column.)

Solid structures
Insulating concrete walls are composed of a concrete product and a thermally insulating material. The amount of concrete used in these walls provides significant thermal mass, while the insulation yields a high whole-wall R-value and low air-infiltration rates that are comparable to SIPs.

There are many different types of concrete walls—defined by their concrete composition, insulation material, construction method, and relative placements of concrete and insulation. The primary types are: insulated concrete forms (ICF), removable form walls, precast and tilt-up panels, concrete masonry units (CMU), autoclaved aerated concrete (AAC) blocks and shotcrete sandwich panels.

ICFs are rigid insulating forms that are assembled on-site and lock together somewhat like Lego bricks. Steel rebar is added and concrete is poured or pumped into the forms, which remain in place to provide thermal insulation for the wall. The forms come in many different shapes unique to the manufacturer, but all tend to sandwich or surround the concrete and produce a wall with a monolithic concrete core. ICFs do not suffer from thermal bridging since the form creates a continuous insulating layer.

Talk of the town
The expanded polystyrene (EPS) ICF is the most popular insulating concrete wall in North America. These products are differentiated by the shape of the form, how the forms join together, and tie design. Ties are usually plastic connectors embedded in the EPS to hold the forms together during pouring. The EPS usually accommodates electrical and plumbing installations.

In order to achieve comparable R-values, ICF walls generally are wider than SIP walls or stick-framed walls, and typically range from 8 to 15 inches with R-values from R-17 to R-26. Some leading manufacturers are Arxx (www.arxxbuild.com), Amvic (www.amvicsystems.com), Logix (www.logixicf.com) and Quadlock (www.quadlock.com).

Another popular form is the composite ICF—interlocking hollow blocks made of a special composite material. The interior channels of the blocks form a continuous, structural concrete grid. RASTRA block (www.rastra.com) has been popular in Europe for some time; it contains 85 percent recycled EPS and 15 percent cement as a binder. Durisol (www.durisolbuild.com) is an EPS-free product containing recycled wood that is mineralized and bonded with cement.

I’ll be back …
In my next column, I will continue this topic with descriptions of other types of insulating concrete wall systems and some interesting research. I’ll also talk about some new ICF home developments in Chico. Stay tuned.