BTU per hour-foot-Fahrenheit
Thermal conductivityexact by definition
The BTU per hour per foot per Fahrenheit degree is the customary US unit of thermal conductivity, equal to 1.730735 W/(m·K). The relation is exact by definition, following from the International Table BTU, the international foot of exactly 0.3048 m, the hour and the Fahrenheit degree of 5/9 K, though the decimal shown is a rounding of that exact ratio.
Watch out: US insulation literature more often quotes conductivity per INCH of thickness, in BTU·in/(h·ft²·°F), which is twelve times this unit. That per-inch form is the reciprocal of R-value per inch: fibreglass at R-3.2 per inch has a conductivity of 1/3.2 = 0.31 BTU·in/(h·ft²·°F), which is 0.026 in this unit and 0.045 W/(m·K). Check the exponent on the foot before converting.
| 1 BTU/(h·ft·°F) | 1.7307347 W/(m·K) |