I believe that most of the heating that occures in the attic is secondary to radiation. The sun heats the roof surface, the building materials conduct the heat through the materials then the heat is reradiated inward. Convection of the air plays a minimal role. If it did, why would the roof get hot in the first place. It gets hot from solar radiation. The inside surfaces get hot from infared radiation. That is why reflective surfaces work so well to reduce heat transfer in the attic ( eg: reflective insulation around Ac ductwork). When it gets dirty it becomes a infared absorber. We all have an intuative sense that dark colors absorbes radiant energy and light colors reflect, just look at the outside world around you. I would bet that if you could take the roof off completely, during the day, the underside assembly would preform exactly the same way. What I mean is that air flow plays a minimal effect. Insulation is all that matters.
Most research shows that a minimum of 1 to 2 inch air gap is required for adequate air flow. So, perhaps it would be better to cut the board 4 inches longer, then score on both sides to create a z fold. This would give an air gap of 1 to 1.5 inches.
There are a lot of online arc calculators to make this job much easier.
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