Hi all,

I came here after reading  “How to build an insulated cathedral ceiling”. I tried posting this question to that article, but the post disappeared. Anyhow, I’m so thankful to the author of that article and to all you knowledgeable folks! My name is Peter and I live in Southern California within 5 miles of the coast (Climate Zone 3B).

 

My roof above an unvented cathedral ceiling is being replaced this week, and I would greatly appreciate any feedback you have on the proposed approach, below.

 

Background: The original roof is cedar shake, and the new roof will be asphalt shingles. The rafters are 2×6″ with 1/4″ skip sheathing spaced every 4″ from roof edge to ridge. The rafter bays are continuous with no blocking or penetrations from roof edge to ridge. I am mostly concerned with preventing moisture issues – the addition of insulation is mainly to prevent condensation.

 

Here is the proposed construction from interior to exterior:

 

1) drywall + plaster + drywall, about 1.5″ thick (existing)

2) Unfaced fiberglass insulation inside 2×6″ rafter cavity filled all the way from drywall to skip sheathing (new)

3) skip sheathing made of 1×4″ strips, spaced every 4″ (existing)

4) 1.5″ of EPS rigid foam sheets, R-6 (new)

5) 1/2″ OSB sheathing (new)

6) roofing paper

7) asphalt shingles

 

Some specific questions/concerns:

A) Do you see any potential issues with this roof construction above the unvented cathedral ceiling? 

B) Does the 1.5″ of EPS foam need any special treatment? taping or caulking at the seams?

C) The skip sheathing creates many non-continuous, small air gaps between the fiberglass insulation and the EPS foam. Is this an issue?

D) If preventing moisture issues is my main concern, would I be better served by either 1) ventilating the ceiling with new soffit/ridge vents or 2) adding vapor diffusion ports? 

 

Thank you so much for your help and wisdom in advance.

 

Be well,

 

Peter