HIGH PERFORMANCE WALL SYSTEMS by Alex Wilson Most builders surveyed now use some combination of 2x6s with: foam on the outside (left) or foam on the inside (right). One builder imports rigid fiberglass (above) from Canada. Energy-minded builders strive to save Btus while keeping construction simple A few years ago, during the height of energy awareness, every builder I knew used a different wall system. Builders were trying every option they could think of to minimize heat loss and produce a building shell tight enough for one of Jacques Cousteau's mini-subs. Complexity was no concern. And as for cost, well... that was sometimes a problem. In fact, because so many extras were added in the interest of energy conservation, the builders sometimes found they couldn't charge