  Light scalar fields play a variety of roles in modern physics, especially in
cosmology and modified theories of gravity. For this reason, there is a zoo of
experiments actively trying to find evidence for many scalar field models that
have been proposed in theoretical considerations. Among those are setups in
which the pressures expected to be induced by light scalar fields between two
parallel plates are studied, for example, Casimir force experiments. While it
is known that classical and quantum pressures caused by light scalar fields
could have significant impacts on such experiments, in this article, we show
that this can also be the case for thermal pressure. More specifically, we
derive expressions for the quantum and thermal pressures induced by exchanges
of light scalar field fluctuations between two thin parallel plates. As
particular examples, we then look at screened scalar fields. For chameleon,
symmetron and environment-dependent dilaton models, we find large regions in
their parameter spaces that allow for thermal pressures to equal or exceed the
quantum pressures. By comparing with earlier constraints from quantum pressure
calculations, we conclude that thermal pressures induced by chameleons are
actually of experimental significance.
