[Greenbuilding] mounting rigid insulation above sloped framed un-vented roofs.

[Eli Talking]

I hope list participants will share their insights and experiences of
mounting rigid insulation above sloped framed un-vented roofs.  Sealing
tight, continuity, avoiding thermal bridges  are some of the appeals of this
approach.  However, this is a departure from common practices.

I am developing a model home for demonstrating a continuous wrapping on all
sides of the thermal envelope of 2" xps foam.  On exterior frame walls, foam
is mounted on exterior sheathing and extend to bottom of roof sheathing at 
the eave.
I am interested in continuing the xps over the roof sheathing with white
(cool) prefinished corrugated roofing panels over the foam.  I was thinking
of using long screws with neoprene washers.  I have used corrugated roofing
for awhile now and find them to be durable and surprisingly reliable.  I am
looking at roof pitches from 7/12 to 11/12.  The examples I have seen
published generally install another layer of sheathing above. The book, "JLC
Guide to Energy Efficiency"showed double layer of foam, with furring strip
in same layer of top foam layer (2x for 1.5" foam) with sheathing nailed to
that.    In the text of the article the author says that where only one
layer is used, he omits the sheathing as I am proposing.

In another project where the final roof is a traditional standing seam, I am
proposing 8" strips of 3/4"plywood at 2'oc above the foam to be the mounting
surface for the clips.  In this way, I am maintaining a gap below the metal
roof.  I think this is an extension of maintaining a ventilated drain plane
behind siding before the thermal envelope starts. (2" foam with sealed
seams, 1/2" osb sheathing, R38 un-faced fiberglass batt insulation, gwb
ceiling (keep the wires out).

Another scenario I am looking at is using attic truss to frame a half story
for finished attic.  Since this would not have finished ceiling following
the roof plane, enclosing the fiberglass batts below would require an extra
layer of sheathing.  I am considering proposing all insulation being rigid
insulation above the sheathing.  In this way, the roof sheathing also acts
as the required ignition barrier for foam and an extra layer of sheathing is
not required.  On top of rigid insulation I would again use the 8" strips of
3/4" plywood.  The roof insulation would meet up with the wall foam as
described above.  The questions I have are installing a thick layer of foam
on a roof pitch of 7/12 or greater, how should it be supported.  I am
looking at building a bracket at the eave for supporting the foam just below
where the vertical foam intersects the roof sheathing.

Eli