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Mass Confusion

Do Poured Concrete and Masonry Walls Deliver?

by Charles Wardell

AN INSULATED CONCRETE WALL SYSTEM consists of poured concrete or concrete block insulated with expanded polystyrene (EPS) or extruded polystyrene (XPS) foam insulation. EPS is by far the more common insulator, mostly because it costs a lot less than XPS. The wall itself can consist of stacked concrete blocks, called concrete masonry units or CMUs, or cast-in-place steel-reinforced concrete. Although CMUs account for half the total market share for concrete walls and are especially popular in the South (Florida estimates run to over 80% of new residential construction), cast-in-place systems have been growing steadily nationwide.

The most familiar cast-in-place systems are the various insulated concrete form products, or ICFs,. These foam forms are assembled on site and filled with rebar and concrete, then remain in place after the concrete pour to serve as the wall insulation. According to Donn Thompson, director of market strategy and positioning at the Portland Cement Association, the economic downturn has eliminated many small ICF manufacturers, but he estimates that about 30 of them are still serving the U.S. market.

Most ICF products fall into one of two broad categories: hollow foam blocks that workers stack like Legos, and flat foam panels that are erected and braced like conventional forms. Foam block systems enjoy the largest market share and require less skill to assemble than panels.

Over the years, manufacturers have offered forms that produce a variety of concrete geometries, from conventional flat walls to post and beam systems, to grids that resemble breakfast waffles. While the latter two use less concrete, Thompson says that the industry has standardized around the conventional flat wall, whose familiarity makes it accepted by contractors and homeowners, alike.

A lesser-known alternative to ICFs is the concrete sandwich, in which rigid foam is placed between reusable forms and concrete poured on either side, leaving the foam at the center of the wall. These systems are very minor players in the residential market, but manufacturers say that 10% to 30% of their business is residential--mostly architect-designed custom homes. “Architects like this system because it’s engineered,” says Brad Nesset, VP of sales at Thermomass. “It has well defined structural properties.”

Why Concrete?
Two things to keep in mind about an insulated concrete wall are that it will be more costly and more massive than a stick frame. Cost premiums range from 1% to 10%, depending on a number of factors. Wall thickness can vary from 8 to 15 inches, ddepending on the particular product and the amount of insulation used, but the wall will be wider than a SIP or stick-framed wall with a comparable R-value.

What’s the payoff? Solidity, for one thing. Reinforced concrete stands up well to high winds, so it’s an asset in places like Florida and the Gulf Coast. The sheer mass of a concrete wall--and a cast-in-place wall, in particular--will deaden street noise, making the home quieter inside than a wood frame. Concrete also doesn’t catch fire or get eaten by termites.

CWS manufacturers also make claims about energy savings, but some industry experts say that the R-value numbers alone aren’t overly impressive. A study by Building Science Corporation in Westford, Mass. points out that the R-20 offered by a 15”.-thick ICF wall with 5” of EPS would not be considered high in northern climates and “would need to be supplemented by interior insulated frame walls, which further raises the cost.” On the plus side, they add that the whole wall R-value of a properly detailed ICF wall will be much closer to its nominal value than with a stick-framed wall, largely because of the lack of thermal bridging.

Thermal Mass and Climate
Masonry wall advocates counter that R-value isn’t the whole story because concrete offers an added benefit that wood framing doesn’t: thermal mass, or the ability to store heat. Because masonry is slow to respond to changes in outdoor temperatures (see Graph 1), temperatures in the house stay more even, easing the load on the HVAC system. In fact, ICF manufacturers point to the fact that the International Energy Conservation Code (IECC) specifies a lower minimum wall R-value for high-mass systems like insulated concrete.

But the thermal mass benefit of an insulated concrete wall--and the R-value discount offered by the IECC--will depend on exactly where the wall is built. The IECC recognizes 8 U.S. climate zones, with zone 1 being the warmest and zone 8 the coldest. As shown in Table 2, R-value discounts vary by zone, from a low of 1 to a high of 10.

In addition, not all hot climates are created equal. During the day, the mass of a concrete wall will soak up heat that otherwise would have gotten into the home; at night, when temperatures drop, any heat remaining in the wall will move back outside. The theory is that this process works best where there are very hot days and very cool nights, as in the Southwest desert.

Greg Kallio, a professor of mechanical engineering at California State University in Chico who specializes in heat transfer, recently tested this theory by modeling “the whole gamut” of wall systems, from stick-built to SIPs to insulated concrete, using industry standard energy analysis programs like EnergyPlus, as well as his own custom software. His conclusion? “The effectiveness of thermal mass is very dependent on diurnal temperature variation. You want nighttime temperatures that get at least 10 degrees cooler than the thermostat set point. If you keep the thermostat at 78, outside temperatures need to fall below 70 degrees at night to really take advantage of the thermal mass.”

While manufacturers don’t claim that thermal mass works as well Minneapolis as it does in Phoenix, they insist that it’s still an asset during cold northern winters. Nesset says this has been documented by computer simulations done at Oak Ridge National Laboratory. “A mass wall might be 3 times more effective in Phoenix than the baseline you would expect from the insulation alone, but in Minneapolis it will still be 1.5 times more effective than the baseline. 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.”

     
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Most window manufacturers have developed specialized products for ICF construction. As with any type of structure, careful sealing against air infiltration allows the mass of the walls an optimal performance.

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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.


Insulated Concrete Wall Systems

Insulated Concrete Forms (ICF)
ICFs can consist of hollow foam blocks that are stacked one atop another, or foam planks or panels that are held together with plastic ties. They can be used to form various structural configurations, such as a standard wall, post-and-beam or grid.

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Concrete Masonry Units (CMU)
CMU walls are built with hollow 8”x8”x16” hollow-core concrete blocks. They’re very popular in the South. Foam insulation can be applied to one or both faces of the completed wall.

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Interior Insulated Concrete
This is a forming system that uses non-conductive ties to hold the insulation at the center of the inner and outer forms. Manufacturers point out that the placement of the insulation protects it from damage. Photo: www.thermomass.com

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Shotcrete Sandwich
Rather than using removable forms, shotcrete panels are built by erecting a foam core with steel reinforcement. A pneumatically applied fiber-reinforced concrete called shotcrete is then blown onto the panels, and the surface troweled smooth. The manufacturer claims that this results in a stronger wall than concrete. Photo: www.solarcrete.com

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