What is the rate of transfer of heat through one square metre of a structure, divided by the difference in temperature across the structure, called?

• A. Thermal conductivity
• B. Thermal transmittance
• C. Thermal inertia
• D. Thermal emissivity

Answer: (B)

This quantity describes how easily heat flows through a building element per unit area for each degree of temperature difference. It is called thermal transmittance, commonly expressed as the U-value. The rate of heat transfer per area per temperature difference has units of W/m^2K, and it reflects the overall insulating performance of the entire structure, including layers, gaps, and surface effects. A lower U-value means better insulation.

Thermal conductivity, by contrast, is a property of a material itself (how well a material conducts heat per thickness) and doesn’t capture the whole assembly. Thermal inertia relates to how much energy is stored in a material and how slowly its temperature changes. Thermal emissivity describes how readily a surface emits radiant heat, not the total heat transfer through a structure.