metal roofing cost & insulation questions

I'm familiar with the feature comparisons between standing seam metal panel and conventional asphalt shingle roofing, but I have a few questions I haven't been able to find answers to.
1. What are the differences (if any) in terms of airflow and insulation required below the metal roof surface (below the decking)? Does the decking get enough airflow from under the metal roof, or does a metal roof still require airflow under the plywood as well? This would be over a finished attic area, so I'd have minimal area for venting and I'm not sure if or how this would affect my need to powervent or ridgevent to have airflow. Ideally I'd love to use that expanding spray insulation to insulate the attic roof space, even if I need to use those vent spacer boards to leave enough airflow on the underside of the decking.
2. It has been a while since I've done price comparisons; what is the general installed price per square (simple roof) as compared to the price per square of asphalt? My old asphalt roof needs to be removed in either case, so it would be for an install on just the decking. I'm not looking for a to-the-dollar accurate answer, just a general estimate of how much more I might pay per square if I went with something like galvalume (I doubt I could afford copper).
Thanks for the info, Keith
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i'll take a shot at your first question. generally, codes say you still need ventilation below the sheathing. this is because if moisture finds it way into the rafter cavity, it can condense on the underside of the sheathing. ideally, a vaulted ceiling (at least in minnesota where i live) would be deep enough for a 2" airspace and 12" of insulation. the venting requirements are based on the square footage of floor area. i don't remember them exactly, but someone who knows my well post here.
however, another option might be a hot roof---that is spray foam (icynene) with no airspace. you will need to talk to your building department to see if they will let you do this...we have had some battles about this issue with our building department.
some people (myself included) feel like venting roofs is a waste of time and money in terms of heat flowing out of your ridge vent. if i was building my own house in a non-inspected area, i wouldn't hesitate to skip the venting, fill the rafter cavities with insulation, and spend my time on an excellent vapor barrier. after all, we don't ventilate walls. building codes don't agree with this view, so we are still venting where i live.
as to your second question, i'm somewhat out of the loop on these prices. and they are going to very widely from area to area. but suffice it to say that a standing seam metal roof is going to be much, much more expensive than shingles. double the costmaybe? if you add to that getting a contractor to blow in icynene, you are looking at a pretty pricey little roof. nice if you have the money, but not cheap.
good luck.
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Keith wrote:

had a gerard metal roof replace a asphalt one about 3 years ago. they put metal type ice guard on the entire roof. the attic kept its normal air flow. talked to the roofer about 2 months ago. i paid 1/3 more for gerard metal than 40 year arch asphalt shingles. however the price of metal has gone up since about 3 years ago. today the price would be almost 3 times what i paid. hope this helps
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