This is why we don't seal our directional lights - they can breath through the knuckle and stem, or shroud assembly. This allows the expanding hot humid air to escape when the fixture is lit, so when the fixture cools, there is less moisture inside the fixture - hence, less condensation to corrode internal parts.
A fixture that is completely sealed (moisture trapped in the fixture) goes through a continuous evaporation/condensation cycle that results is far more corrosion. In fact, standard laboratory tests for corrosion put test samples through this exact same scenario - repeated cycles of heating and cooling (in humid air)to accellerate the effects of condensation corrosion.
Our early experiments with completely sealed fixtures, and observations of corrosion in sealed fixtures confirmed this for us.
Communication and Marketing Director
CAST Lighting LLC.