Pipe Insulation

Insulation is a resistance in series with the pipe wall and the outside air film. Two results usually matter more than the heat loss itself: the surface temperature that decides whether the pipe is a burn hazard, and the critical radius — below which adding insulation to small tubing increases heat loss, because the extra surface area outruns the extra resistance.

Bare pipe loss
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Insulated loss
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Reduction
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Insulation surface temp
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Bare surface temp
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Critical radius
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Still-air natural convection is around 10 W/m²·K; outdoors in wind it can exceed 25, which cuts the surface temperature but raises the loss. Bare-metal surfaces above roughly 60 °C will burn on contact — EN ISO 13732-1 gives the time-to-injury thresholds by material. Radiation is not included here, so outdoor and high-temperature cases lose more than shown.

Design-stage estimates. A real project needs a full load calculation to the ASHRAE/CIBSE procedure, and ventilation rates are code-mandated — check the standard and jurisdiction that apply to your building. A helper, not a substitute for engineering judgment.