  This paper reports on the methods and results of a theoretical analysis to
design an insulator which must provide a thermally quiet environment to test on
ground delicate temperature sensors and associated electronics. These will fly
on board ESA's LISA PathFinder (LPF) mission as part of the thermal diagnostics
subsystem of the LISA Test-flight Package (LTP). We evaluate the heat transfer
function (in frequency domain) of a central body of good thermal conductivity
surrounded by a layer of a very poorly conducting substrate. This is applied to
assess the materials and dimensions necessary to meet temperature stability
requirements in the metal core, where sensors will be implanted for test. The
analysis is extended to evaluate the losses caused by heat leakage through
connecting wires, linking the sensors with the electronics in a box outside the
insulator. The results indicate that, in spite of the very demanding stability
conditions, a sphere of outer diameter of the order one metre is sufficient.
