R-value is defined as which of the following?

• A. The Rate Of Heat Flow Through A Material
• B. The Reciprocal Of The Rate Of Heat Flow Through A Material
• C. The Reciprocal Of The Time Rate Of Heat Flow Through A Unit Area Induced By A Unit Temperature Difference Between Two Defined Surfaces Of Material Or Construction Under Steady-State Conditions
• D. The Reciprocal Of The Thermal Conductance

Answer: (C)

R-value measures how much a material resists heat flow. It is defined as the reciprocal of the rate at which heat would transfer per unit area for a unit temperature difference, under steady-state conditions. In practical terms, if you impose a temperature difference across a layer, the heat flow per area q" is proportional to ΔT, and R = ΔT / q". When you set the temperature difference to one unit and look at heat flow per unit area, the reciprocal of that rate gives the R-value. This is why higher R-value means better insulation, and why resistances add for multi-layer assemblies (R_total = sum of individual R-values).

The other statements describe either the heat flow rate itself or omit the per-area and unit-temperature-difference framing, so they don’t match the standard definition. While reciprocal of thermal conductance is related, the precise definition used in building science emphasizes the per-area, unit-temperature-difference, steady-state formulation shown here.