OK this one has been bugging me, I'm pretty sure it's wrong but couldn't quite see why.While my thermodynamics is pretty rusty I think I've got it sorted now but I stand to be corrected.

The Energy flow by conduction per second is directly proportional to the area and the temperature gradient i.e.

2 pi.r l (T1-T2)/r where r is the lagging thickness, l the length of pipe and T1 and T2 the inside and outside temperature respectively.

so the energy flow per second of the lagged pipe *by conduction* is not dependant on the thickness of the lagging.

However the temperature at the surface of the lagging is much reduced due to the larger surface area, and the energy lost by radiation is directly proportional to the surface area (which is directly proportional to the thickness of the lagging) but proportional to the fourth power (ie the square of the square) of the temperature.

As the energy lost by a pipe is mostly by radiation it is highly dependant on the surface temperature and that is why thicker is better.

I'm grateful to this website for aiding my failing memory

http://online.cctt.org/physicslab/content/PhyAPB/lessonnotes/thermodynamics/lessonthermodynamics.asp