On Sep 1, 11:33 pm, 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.

You need to:

1) Put in a ridge vent, soffit vents, and chutes to connect them.
2) Put in a plastic vapor barrier before you put the tongue & groove
ceiling back up.

Polyisocyanurate will get you better than the R-5 per inch that you
are planning on for the styrofoam.

JK