I have a small shed, 8x7, near the house that I use as a pump/filter house for a koi pond. This building has a 7 foot deep basement made of cinder block which houses an open top settling tank, so humidity will be high. The basement and main floor have a grate between them. I was planning on having an air intake near the ceiling on the main floor and an inline fan in the basement to vent outside. Is this backwards? Do I need the intake coming into the basement and the fan exhuasting from the mainfloor? This building has R-19 insulated walls and R-30 ceiling.
Ventilation for summer only makes it easy. In at the bottom, out at the top - or else close off the grate (use thick glass if you need to see through there) and run an in and an out on the bottom.
A "hula hoop" (not really, I don't think they are that big - but same thing made from ~12'8" of heavy black poly well pipe and a coupling) plus a sheet of plastic would be one lightweight, non-rotting approach to a cover. If you handled it with care, you might be able to use the shrink-wrap plastic "winterizing film" for patio doors and have it be quite clear, if looking through the grate into the tank is important for monitoring. It will probably still get water droplets condensing on the bottom of it, so it might not be all that see-through.
Unless I am missing something, this seems a perfect application for an air-to-air heat exchanger. Fully insulate the building, seal it up, and install the heat exchanger in a convenient location. A blower through it could be controlled by a humidistat.
If winters are very cold, some supplemental heat might be needed inside the building unless there is quite a large mass of water.
A reasonably effective plate heat exchanger could be built by anyone with basic sheet metal experience.