The typical wall built in Davis, California, uses 2x4 studs spaced 16-in. on-center. Designers at Davis Energy Group, working as consultants to Pacific Gas & Electric Co., designed an alternative wall system for PG&E's ACT Squared demonstration home that was energy efficient, cost-effective and structurally sound. The wall was first built at the ACT Squared house in Davis and a refined version was employed later in Rockland, California. The design displays several innovations.

Engineered Wood Studs

The studs are made of an engineered wood product called Oriented Strand Wood (OSW) from Trus-Joist MacMillan. To make OSW, small fast-growing trees are shredded into 12-in. strands. The strands are saturated with polyurethane resin and packed into a steam-injection press. A sheet of OSW formed by the press, which can be up to 35 ft. x 8 ft. x 5-1/2 in. deep, is sawn into lumber. Like other engineered wood products, the OSW lumber is more stable than "real" wood. It arrives on the job site straight and won't warp over time. It's also more expensive than standard lumber, although the price is expected to drop as its market matures.

Because they were stronger than pine or fir, the OSW studs were only 1-1/4 in. x 3-1/2 in. They reduce the amount of wood in the wall, making room for more insulation. Studs were spaced 24-in. on-center, instead of the more typical 16-in. spacing. Headers typically fill a 2x4 wall cavity. The engineered wall used a continuous header mounted to the inside face of the studs.

OSW furring strips were attached to the inside face of the framing to support the drywall around window and door openings. A horizontal nailer was also placed along the floor and another about halfway up the wall. The remaining stud faces received a strip of foam insulation for extra R-value.

The furring strips created a 1-1/2 in. air space behind the drywall, which boosts R-value a bit. Wiring runs through this space without damaging the insulation.

The framing-to-wall ratio for the engineered wall was 9 percent. Conventionally framed walls typically range between 15 and 30 percent.

Structural tests performed on 8 x 8 ft. wall sections showed the engineered wall section to be stronger than a conventional wall. The engineered wall moved and cracked less, while the conventional wall pulled off its sill plate.

Foam Boards Between Studs

Foam boards are frequently attached to the outside of a frame wall. However, the ACT Squared builders put 3-1/2 in. Celotex Blackore inside the wall cavity. The foam boards were installed as the wall was assembled, which promoted proper stud spacing and reduced wall layout time. The insulation boards also added to the wall's rigidity. The overall insulating value of the wall was R-25.6 (U-0.039).

Design Changes

The framing crew struggled with the unfamiliar system at the Davis house. Several changes were made to this system at a second project in Rockland. The continuous header was replaced with conventional headers over wall openings. The walls were framed and the foam boards installed afterwards. Each piece of insulation was sealed in place with expanding foam. Foam spacers on the face of the studs were eliminated.

ACT Squared

The ACT Squared demonstration homes also contained innovative mechanical equipment, lighting, appliances, daylighting and other design features. The homes were so energy efficient that conventional cooling equipment was not needed.