How to use this calculator
Enter the insulation conductivity, the outside convection coefficient, the bare radius and the insulation thickness. The chart shows the heat loss as the insulation gets thicker.
Formulas
Cylinderrcr = k/h
Spherercr = 2k/h
Heat lossQ = ΔT/[ln(r₂/r₁)/(2πkL) + 1/(h2πr₂L)]
Worked example
3 mm electric wire (r₁ = 1.5 mm) with 2 mm of plastic insulation, k = 0.17 W/m·K, h = 12 W/m²·K (the default)
- rcr = 0.17/12 = 14.2 mm, much larger than the wire.
- Adding insulation up to 14.2 mm raises the heat loss, so the insulated wire runs cooler.
- Here the 2 mm layer raises the loss by about 93%.
Frequently asked questions
Does insulation always reduce heat loss?
No. Below the critical radius, the extra outer surface area outweighs the added conduction resistance. For most pipes r₁ is already larger than k/h, so insulation helps.