There, an insulated concrete wall with a material value of R-11 could have the same heating and cooling loads as a wood-framed wall insulated to R-20.”
Nesset says that thermal mass can be very advantageous in electrically heated homes located in service areas with time-of-day pricing. There, he says, you can set the thermostat to turn the heat on in the pre-dawn hours when rates are lowest. The walls will hold that heat for some time.
Insulation Placement: Does It Matter?
Nesset and other sandwich system manufacturers also claim that putting the insulation in the center of the wall makes the thermal mass more effective. Their reasoning is that when the inner and outer surfaces aren’t covered by insulation, they’re more efficient at absorbing and releasing heat without transferring it through the wall.
However, if Kallio’s models are correct, insulation placement doesn’t seem to be a major factor in the effectiveness of the thermal mass. “My findings show that it does help a little to have the concrete layer on the inside, but that it’s not a huge advantage. What’s important is that you have the concrete, not where it’s located.”
Thompson says that a PCA study came to a similar conclusion. The Association tested different wall configurations in 25 different climates and found that, even with the concrete encapsulated in the foam, there’s a still thermal mass effect. “A [high-mass] system with lots of insulation will out-perform one with less insulation, regardless of where the concrete and insulation are placed.”
AUTOCLAVE AERATED CONCRETE (AAC)
An alternative to concrete and foam is AAC: a concrete block product made by mixing Portland cement with aluminum powder and other additives, including recycled fly ash (a byproduct from coal-burning power plants). A chemical reaction between the aluminum and concrete forms microscopic hydrogen bubbles in the concrete, causing it to expand to several times its normal volume. It’s then steam-cured in a pressurized chamber called an autoclave. One advantage of AAC is that, because the bubbles serve as insulation, no foam is needed. R-values of approximately 1.25 per inch are better than concrete or CMUs, but not as high as an insulated concrete system for a given wall thickness. According to Greg Kallio at California State University, Chico, “you need a 12” wall to get the same R-value that you could achieve with a 6” or 10” ICF wall.” An example of AAC can be explored here:
www.e-crete.com
IS CONCRETE SUSTAINABLE?
In addition to its thermal mass benefit, the concrete industry also claims that concrete is a sustainable material, citing obvious attributes such as durability (concrete buildings should last a long time) and the use of recycled materials. Environmentalists, on the other hand, base their analyses on more encompassing factors like embodied energy: total energy needed over a material’s life cycle to harvest or mine, transport, and dispose of it. Some believe that, when this criteria is used, concrete falls short.
“It’s hard to justify building a home with concrete walls,” says Tristan Roberts, editorial director at
Buildinggreen.com. “For one thing, the making of Portland cement is very energy intensive.” Roberts admits that a definitive answer will require more data, but that the situation is improving. He says that with recently introduced tools like the Athena Institute’s Eco Calculator (
www.athenasmi.org/tools/ecoCalculator/), which helps designers assess the life cycle impact of various building assemblies, more quantitative data is quickly becoming available.
MANUFACTURERS
ICFs
www.amvicsystems.com
www.arxx.com
www.buildblock.com
www.foxblocks.com
www.greenblock.com
www.nudura.com
www.quadlock.com
www.rewardwalls.com
www.superformicf.ca
www.tfsystem.com
Interior Insulated Concrete
www.thermomass.com
Shotcrete System
www.solarcrete.com
Autoclave Aerated Concrete
www.e-crete.com
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