[Greenbuilding] thermal implications of a vault vs flat ceiling at R-38?

[Reuben Deumling]

I'm restoring an 1878 farmhouse here in Western Oregon and have pretty much
started over with the roof. The rafters extend the original 12-in-12 pitch,
but rather than terminate in (the original) square flat roof section I took
them all the way up to a standard ridge. The new rafters are 2x6 and I'm
going to run 2x4s perpendicular to them, attached to the rafters at the
points of intersection with those little hurricane ties, the twisted ones
that attach only to one side of a joist or rafter, leaving an air gap that
will allow me to run R-19 batts vertically and horizontally for something
close to R-38 with very little thermal bridging. (This part I've done
before and am quite pleased with how it works.) What I haven't decided is
whether to leave the ceiling vaulted or frame in a normal flat ceiling at 8
or 9 feet, and am curious for your thoughts on the thermal implications of
both choices. In our climate heating the space seems less of a concern than
dealing with summer heat gain. With a hip roof, and a ridge 13' above the
upstairs floor, the opportunities for passively venting are somewhat
limited. I am putting a standing seam metal roof on the house, and although
a vented ridge is easy enough to install with a flat ceiling, with a
vaulted ceiling the venting opportunities seem far more limited. Or? I'm
planning to make wooden shutters for the East and West facing dormers to
reduce direct heat gain, but no AC is planned.

Thoughts?
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