  A finite volume allows tunnelling between degenerate vacua in Quantum Field
Theory, and leads to remarkable energetic features, arising from the
competition of different saddle points in the partition function. We describe
this competition for finite temperature at equilibrium, taking into account
both static and (Euclidean) time-dependent saddle points. The effective theory
for the homogeneous order parameter yields a non-extensive vacuum energy at low
temperatures, implying a dynamical violation of the Null Energy Condition.
