Are you saying that there's NO ventilation in the attic??! I mean, with a metal roof, you DEFINITLEY need it. There should be AT LEAST soffit and gable venting. The BEST scenario is soffit and ridge venting.
Installing the vapor barrier could be your PROBLEM. The barrier is suppose to be the 'layer' between the attic floor joists and the drywall (from the rooms below) that are screwed to the joists. Now this is what's done IF it's done. Usually, the attic isn't 'barriered' from the rooms below. BUT, if it's done, it should be installed where I mentioned. (Hopefully, there's a vapor barrier on the walls of the rest of the main house ...)
So, where does this leave you? Well, I'd consider removing the barrier. Go ahead w/the blown insulation making sure that you DON'T compress it! If you do, it will lose it's insulation R-30 'value'. (If you want to store stuff up there, consider building an additional flooring system that's OVER the insulation. It's OK if the 'runners' of the added floor rest on the joists and compress a little of the insulation. You can build them in 4 x 8 'sections' so you can move them if you have to get access to the ceiling from the attic. You'd lay these 'sections' PERPENDICULAR to the attic floor joists. WHEW!!!) Now, talk to the Builder about ventilation. Better yet, have him hold off and YOU call the MFGR. of the metal roof, and find out what 'ventilation recommendations' they have for you particular roofing situation. This is the BEST that should be done.
You see, by 'trapping' the condensation w/a vapor barrier, you just promote mold, mildew, and rot. By ventilating the attic, in a sense you allow the moisture a way out of the house. You also need to find the SOURCE of all the moisture. You say you're in the process of building the home, yet, you're talking about moisture problems. I mean, what's the EXACT status of the house? What's done and what's yet to be done?
My best to ya and hope this helps.
Jay J -Moderator
PS: I wouldn't put a dehumidifier up in the attic. Where would you run the drain line??? I hope you're not thinking of emptying the 'pail' every day!!!