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

[Clarke Olsen]

I think that standing seam needs more continuous support: the 16" space might be too much.
Corrugated panels can cheerfully span 2x4's @ 24"oc.
My favorite roof pitch is 8.5 in 12, aka 10 in 16: the golden mean, used on all 19th cent. barns, 
houses, and churches in New England. With a metal roof, it's the steepest I can stand on and 
the shallowest to shed snow.
Roxul is a better choice then fiberglass.
We obtained an attic with a standard pitched truss,w/2x6 bottom chord, insulating & drywalling
that inner triangle.
Clarke Olsen
373 route 203
Spencertown, NY 12165 
USA
518-392-4640
colsen at fairpoint.net




On Jul 5, 2012, at 12:10 PM, Eli Talking wrote:

> 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 
> 
> 
> 
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