Nick Mueller wrote:

Pete C. wrote:

Again with the steel container, EPS insulation and FRP paneling you have
a triple layer vapor barrier. Moisture outside stays outside, moisture
inside stays inside (normal vents remove it).

You didn't understand what ventilation does, or you suppose that there is no
need for ventilation.

No, I understand what ventilation does just fine and I've indicated the
OP should be installing proper vents.

You don't seem to understand the difference between conventional frame
house construction where pretty much all the wall construction materials
are moisture permeable and an additional non permeable vapor barrier
(usually plastic sheeting) is used to prevent interior moisture from
humans, showers, kitchens, etc. from getting passing into the wall
structure and saturating the fiberglass insulation. The permeability of
the materials on the other side of the vapor barrier allows the moisture
level on that side to settle to the average humidity of the exterior
environment.

The steel of the container, extruded polystyrene insulation and EPS wall
paneling are all non permeable - all are vapor barriers. There will be
absolutely no moisture migration through these materials. The humidity
level in the small spaces formed by the corrugated steel side and the
EPS insulation will stay constant since they are sealed cavities
(subject to reasonable care with the adhesive when installing).

During the summer when you have A/C running in the 1,280 cubic foot
shop, the likelihood if the interior humidity level and temperature, and
wall surface temperature being just right for moisture to condense on
the walls is very, very low. The point where the temps might be at dew
point will be somewhere within the non permeable wall structure where it
will have no effect at all due to the inability of moisture to reach
that point since all the wall materials are vapor barriers.

The OP's container a.k.a. steel building lacks most of the moisture
sources found in a residence (no shower or kitchen), lacks the permeable
wall materials found in a residence, and lacks the materials such as
fiberglass insulation that could be damaged by moisture passing through
permeable wall materials. Installing some sort of plastic sheeting vapor
barrier over the steel wall and EPS insulation would be positively
idiotic since it would accomplish nothing putting it over two other
vapor barriers, and under FRP paneling, a third vapor barrier.