It's more effort, but here's what you might do:
OD = new OD
OD* = OD as input into CAESAR
WT = new WT
WT* = WT as input into CAESAR
ICA = internal CA
ECA = external CA
INS = Insulation thickness

CA = ICA
OD* = OD* - ECA
WT* = WT* - ECA
INS = ECA

Insulation density = metal density.

Note: You will want to model bends and tees the "normal" way and this way and compare calculated stiffnesses and SIFs, to ensure this approach makes sense.