California is a mixed climate region that requires both heating and cooling. To determine the proper energy efficient replacement windows for your California’s home, the state is separated into two zones; Northern/Central and Southern/Central. Efficient Windows Collaborative (EWC), a coalition of private and public organizations and agencies that seek to expand the market for high-efficiency window products, created a U-factor and Solar Heat Gain Coefficient (SHGC) guide based on various climate zone in the U.S.
Focusing on homes in the North/Central zone of California, choose replacement windows with as U-factor equal to or less than 0.32 and an SHGC equal to or less than 0.40. Homes that lie in the Southern/Central zone, choose replacement windows with as U-factor equal to or less than 0.35 and an SHGC equal to or less than 0.30. The larger your heating bill is, the more important it is to have a lower U-factor. Windows with low SGHC values help reduced summer cooling demand, but they also reduce free winter solar heat gain.
Below is a chart comparing window performance in San Francisco, California: (click to view the chart)
Source: http://www.efficientwindows.org/factsheets_existing/California.pdf
By: Paul Doh