The double wall has been used to build energy efficient homes for a long time. While there are many ways to do it, the basic premise is to build one structural wall (typically the inside wall), build a second wall around it, and fill the space between with insulation.
It has several advantages over a conventional stick-frame wall: it provides more room for insulation, reduces thermal bridging through the studs, and avoids moisture problems that can arise from using vapor-impermeable materials like foam board.
CCHRC has tested and designed several different double-wall techniques. The key to this system is good air sealing (so warm air is not lost through the wall) and placing enough insulation outside the air barrier so the structural wall sheathing so that it stays warm and dry.