Very low humidity levels in new construction home - over ventilation?
If you have an energy saving, very tight home, then why would you run an exhaust fan of any size 24/7?  Bathroom fans should only be run when the occupants are showering or doing the other thing that requires ventilation. I would allow the fans to run when the bathroom is occupied either by the use of a timer, or by a motion detector. If you need that much ventilation just open a window! There are energy recovery systems that move air in and out of the house and maintain a balanced pressure in the home. They have a type of heat exchange mechanism,  to transfer heat between the outgoing and incoming air to make the units "more green" or energy friendly. Oh, by the way, contrary to some peoples opinion, forced air heating systems or any heating systems do not "dry out" the humidity in any home. The humidity is relative to the temperature,  raising the air temperature lowers the relative humidity and visa/versa. For your house, you will have to add  1 or maybe 2 whole house humidifiers to the heating systems. Raising the homes humidity will yield a more comfortable feeling at a lower temperature.