"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
1 | |
R-value = | _____________________________ |
conductance of the material |
where the conductance of the material is: Btu (hour × square foot × Fahrenheit degree average temperature difference between two surfaces).
Ca. Bus. and Prof'l. Code § 19020