I think the humidity is less important than the dew point reading, which is the temperature at which water condenses from the air - as long at the temperature inside the building is higher than the dew point, you shouldn't have problems with condensation.
I have experimented with my home garage with this same concept in mind, condensation is the thing to try and avoid, not necessarily the absolute humidity level.
I have an uninsulated wood 2.5 car garage and was concerned about rusting of stored items, I don't usually work there just storage.
In MI during the winter we get a lot of temperature swings which causes daily condensation when the outside temperature crosses the dew point.
My initial idea was to install a panel infrared heater with a fan to circulate the air. The idea was to run the heater and fan as little as possible to keep the operating cost down and provide just enough heat to keep it 5-10 degrees warmer than outside, so it would not condense in the garage while the temp dropped to the dew point outside. Once the temp drops below the current dew point, usually not much more condensation occurs until the next cycle.
At that time the garage had no vapor barrier under the slab, and had a cupalow in the roof that was half way closed off. For the automatic temperature and ventilation control, I built a little home-brew controller from parts I had in my collection. It has some switches for automatic/manual operation. A switch to disable the heater and just run the fan. A thermostat to sense inside temps and a timer to cycle the system in automatic mode.
The thermostat was set to 45 deg F, anything below that the heat would be cycled on with the timer. The timer I used was for a lawn sprinkler, it had many more programmable setpoints than a normal 24 hour timer. The fan and heater ran at the same time, controlled by the timer. The timer was programmed to run the system 10-15 minutes every hour when the temp was below 45 deg. Again the idea was to keep the inside temp a little higher than outside, and simultaneously run the fan to keep the air from stratifying. My thoughts were that any existing moisture would be picked up by the heat and carried out through the partially open cupalow vent, leaving dry air below.
The thing worked as intended, but produced disastrous results. I worked in the garage a little that first winter, and put the controls in manual mode to jack up the heat while working for a day or two. The heat was working great, but it didn't take long for a major problem to appear. The heat wicked a tremendous amount of moisture through the raw concrete floor, even though it was below freezing outside. The water soon began to puddle in the floor. It felt great, it was like working in the summer, heat and humidity, usually heated buildings are very dry during the winter.
At the time I didn't think much about the moisture coming through the slab. I figured that it was due to me jacking up the temperature temporarily and would cease once the temp was turned down again for normal storage operation.
I couldn't have been more wrong. Within a week the fan had blown that wicked moisture straight through the wood siding and removed six layers of paint in sheets. The fan was mounted to blow the air horizontally across the top of the panel heater to prevent the wood roof from becoming too hot. The wall opposite of the fan was almost completely stripped of 60 years of paint. The others weren't too far behind. The garage became so raw that I had to cover it with Tyvek building paper to shield it from mother nature for the rest of the winter.
My experiment to stop condensation, created condensation, and like most human discoveries (accident) turned into the best wood siding paint stripper that you could find, removed all layers from 1940 without a scratch. All is not lost though, continue reading, you can learn lots from your mistakes.
The next year I replaced the garage floor along with the driveway. I installed a heavy vapor barrier over compacted sand. Along with 2" foam insulation all around the perimeter foundation vertically, and horizontally for two feet under the 6" slab, to make a thermal break from the ground. Before I painted again I wanted to do some more testing, to check the condensation during the winter. The vapor barrier and insulation solved the wicking problem, no more water on the floor, and no more paint was coming loose on the other three walls.
The next season I finished stripping the remaining paint with a steam cleaner to get it all down to the raw wood siding. Primed with oil based primer and top coated with Sherwin Williams Duration product.
I then decided to test for condensation caused rust without running the heat, just the ventilation fan on the timer described earlier. For my test I took a large 8' long log from my freshly cut maple tree (green) and placed it horizontally in the floor at the rear of the garage. On top of the wet log I placed some steel fabricated plates I had made for another project. These plates had some mill finish, some welding, and some raw ground sections. I'm happy to report after about 5 years of storage without any heat the plates were rust free, even the raw sections. And the log was fully air dried, ready for milling into a project. I put the dry log away in the corner for future use and kept the plates in the same area on a steel cart to monitor their rusting into the future.
It's been many years now (15) and those plates finally started to develop some minor surface rusting on the raw areas, the mill finish is still clean. From my observations nothing else stored has corroded in the following years, there is a unused car, tractor, tools,and yard implements. I think that the biggest moisture source now is from my 48" snowblower mounted on the tractor. It comes back in with lots of snow stuck inside of it and only begins to melt when the outside temp goes above freezing.
In closing, my observation is this. I think that the combination of slightly heating the building with the sun (darker color roof, brick red paint) and keeping the air from stratifying (stagnating) and continuously venting the warmest (humid) air through the roof cupalow, gets rid of the most water soaked air before it can condense and rust my items. I had to eliminate the ground moisture first. The fan I run to destratify/ventilate is about 1/15 HP pulled from a large old window A/C unit. It still runs about 15 minutes every hour year round. And the operating cost is so low that I don't really notice anything extra on the utility bill. By the way I still occasionally will work in the space and use the heater with no problems. My paint job is still looking mint after it's first fifteen years of service.
That's what works best for me in SE MI, your mileage may vary.
