  We propose a new method to compute glueball masses in finite temperature
Lattice Gauge Theories which at low temperature is fully compatible with the
known zero temperature results and as the temperature increases leads to a
glueball spectrum which vanishes at the deconfinement transition. We show that
this definition is consistent with the Isgur-Paton model and with the expected
contribution of the glueball spectrum to various thermodynamic quantities at
finite temperature. We test our proposal with a set of high precision numerical
simulations in the 3d gauge Ising model and find a good agreement with our
predictions.
