"This might go against your definition of "built correctly" but there
are some cathedral ceilings which are built where you see the decking
(which is usually some "nicer" wood) of the roof. Sucks for
insulation mind you. (you can only insulate tween the roof decking and
shingles) My friend has a "great hall" built like this. I think this
is what George might be picturing. I believe the OP mentioned
drywall, so unless the ceiling beams are exposed, and the drywall is
directly attached to the sheathing between them, there is likely a gap
like you describe. (perhaps not insulated/vented though) "
Yes, you could have exposed rafters, but like you said, I think the OP
has a finished sheetrock ceiling, which is the most common. There may
not be an air gap channel though. With vent baffles there will be, but
without, if you put too much insulation in, ie fill it all up anywhere,
then the air is blocked.
I plan on opening up my attic space ceiling, so I want to understand
your reasoning. From roof towards the inside:
sheathing, 1.5" air gap made by vapor barrier, insulation, insulation's
vapor barrier, and sheetrock?
"I'm a little confused about the double vapor barrier.
I thought the VB went on the "hot side". In this case it seems like
you are considering both to be the hot side. ;) Perhaps the plastic
sheet isn't meant as a VB, but rather just a vent channel? "
Exactly, they are vent baffles/channels, etc. Basicly a plastic chute,
designed to keep an open channel for air to flow, not to insulate or be
a vapor barrier. Available at home centers, building supply, etc.
They should also be used in conventional attics for the couple of feet
at the lower end of the rafters. There they keep that area open and
free of insulation. Without them, what typically happens is the
monkeys installing attic insulation push it all the way into the end,
blocking the soffit vents. Had that exact problem in my house.
"My plan was going to be sheathing, preformed "C" vents, then spray
foam, then sheetrock. The "c" vents would meet at the peak where I
would pump the heated air to a location where it helps me most. (heat
exchanger if interior calling for heat, outside at other times). The
spray foam is a VB of it's own... so perhaps i could use the sheeting
as you suggest to be a less expensive alternative to the preformed "C"
I don't know if you'd get enough benefit from this to make it
worthwhile. In the coldest weather, the air would probably be colder
than the inside air. It might be of some value on moderately cool
days, but then you have the issue of controlling it, ie opening
it/closing it depending on whether the air coming out is going to help
or make it worse. If it's not automatic, then what? Likely you will
lose more heat by leaving it set wrong, etc.
I've also considered a "hot" roof (insulation all the way to the
sheathing), but havn't been able to find enough info on it to
determine appropriateness for my climate (Pittsburgh PA, USA).