U-factor is related to R-value how?

• A. U-factor equals the total R-value
• B. U-factor is the reciprocal of total R (U = 1/R_total)
• C. U-factor is the sum of R-values of all layers
• D. U-factor is unrelated to R-value

Answer: (B)

U-factor measures how easily heat flows through a building element, and it is the reciprocal of the total thermal resistance (R-value) of that element. In practical terms, the total R for a layered assembly is the sum of the individual layer R-values, and the U-factor is U = 1 / R_total. So increasing the total R lowers the U-factor, meaning better insulation. The other statements don’t fit because the U-factor is not equal to the total R-value, not the sum of the layer resistances, and it is indeed related to R-value through this reciprocal relationship.