maybe you could use 2 13mm thick insulations (two pipeinsulations, one over the other)? could do the trick if 19 isn't enough. But the required insulation thickness is possible to calc on, 125-150K is a very big difference in temp and require decent insulation, I always use 13mm unsulation on -50> units since it's enough.

nice drawings for case you got, what program do you use for drawings? autocad?