Achieving a LEED platinum rating, the highest level, for a home means doing everything feasible to make it energy efficient and sustainable. For the Gallo residence in Minneapolis, which the owners named 5ive, that included concrete sandwich panel walls.
This simple structure packs a lot into 1760 square feet. The exterior walls are loadbearing concrete panels infilled with double-pane, argon-filled glass. “Essentially we chose the technology before we designed the building,” says Jeff Dwyer, Shelter Architecture, Minneapolis. “The client had a very small budget and wanted a very modern home that would be as sustainable as possible. Our economical sustainability model is to get the envelope really efficient. You can do so much more with an envelope than you can with alternative technologies and we knew we were going with a commercial glass system so essentially the entire envelope was prefabricated.”
The concrete wall panels that form the house's structural shell were precast using Thermomass, a sandwich panel system with concrete wythes on the outside and rigid polystyrene foam insulation in the middle. Fiberglass composite connectors—which eliminate thermal shortcuts—connect the two wythes. The panels for 5ive have a 4-inch structural interior wythe, 3 inches of insulation, and a 2-inch-thick exterior concrete facing, resulting in an R-33 wall—double that of timber construction. “We started with the idea of using that technology [Thermomass], then designed everything with vertical continuity,” says Dwyer. “All of the panels and windows were completely vertical and we designed the house aesthetically to work within that logic. The panels have great thermal properties that even go beyond insulating concrete forms, so we were really excited about that."
“The insulation in our sandwich panels is always a complete edge-to-edge installation, meaning the insulation goes from top to bottom of the panel and right to the edges,” says Mark Finholt, owner, ForeCast LLC, Lakeville, Minn., which served as the consultant for the concrete wall systems. “We are very conscientious to create a complete insulation envelope that is not compromised by solid sections of concrete or conducting ties.” To accomplish this, walls extend past the roof into a parapet and 16-inch-thick R-60 roof insulation runs up the inside of the parapet and over the top of the concrete sandwich panels, where it was capped with Corten steel flashing. “That way the insulation all ties together and there are no thermal bridges that can transfer heat from the interior living quarters,” says Finholt.
“The window frames and windows are the weak point of the envelope at only an R-3 or R-4,” says Dwyer. Shelter's building design takes advantage of natural lighting, but reduces solar gain and lowers summer cooling costs by relying on the mechanical system rather than the sun's heat. “We used the windows in any way we could to our advantage for energy efficiency, other than them performing thermally,” he says. “The position of the windows was deter mined by the system, so we went with the mechanical strategy instead of the passive strategy and got the most efficient mechanical system we could find. When you get through all the science and look at all the alternatives, you can't beat a good envelope and a good mechanical system.”