Excellence in Sustainability: County of San Mateo, County Office Building 3 (COB3)

Sustainability was top of mind for county officials and their project team working on this $195-million mass timber office building. The 208,000-sq-ft, five-story H-shaped building is the first net-zero energy, LEED Platinum-designed, mass timber civic building constructed in the U.S. It features a mass timber and glulam frame with a cross-laminated timber deck structure, which helped achieve an 85% reduction in embodied carbon compared to a conventional steel structure.

“The County of San Mateo aimed to create a bold civic building with strong sustainability goals,” said Mark Whiley, project executive at Truebeck Construction, in a statement.

Photo by Reid Schultz/Truebeck Construction

Truebeck sought firms that could install the mass timber package turnkey, and locked in material pricing in three phases. This saved more than $1 million in materials costs. The design allowed for additional savings by using five-layer CLT rather than seven-layer, supplementing Douglas fir on the exposed top and bottom layers with lower-cost wood for the middle.

Photo by Rowan Jack

The all-electric building features 333 kW of on-site solar arrays. MEP systems were designed for efficiency and have redundancy providing greater resiliency and future flexibility. The building has 190 automated operable windows that provide natural ventilation and passive cooling each night when outdoor conditions are mild.

The mass timber building features a high-performance envelope, on-site solar and passive design strategies.
Photo by Rowan Jack

To minimize heat gain and reduce the need for artificial cooling while also optimizing daylight, the exterior glass on the north facades is recessed and shaded, while the glass on the east and west facades is protected by fins. The site also features bioretention planters that absorb and treat all of the site’s stormwater runoff. Compared to other buildings this size, COB3 uses about 610 MW less power and 250,000 fewer gallons of water per year. “This is not only the greenest civic building in the United States, but one of the most aesthetically pleasing,” San Mateo County Board of Supervisors President David Canepa said in a statement.