attic roof fan and insulation for flat-roofed rowhouse
Earlier I posted a question about my CAC being unable to cool the third floor of my row house, and it was suggested that I may need to better insulate the attic. So, I've been looking into that. I've learned more about the situation in my house now and it has been further suggested that I consider an attic roof fan. I'd really appreciate some feedback/advice from you folks on this site, as I'm a novice on this.
Here's the situation: Largish, old rowhouse, with a flat (actually just very slightly pitched) roof, that seems to be some sort of shingles/tar. LIttle or no access to what attic there is...the only opening to the attic, in the ceiling of a closet, is blocked by the CAC unit for the third floor (it's an all-inside, high velocity type system).
One insulation guy who says he's worked on these old rowhouses for 20 years, says insulation may help somewhat, but that it will be fairly costly and involve creating two access points in the ceilings. And, he says, the real problem is that there's no way for the hot air in the attic to get out.
Can anyone give me an idea about whether putting a fan on an old flat roof is a good idea, and if so, what kind? A turbine fan? I also found one called an aura fan, which is like a turbine, but no moving parts. And is this likely to be worth the expense and risk of roof leaks? And should I still go ahead with the insulation?
Any other advice?