Thanks for replying to my query so quickly. Would it be possible for you to expand on your statement, "That would certainly be a second vapour barrier and that is about the last thing you want in a cathedral ceiling." I would like to understand why this would be the last thing I would want in a cathedral ceiling, in particular - condensation formation (if so, where), heat accumulation, etc.?
Answer:
Once rigid foam insulation is an inch and a half thick or thicker it becomes its own vapour barrier and you can compare that to having left a green garbage bag out on the lawn last summer. In the morning you found that the inside was not only wet but it held that moisture and your two vapour barriers can do the same thing. There are people who do thermal photography in the area and I'm sure there are insulation companies who know how to get a hold of them.
