U-value, also known as thermal transmittance, is a measure of heat transfer across a building element, such as a wall or roof. It represents the amount of heat that passes through unit area in unit time induced by unit temperature gradient. Some synonyms for U-value include thermal conductivity, insulation value, heat transfer coefficient, and R-value. R-value is a synonym that is commonly used in the United States and represents the thermal resistance of a building element, while U-value represents the inverse of this resistance. Other synonyms for U-value include thermal resistance, thermal transmittance, and thermal insulance. Each of these terms describes the ability of building elements to resist heat transfer and maintain comfortable indoor temperatures.