R-Value (Thermal Resistance)
Definition
A measure of thermal resistance to heat flow, calculated as R = L/k where L is thickness and k is thermal conductivity. Higher R-value means better insulation. R-values are additive for layers in series.
Practical Note
Metric R-value uses m²·K/W. Imperial uses ft²·°F·h/BTU (multiply metric by 5.678). Building codes specify minimum R-values for walls, roofs, and floors.