Lampost, IMO bringing in outside air just to "dehumidify" your grow is probably a bad idea unless you have state-of-the-art filtration.

55% RH ain't that bad. And it's relative. Hot air can hold a lot more water than cooler air. So you need to look at your RH both lights-on, and lights-off--at your temperature extremes.

This "dry" air you're considering bringing in from outside: what's its temperature? If it's warm enough, it could actually be holding more water than cooler air whose relative humidity is higher. (The cooler air is nearer its "dew point.")

When you're talking about relative humidity, temperature is kinda what it's "relative" to. Gotta consider temp and RH together.

For air circulation, I like a few $5 computer fans in amongst the plants. Run them with cheap wall warts or a DC fan controller ($20-40) to just keep the air moving across / around the leaf surfaces to facilitate O2 / CO2 exchange. At some point in trichome development you may want to stop blowing the leaves so hard that they grind each other's trichs off. :thumbsup: