On 2008-09-06, grasshopper wrote:

Ceiling will be ~R50 with perforated radiant barrier laid over top.

Radiant barriers can't be used facing up, as they will collect dust.
They must also face an air space, otherwise they are defeated by
conduction. For new roof construction, I think a radiant barrier on
the underside of the roof sheathing works best.

Cheers, Wayne