The "R" value of an insulation product refers to

• A. Conductivity
• B. Absorptivity
• C. Reflectivity
• D. Resistance to heat flow

Answer: (D)

R-value measures how much a material resists heat flow. It’s a measure of thermal resistance: the higher the R-value, the less heat passes through for a given temperature difference, meaning better insulation. This resistance comes from the material’s thickness and its ability to impede heat transfer (often thought of as thickness divided by thermal conductivity). It’s not about absorbing or reflecting radiant energy, which relates to absorptivity and reflectivity, nor is it the same as conductivity itself (conductivity is the property that determines how easily heat moves, while R-value is the resistance to that movement). So the R-value is best described as resistance to heat flow.