The attic ventilation I have is fine. The problem is in the winter months. If I run the bathroom fan and exhaust it into the attic. I get frost on the underside of the roof. I have also heard of people that vented a bathroom fan straight up and out the roof only to have the moist air freeze on the outlet chimney. It then warms up, melts and runs back down through the fan onto the bathroom floor. I have seen newer houses vent the bathrooms through the side of the house, the fans probably are the models that the hose hooks onto the side of fan and not the top though.