Which statement best explains the relationship between R-value and U-factor in wall assemblies?

• A. R-value measures resistance to heat flow; the U-factor is the reciprocal of total R-value for layers in series.
• B. R-value and U-factor are the same metric.
• C. Higher R-value directly lowers U-factor without relation.
• D. U-factor increases as R-value increases.

Answer: (A)

The main idea is how resistance to heat flow and actual heat transfer relate. R-value measures how much a wall resists heat flow—the higher the R-value, the more the wall resists heat. U-factor, on the other hand, tells how much heat passes through the wall and is the reciprocal of the wall’s total resistance. When you stack layers, you sum their R-values to get the total resistance, and the overall heat transfer rate is U = 1 / Rt. So, a higher total R-value makes Rt bigger, which lowers the U-factor, meaning less heat loss. This is why the statement that R-value measures resistance and U-factor is the reciprocal of the total R-value for layers in series best captures their relationship. The other choices misstate the relationship or equate the two metrics.