Re: New steel roof, has condensation on it...
There should be a continuous air gap from the eave to the ridge between the roofing and the insulation -- the insulation should not be in contact with the roofing. This will allow air to flow and remove moisture.
Normally, rather than tar paper, a heavy plastic sheeting product called 'scrim' is used. The scrim protects the metal roofing from any moisture that does condense on the underside. Tar paper isn't recommended because it's not fully supported by full plywood sheathing; over time it will become brittle and fail when it's only supported by the purlins.
Since it's rather difficult to install the scrim after the fact, you can remove the existing insulation and have expanding closed-cell spray foam applied on the underside of the roofing to 1" thickness. This will provide the necessary protection to the roofing. Then reinstall your insulation. An advantage is that you won't need to provide any air space or vents at the eaves and ridge, though you will want to provide a bathroom-style exhaust fan on a timer (or a heat recovery air exchanger on your furnace) to provide periodic air exchange to get rid of the moisture that will inevitably build up inside the house.
Of course, this is all speculation based solely on your post. There may be more relevant information that renders my recommendations invalid.
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.