What is the critical radius of insulation?

Yeah, this is a bit of a weird one, mate.

You'd reckon adding insulation always stops heat loss, yeah? But with thin pipes or wires, adding a little bit of insulation can actually make it lose more heat.

It happens because you give the pipe a bigger surface area. This helps the heat escape into the air, which is called convection. The critical radius is the point where this effect is biggest and you get the most heat loss.

There's a formula for it, it's k divided by h. 'k' is for the insulation and 'h' is for the air. So if your pipe is smaller than this radius, you need to add heaps of insulation to get past that point. Otherwise, you're actually helping the heat escape.