Re: [GBlist] Qustn: Rigid Insul above exist'g roof

[John Salmen <terrain@shaw.ca>]

Hi Jim

welcome to the list

I generally prefer expanded foams to the extruded. Part of the reasoning is
the friendliness of the technology used the other is about performance. Most
extruded foams tend to have a lower R value in lower temperatures while
extruded foams tend to gain in value as the temperature drops, which is what
you would prefer to have happening. Expanded foams seem to have a longer
life as well, something to do with cell collapse in extruded. Extruded foams
(bead board) are available in a wide range of compressive strengths, but for
a roofing application the typical type 1 offers 10-15 psi rating and higher
ratings are available.

I would never really assign a higher value to foam in situ than a R5 per
inch. Polyisocyanurate has an aged value of 7.2 (which theoretically adds up
to R21.6 for 3") so there could be some benefit there but at that range the
additional cost value is dubious and I would like to see those values 20 yrs
hence - they can be compromised substantially by any thermal bridging which
can substantially reduce the real value of the assembly. In a situation such
as Barbara describes I would prefer to see a completely sealed insulative
layer with all gaps foamed or sealed (complete to the overhangs) with no air
gap between insulation and roofing. Typically I would use the thickness
needed in a basic eps rather than considering a thinner profile in a more
chemically complex foam.

The elimination of strapping reduces material usage and labour allowing for
an increase in insulation, a small increase in material which provides some
actual benefit. The use of a lighter coloured roofing material would
eliminate the need for a foil facing and an increase in foam thickness would
further that.

John Salmen
TERRAIN E.D.S.





 ----- Original Message -----
From: "Jim Coate" <jbc@coate.org>
To: <greenbuilding@crest.org>
Cc: "Barbara Lamprecht" <mac@cacr.caltech.edu>
Sent: Monday, November 26, 2001 7:16 PM
Subject: Re: [GBlist] Qustn: Rigid Insul above exist'g roof


>
> I don't have actual psi numbers at hand, but rigid foam is strong in
> compression. Note this is "Styrofoam" (extruded polystyrene, the blue or
> pink sheets) NOT "bead board". I seem to recall applications where a
> concrete footing is poured, rigid foam is laid on top, and then concrete
> walls formed above that, so the foam holds the weight of the foundation
> wall.
>
> The advice you quoted from Ralph makes sense. If the sleepers are laid
> on the foam directly over the existing rafters, the foam will only be in
> compression, with no bending or shearing so should be fine. And with the
> stiff 2x6 t&g decking, it probably would work even if the sleepers
> weren't aligned to the rafters. Making a solid grid adds more work and
> eliminates a nice thermal break. If you use long 'sheet rock' screws,
> either system could be disassembled at a later date. The biggest worry
> I'd have is to make sure there is a plan for boxing in the edges neatly.
>
> PS I'm new to this list, so "hi". Searching for radiant floor
> information led me to the archives and lots of other great stuff so far!
>
> _________
> Jim Coate
> 1992 Chevy S-10
> 1970s Elec-Trak E20
> http://www.eeevee.com
>
>
> Barbara Lamprecht wrote:
> >
> > Dear Listers,
> >
> > In order to turn a room with single-pane glass on
> > three sides  we would like to apply rigid
> > foam insulation to the
> > ceiling/roof (flat; that is,  this 15' x 24' roof has 1/4 -- 12 slope,
> > in order to preserve the interior
> > look of 4x6 rafter joists supporting
> > 2x6 t&g; only rolled asphalt
> > above that.
> >
> > The general contractor wants to lay a sea of
> > rigid above (we are shooting for at
> > least 3" for an R value of about 24/25), with
> > 1x4s on top of the rigid, about 24
> > oc, and 1/2" sheathing above that. His reasoning
> > for the insertion of the 1x4 is
> > to create a venting area. I am concerned with the
> > rigid performing a structural
> > role, in a way, since at some point in the future
> > it may be under compression;
> > my idea was to rip the 4x8 sheets in half and
> > separate them with 2x4 studs laid
> > on the narrow side, creating a structural grid of
> > support for the CDX and the
> > 1/2" (3 1/2" stud height minus 3" rigid
> > insulation thickness) So, my thinking
> > creates a grid, his creates a "sea", to use a
> > metaphor.
> >
> > Am I right to be concerned with compression, and
> > also demountability over time,
> > another element in thinking sustainably?  The
> > roof edge will be seen from the
> > street, and I have not had success with "spray
> > foam" companies whose minimums
> > are 1,000 sq. ft. I think this job is too small for Icynene contractors.
> >
> > Is venting important? I assume it is, and am concerned with how it
> > should be detailed. We are holding back the smaller roof from the
> > larger "L" of the gables that surround it.
>
> ______________________________________________________________________
> This greenbuilding dialogue is sponsored by REPP/CREST, creator of
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> ______________________________________________________________________
>


______________________________________________________________________
This greenbuilding dialogue is sponsored by REPP/CREST, creator of
Solstice http://www.crest.org, and BuildingGreen, Inc., publisher of
Environmental Building News and GreenSpec http://www.BuildingGreen.com
______________________________________________________________________