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

[Barbara Lamprecht <mac@cacr.caltech.edu>]

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.

Below I have cc'ed the helpful info that Ralph DiNola of Green Building
Services provided to me:

"... I think that it will
  be fine for you to use the method that the contractor is recommending,
here are a
  few finer points:
      I would install at least three inches if you are going to that
much effort in the
      first place.
      You could use 1x or 2x for the sleepers over the insulation for
more of an
      air space.
      I would ask that the sleepers bear over the structure below to
transmit the
      load, if evenly distributed there should be no problem with the
insulation
      crushing.  If you are very concerned with that you could use a
structural
      insulated panel which has foam sandwiched between OSB panels.
They
      are much more expensive.
      Select isocyanurate foamwith foil facing, when used with an
airspace it
      helps to radiate heat away from the roof.
      With only a 1/4" per foot slope you will not be able to use metal
roofing
      unless it has soldered seams.  Check out energy star rated roofs
at the
      energy star website, they have a list of approved
systems/companies.
      You can cover up the added thinkness to the roof with additional
facia
      boards at the eaves, be sure to add vents though."

Much obliged,
barbara lamprecht


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