Section 19020.

Cal. Business and Professions Code - BPC § 19020.
in effect
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“R-value” means a measure of thermal resistance of material or composite materials. “R-value” is equal to the reciprocal of the conductance of a given material, or R-value = 1 conductance of the material where the conductance of the material is: Btu (hour × square foot × Fahrenheit degree average temperature difference between two surfaces).
Source: CA Legislative Counsel Bulk DataURL: https://downloads.leginfo.legislature.ca.gov/pubinfo_2025.zipFetched: 6/27/2026

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