The page http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling is a compendium of several other pages covers the subject pretty well, though it is targeted at cathedral ceilings, all of it applies equally well to a moderately sloped or level ceiling.

Keeping moisture out is the prime consideration for unvented rafters. To do it right, you'll have to either remove the ceiling or tear off the roof (and rebuild it afterwards).

For those of us in temperate areas, 2.9 seems pathetic—almost a Why bother? Indeed, three layers of 3/4 inch plywood achieve R2.82, so adding almost any roofing material over that would meet the requirement.

The page http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling is a compendium of several other pages covers the subject pretty well, though it is targeted at cathedral ceilings, all of it applies equally well to a moderately sloped or level ceiling.

Keeping moisture out is the prime consideration for unvented rafters. To do it right, you'll have to either remove the ceiling or tear off the roof (and rebuild it afterwards).

For those of us in temperate areas, 2.9 seems pathetic—almost a Why bother? Indeed, three layers of 3/4 inch plywood achieve R2.82, so adding almost any roofing material over that would meet the requirement.

Edit: Since R2.9 metric is R16.5 non-metric, this is indeed a worthy level of insulation and should be done right.