[Greenbuilding] mounting rigid insulation above sloped framed un-vented roofs.
Clarke Olsen
colsen at fairpoint.net
Wed Jul 4 11:28:28 CDT 2012
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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