In all honesty, I think best bang for the buck is THICK insulation on the attic floor, and AIRTIGHT ductwork if it is in the attic. Bottom line. Unless you insulate the roof deck, or unless you employ a radiant barrier with a 99.xx% radiant heat reflectivity, a ventilated attic will not stay near ambient temperatures, which is the ideal goal of any attic ventilation scheme.
Originally Posted by rnevil
For much residential construction, the largest contributors to heat gain into the structure (which determines the quantity and capacity of cooling needed to keep the interior comfortable) are windows and infiltration (leakage). A nominally insulated attic floor will keep heat gain from the attic to the interior below the percentages gained through single pane windows (particularly unshaded windows) and infiltration, Airtight, well-insulated ducts that run through the attic will assure the a/c system delivers more of its actual cooling output to the rooms vs. being lost to the attic via air leaks and heat gain into the ducts from the hot attic air and surfaces.
Building Physics Rule #1: Hot flows to cold.
Building Physics Rule #2: Higher air pressure moves toward lower air pressure
Building Physics Rule #3: Higher moisture concentration moves toward lower moisture concentration.