There is no easy or cheap answer but I'll give some choices you can pick from. Houses are being built to be extra tight on the outside (tyvek, foil, foam, etc.) but nothing much new has been done over the years from the ceiling and up so your house is like a chimney with everything going up. Get the builder back to help you fix some of the moisture problems before you are replacing insulation, drywall, and rotting trusses. Moisture gets in under paint and popcorn on the ceiling and it will come loose and fall like rain. Like the other poster said, I have also seen it run down inside the wall cavities and into windows and below. Go buy some humidity guages from walmart or somewhere and put one on each floor and the attic and one outside and monitor them and right down the humidity levels over a period of time that you have the problem.
I would never seal off the attic vents as you will end up with more moisture up there and under your roofing and sheeting under the roofing even when heated except that the heating changes the dew point tempature and hopefully it won't get to that.
Do you have bathroom fans with moisture laden air vented into the attic instead of outside with insulated ducting like they are supposed to?
Do you have fireplaces or unvented gas fireplaces which really create a lot of humidity?
Are use using a furnace or boiler using gas or oil from which the combustion therefrom creates a lot of humidity.
Sealing off the ceiling so moisture doesn't travel through the ceiling by having a moisture barrier and sealing it and caulking around any ceiling electrical boxes and vents, etc. What type of insulation? blown in or kraft paper faced fiberglass batts. With blown in and even with paper faced batts, if you have a moisture problem, you may want to move the insulation over part at a time and lay down sheets of 6 mil plastic on a roll. Form it over any joists and down in the cavities and refill with insulation while having plenty of overlap and sealing of the plastic at edges and laps.
You have a new tight house and opening some windows to let the air equalize and get rid of the moisture before the attic would help.
Install some dehumidifiers in the house or run the whole house dehumidifier (which is the a/c system).
Install a whole house ventilator system (energy recovery ventilator) into your furnace to mix in some outside air and get rid of the moisture before it gets to the attic.
Install a power exhaust fan in an attic vent run off of a humidstat exhaust the attic.
Install a whole house exhaust fan in the ceiling run off of a humidstat to move humid air into the attic quicker and when combined with the powered roof vent to get it out of the attic.
People also create humidity so tell (if you have any) your deadbeat relatives and/or brother in law and family to move out if they are there. lol
This is a weird winter weather year around the country with the moisture and humidity patterns. I would shoot for sealing off the ceiling, a heat recovery ventilator system added to your furnace for the long run. Ceiling and attic exhaust fans are next and dehumidifiers where you need them.
If I left out any other options or am wrong on something then someone else can add their 2 cents worth.