How did it go with the project?
I have never worked with arduino, but I am just about to order the pieces for a similar project.
My problem is that ventilation in the summer brings hot humid air into the colder, poorly isolated basement, which means that the water condenses on the walls and increases the humidity with all colourfull kinds of fungi as a bonus. I could of course invest in a dehumidifier, but that uses a lot of power.
It sounds like you have the same aim: only ventilate, when it would decrease the humidity. However, since most sensors (like the DHT11) measures the relative humidity, I think you will also need to take the temperature into account (which DHT11 also measures). The relative humidity gives the amount of water in the air compared to saturation at the given temperature, but for instance at 21 degree C. the air can hold roughly twice the amount of water as at 10 degree C. I would therefore compare the actual partial pressure of water vapor inside and outside to determine when to ventilate. This can be calculated from the relative humidity and the temperature. In theory this is also affected by the air pressure, but this should be of minor importance. You can read more about how to do the calculations here: http://www.engineeringtoolbox.com/relative-humidity-air-d_687.html