BTU per hour square foot Fahrenheit
Heat transfer coefficientexact by definition
The BTU per hour per square foot per Fahrenheit degree is the customary US unit of the heat transfer coefficient, and it is the U-value that appears on every North American window label. One of these equals 5.678263 W/(m²·K), a relation exact by definition from the International Table BTU, the international foot and the 5/9 K Fahrenheit degree, with the decimal shown being a rounding of that exact ratio. It is the exact reciprocal of the imperial R-value.
Watch out: This unit is the reciprocal of R-value, so lower is better here while higher is better there. A U-0.30 window is an R-3.3 window. Also note that a window's U-factor is a whole-assembly number including frame and edge effects, which is why a triple-glazed unit with a superb centre-of-glass number can still test at U-0.25 overall.
| 1 BTU/(h·ft²·°F) | 5.6782633 W/(m²·K) |
About the btu per hour square foot fahrenheit
This is the U-factor of North American fenestration, and the number that most building codes regulate directly. It is defined by \(Q = U A \Delta T\): a window of area \(A\) square feet with \(\Delta T\) Fahrenheit degrees across it loses \(Q\) BTU per hour. A 20 ft² window at U-0.30 with 60 F° across it loses \(0.30 \times 20 \times 60 = 360\) BTU/h, about 105 watts, or roughly the same as leaving a bright old-fashioned light bulb burning all winter.
The conversion factor to SI is 5.678, the same number that connects imperial R to RSI, because these are reciprocal quantities and the factor simply flips. So U-0.30 BTU/(h·ft²·°F) is 1.70 W/(m²·K), and the good European triple glazing quoted at 0.8 W/(m²·K) is U-0.14 in North American terms. That comparison is worth doing before anyone claims a window is world-class: the two labelling systems differ by nearly a factor of six, and a specification quoting "U 1.4" is metric and describes a decent double-glazed unit, not a catastrophically leaky one.
The other half of the unit's life is in heat exchanger work, where it appears as the overall coefficient in \(Q = UA \cdot LMTD\). Shell-and-tube water-to-water exchangers run 150 to 300 BTU/(h·ft²·°F) clean, and the fouling allowance that designers add is a resistance subtracted from that, which is why a scaled exchanger loses capacity long before it loses flow.