[aerohead]

I'm late to the discussion, so sorry if I'm repeating info.
When in the West Texas Solar Energy Society, the common metric for adequate attic ventilation was that, when measured directly above the insulation surface, the dry-bulb temperature should not exceed 10-degrees above that of the ambient outdoor dry-bulb temp..
If it's 100- F outside, 150-F inside the attic under the roof, but 110- F just above the attic blanket, it's okay. The insulation is 'blind' to heat stratification that your head experiences when standing there. Measure at your feet.
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If the HVAC system and ductwork are in the attic, then long-wave, infrared radiation is an issue ( its in an oven now ), and a radiant barrier can be just as much an issue.
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If all the HVAC system is in a furdown and closet, below the ceiling, or under the house, it doesn't matter so much about the radiant load, unless you have finished living spaces right below the roof.