Insulated Pipe Heat Loss (Radial)
Radial (cylindrical) conduction heat loss per linear foot and total through pipe insulation via the log-mean form, from outer diameter, insulation thickness and k-value, and the temperature difference.
Formula and source
Q/L = 2*pi*k'*(T_hot - T_amb) / ln(r2/r1), where r1 = OD/2, r2 = r1 + thickness, and k' = k_value / 12 converts BTU-in/(hr.ft2.F) to BTU/(hr.ft.F).
Fourier conduction through a cylindrical shell (public heat-transfer formula); insulation k-values per ASHRAE Fundamentals / ASTM C335, by name.
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