What is the SI unit of the R-value?

What is the SI unit of the R-value?

• A. ft^2·°F·h/BTU
• B. W/mK
• C. K/W
• D. m^2 K/W

Answer: (D)

R-value measures how well a material resists heat flow. It comes from the relationship between temperature difference, area, and heat transfer: R = ΔT / q'', where ΔT is the temperature difference and q'' is the heat flux per unit area (W/m^2). If you express q'' as the total heat flow Q through an area A, then q'' = Q/A and R = (ΔT × A) / Q. The units then are (K × m^2) / W, which is written as m^2 K/W. That is the SI unit for R-value. A common non-SI unit used in practice is ft^2·°F·h/BTU, but the SI unit remains m^2 K/W. W/mK is a measure of thermal conductivity (k), not resistance, and K/W is not the standard unit for R-value (it corresponds more to a conductance-like form not used for insulation rating). Also, the reciprocal of R (per area) is the U-value, with units W/(m^2 K).

R-value measures how well a material resists heat flow. It comes from the relationship between temperature difference, area, and heat transfer: R = ΔT / q'', where ΔT is the temperature difference and q'' is the heat flux per unit area (W/m^2). If you express q'' as the total heat flow Q through an area A, then q'' = Q/A and R = (ΔT × A) / Q. The units then are (K × m^2) / W, which is written as m^2 K/W. That is the SI unit for R-value.

A common non-SI unit used in practice is ft^2·°F·h/BTU, but the SI unit remains m^2 K/W. W/mK is a measure of thermal conductivity (k), not resistance, and K/W is not the standard unit for R-value (it corresponds more to a conductance-like form not used for insulation rating). Also, the reciprocal of R (per area) is the U-value, with units W/(m^2 K).