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

[Jim Coate <jbc@coate.org>]

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.

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