Not sure about a "ruck fan" but it needs to be the type of fan where it creates a vacuum by pulling air through it rather than just a propellor that makes wind. A lot of people (myself included) get taken by the claims of the "duct blowers" and think they will work for ventilation but they don't work because they just create wind rather than pulling a vaccuum on the system.
That is why the centrifuge type fans work so great. Air can't get around them and that causes them to build vaccuum. You don't want or need a fan on the input side of your ventilation because of the nature of the vaccuum effect of the exaust fan. Having the vaccuum effect, you create negative pressure on your grow space which keeps air coming in and allows you to control where smells go.
If you are moving your air fast enough(replacing the air in the grow space more than 1 time a minute, then higher humidity will be less of a problem unless the air that you are pulling in from outside is very humid. If that is the case then you may need to use a cooling/dehumidifying unit. I have a mobile air conditioner that cools and dehumidifies the air in my building before it goes into the grow spaces.
However, it sounds like your problem is that your ventilation system isn't replacing the air in the grow fast or often enough. How big is your space? what is your fan rated to move in cubic feet per minute or cubic meters per minute? What is the air like that is being pulled into the grow space?
