On Sep 2, 1:55 am, mm wrote:
On Sat, 01 Sep 2007 21:33:40 -0700, wrote:
I have a cathedral ceiling with no insulation. Construction is 2x6
pine tongue and groove sitting on 4x10 beams. These beams are spaced 4
feet on center. The roofing material is Cedar Shake. What I would like
to do is install 4" styrofoam sm (R20) between the beams and finish
with pine again to retain the original look. Will I encounter
condensation problems? If so what needs to be done to rectify the
problem? Thanks.

I assume you are in a cold climate since you mention condensation in a
cathedral ceiling. I would go with polystyrene over polyiso for two
reasons: polyiso can absorb water, so if you do get any condensation,
your foam could get soaked, and second, poly iso isn't much fun to cut
with a circular saw. It is true that it gives you more R per inch. A
difference between the 2 types is that polystyrene will act as a vapor
barrier as it is (in thicknesses over 1 inch),,,polyiso has to have a
foil face or some other coating to act as a vapor barrier.

Your trick will be to get an airtight fit with the foam to the 4x10
(if I am picturing this right). Definitely buy a pro grade foam gun.
Might even be best to leave a gap between the foam and the 4x10 and
fill this with expandable gun foam. Alternately, you could go for a
tight fit, and caulk the foam to the 4x10. You MUST use a flexible
caulk. Acoustical sealant is the preferred product. Also tape the
seams. If you do a good job, a poly vapor barrier is redundant. By
the way, a poly vapor barrier would have to be caulked to the 4x10's
as well, with acoustical sealant. .