Sandwich panels: a tasty course
Continuing the discussion on insulating concrete walls
One might think that I’m biased toward insulating concrete walls since this is my third column on the topic! That’s really not the case—it’s just that there are so many different options that a homeowner or builder has to choose from with concrete. As I’ve discussed, they can be poured-in-place (PIP) into forms at the construction site, delivered as precast walls (this is more common for commercial buildings), or built from mortared blocks.
Yet another type of insulating concrete wall is the Shotcrete Sandwich Panel. These walls consist of a core of expanded polystyrene (EPS) insulation that is encased in a pneumatically applied fiber reinforced concrete, known as shotcrete. The prefabricated panels of EPS insulation are typically sandwiched between two parallel grids of steel wire or rebar and sprayed with concrete on-site. Incredible strength comes from the ties that pierce the insulation and attach to the steel grids to create a truss-like support system.
How it works
Shotcrete is a process where concrete is projected or “shot” under pressure from a “gun” onto a building surface. The surface can be troweled smooth while the concrete is still wet. When combined with reinforcing steel, the hardened properties of shotcrete are similar to conventional cast-in-place concrete and form a structural wall component.
The properties of shotcrete can be enhanced through the addition of many different additives or admixtures to increase strength and resistance to impact, chemical attack, cracking, and freeze-thaw cycling.
Solarcrete Energy Efficient Buildings (www.solarcrete.com) manufactures insulated panels for shotcrete application. Instead of a steel wire mesh, the Solarcrete panel employs nonmetallic composite wall ties between vertical steel rebar to ensure that the concrete layers are structurally connected without thermal bridging.
The standard Solarcrete concrete wall is 12 inches thick and includes 7-1/4 inches of EPS foam and 2-3/8 inches of fiber-reinforced shotcrete on both sides of the foam. The resulting wall has an R-value of 36 with significant thermal mass.
A related product that utilizes more EPS and much less concrete (only a quarter-inch per side) with higher R-values is the Saebi Alternative Building System, or SABS (www.strataus.com).
The real cost of any wall system consists of material, labor and operational (energy) costs. It should be noted that all these costs have significant geographic variation and the labor costs depend strongly on the experience level of the building crew. Furthermore, operational costs are tied closely to the choice of HVAC system and whether it has been sized according to the enhanced energy efficiency of these wall systems.
Most of the available concrete-wall cost data are for EPS Insulating Concrete Form (ICF) walls, since these systems are the most popular; it is assumed that the data are roughly indicative of most other formed concrete-wall systems. The cost data have a large standard deviation, but most indicate a 5 percent to 10 percent increase in total construction cost and a 2 percent to 5 percent increase in final sales price of the home when compared to wood-frame exterior walls.
Operational cost savings depend heavily on location, but data show a reduction of approximately 20 percent to 50 percent in heating and cooling costs compared to wood-frame walls with glass fiber insulation. The higher values are typically found in hot climates where cooling loads are significant.
Confused by all the choices of wall systems? Optimum Value Engineered (OVE) advanced framing, structural insulated panels (SIPs), and insulating concrete are all affordable choices that will provide high R-value with lower air-infiltration rates. But above all, it is important to find a contractor who has an experienced crew specifically trained for the wall system that you choose. You don’t want your home to be part of their learning curve!