We had the house built in 1985. Plastic piping, 4 baths, two roof vents. Occassionally in the base, usually in the winter, we have a very smell basement, usually associate with outdoor drops in temperature, Illinois. Basement is finished, pipes are not accessiable. Any ideas? Two baths on second floor, one main floor, one in basement. Septic tank is eight feet underground, gravity feed below basement floor, field is down the hill from the tank of course.