Today's building practices encourage "tight" building envelopes. This occurred because of rising energy prices. But as with anything else, changing the way we enclose our homes also brought about a whole new batch of issues, mostly concerning movement of moisture.
I have read many, many articles about moisture and air issues in buildings. It sounds to me like warm, moist (at least comparatively) air is escaping your living space and going up into the attic. Moisture always seeks the coolest area, hence condensation on window glass. In this case the insulation in your attic is weakest at the outer walls, because there isn't enough vertical space to accumulate insulation to make an effective R-value.
It sounds to me like the builder is trying to shine you on. If he truly cares about his reputation, he'll at least try to solve the problem for you.
And these posters are correct - 20% RH in winter is extremely low. Do you have a lot of static electricity?