Re: SB: Venting cathedral ceiling/roof insulated with cellulose?

[nrk <alucinante@web.de>]

Hi Tristan, 
Iīm not sure I understood everything right, but I just try and tell how
it could be right:
Your inside sheathing which is sheetrock has to have a higher moisture
resistance than you outer sheathing, which I donīt know what it is, but
there is hardly anything with a lower resistance than sheetrock. So you
need something on the inside to raise the resistance, I trust in
building papers, others in plastics, some even in special paints.
But the second thing you need on the inside is airtightness. While there
is sufficient reason to consider correctly installed cellulose
"windtight"(although german codes donīt recognize it) it is by no means
"airtight". So here Iīm with my building paper again, but in
Northamerica people seem to do something called "Airtight Drywall", what
should be easily found with a websearch.
And as metal roofing is absolutely vapor impermeable, you DEFINITELY
need to vent the space underneath it. Even if you chose to ignore all
else, thatīs the one thing you should really do, to the best of my
knowledge. Even if you consider your envelope vapor tight, metal roofing
has condensation underneath. 
This is neither a contradiction to a "breathing" building envelope nor
to plain legal building physics.

Ok, now Iīll check the bread dough...

Later, Alex

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