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

[Eli Talking]

At www.Buildingscience.com article   RR-0108_Unvented_Roof_Systems 
describing unvented roofs they reference their concern that the foam layer 
be of sufficient thickness to insure that when air reaches the bottom of the 
foam, the surface temp will be above dew point.  The assumption, I guess is 
that air will penetrate the rest of the assembly and show up at the bottom 
of the foam.  However, if the framed rafter was filled with dense pack 
cellulose or fully stuffed fiberglass or roxul, no wires or plumbing are 
installed, and the drywall is managed to be an air barrier, where would the 
air come from?  With no holes in the foam layer, there will be no driving 
pressure to deliver moisture laden air to the bottom of foam.  However, more 
typical rafters with wires, recessed electric boxes, unsealed plumbing 
vents, this would provide a path for air to get to the bottom of foam, 
particularly if cold.

If this is a problem, this would be a good reason for all the insulation to 
be rigid above the roof sheathing, one of the scenarios I described for use 
with an attic truss.  See below

They use a formula of Ratio of thermal resistance above condensing surface 
to
total thermal resistance:
16 ÷ 46 = 0.348
Temperature of Condensing Surface = (ΔT x 0.348) + Outdoor Temperature

Any thoughts on this recommendation?

Clarke:
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.
Eli:
The high slope greatly reduces the load on the panels.  I do like corrugated 
roofs better for their spanning capability.  However, sometimes we must pay 
homage to the past with traditional materials in our market.  Standing seam 
roofs are very good roofs with good recovery potential as do the corrugated 
roofs.

Thomas Lewis:
We use sheets of poly iso with a fiberglass facing rather than polystyrene. 
It is much less susceptible to cracking during install and much more 
friendly to the environment.  (although I'm pretty sure they all contain 
bromide fire retardants, so make sure you use a vacuum when cutting any of 
them) It had a higher density so great for walking on and I think it has a 
better r value too.

Eli:
The extruded polystyrene(xpx) is very good about not cracking the expanded 
polystyrene(eps) is much more vulnerable to cracking.

I used 3"fiberglass polyiso over the walls of my house.  According to JLC 
Energy Efficiency Manual, the polyiso has a higher R value initailly, 
however, thermal drift from outgassing results in a long term R value of 5 
to 5.5 according to JLC Energy efficiency manual.  This is comparable to 
extruded polystyrene(xpx).

Thomas Lewis:
the last roof we did had existing fiberglass in between the roof sheathing 
and ceiling for much of the roof area, the owner didn't want to tear up 
everything in the house so we blocked up the rim with spray foam, applied 
ice and water shield to the whole roof (now we have air barrier) laid down 
two layers of the 2" poly iso (should keep dew point inside foam board), 
sheathed over the whole thing, then breathable synthetic felt, then standing 
seam.
Eli:
Not tearing up the interior makes this a good retrofit strategy for when the 
roof shingles expire.  This avoids the interior mess in an occupied 
building.

Thomas Lewis:
I would definitely recommend against using furring strips over foam board, 
especially over eps.  I've never tried it, but I think it will make for 
tricky furring strip installation.  With the reduced surface area you will 
find the strips will pull into the foam board and rock unevenly.  I've found 
that even with using full sheets of ply over eps, the screws had to be kept 
in from the ply edge ~1 3/4 in order to keep the ply from bending into the 
foam.

Eli:
My polyiso walls are installed this way.  I put 3"x3/4" osb (plywood on 
future projects)furring strips at 2'oc.  After an initial struggle with the 
wrong long screw, we ended up with 5 5/8"GKR bugle head screws that were 
wonderful.  It would pull the furring strip tight, but it did not compress 
it.   I put fasteners in the middle of the strip.  I also dab caulk where 
the screw is going to be placed to seal these new holes.  The GKR screws can 
countersink into the wood.  Therefore, you could put a dab of foam in a 
recessed hole to reduce the fastener thermal bridge.

According to JLC Energy Efficiency Manual, the compressive strength of 
polyiso ranges from 16 to 24 psi. Extruded Polystyrene (xps) has compressive 
strength from 15-100 according to Owens Corning.  The load polyiso could 
take if mounted over structural sheathing would be 8inx12inx16#/si=1536#/lf.

Thomas Lewis:
If you are going to be using long screws to attach the roofing, then what 
are the furring strips for?  We put down a complete layer of ply so that 
like any typical metal roof, the roofer can screw to the sheathing. for you, 
couldn't you just tape the seams and go?

Eli:
Where I am using the corrugated roof, you ask my same question.  Why not 
skip the sheathing above the foam.  Also the corrugated ribs can provide 
some ventilation below roof to allow drying.  However, in our are, we must 
pay humage to the past with traditional standing seam roofs.  The furring 
strips as I described would provide the mounting surface for screw fastened 
cleats for standing seam.

Clarke Olsen:
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.
Eli:
You must have sticky feet.  I consider 7/12 with regular shingles my 
steepest confident foot pitch.  Metal roofs would more slippery.
Clarke Olsen:
Roxul is a better choice then fiberglass.
Eli:  What makes it better?



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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