Even though there's not near the same amount of heat that a hid produces, proper ventillation/replenishment is still needed.. just not at jet turbine speed. Humidity rising is probably due to less heat.. vapour being transpired by the plants is not being evaporated like it would be under a hid. I've heard led grows smell more.. I wonder if this is why. Similar to what a garden smells like in the middle of a night period, as compared to the warmer day period.
I've heard that using those "tidy cat" pellets in a carbon filter/cannister, not only cleans the odours, but also sucks up some of the humidity. Worth looking into.