Cal. Pub. Res. Code § 25910

The commission shall, by regulation adopted no later than July 1, 1978, establish minimum standards for the amount of additional insulation (expressed in terms of R-value) installed in existing buildings. One year after the adoption of those standards, no insulation shall be installed in any existing building by a contractor unless the contractor certifies to the customer in writing that the amount of insulation (expressed in terms of R-value) meets or exceeds the minimum amount established by the standards. The minimum standards may vary for different types of buildings or building occupancies and different climate zones in the state. The minimum standards shall be economically feasible in that the resultant savings in energy procurement costs shall be greater than the cost of the insulation to the customer amortized over the useful life of the insulation.