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

[Thomas Lewis]

Hi Eli, 

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.

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.

For all of our foam board installation we make a tray out of spf that follows the perimeter of the roof and boarders skylights and chimneys, this is screwed down to framing over the roof's structural sheathing, infill it with foam (just keeping it tacked in place with a few button nails, filling gaps over 1/8" with sprayfoam and taping seams if the installation isn't over an air barrier), then covered entirely with 5/8" ply screwed down to the framing.


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.  


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?

I hope this helps, 

Tom Lewis



"Time makes more converts than reason."  Thomas Paine, Common Sense



Message: 3
Date: Thu, 5 Jul 2012 12:10:49 -0400
From: "Eli Talking" <elitalking at rockbridge.net>
To: "Green Building" <greenbuilding at lists.bioenergylists.org>
Subject: [Greenbuilding] mounting rigid insulation above sloped framed
    un-vented roofs.
Message-ID: <3EAC4D99D35E4A80887148BA45F9252D at NewCentury>
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
    reply-type=original

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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