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

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.

Keith wrote:
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
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