I spent 35 years in the roofing and insulation business with a major company. There are three ways to retard heat flow..(conduction, convection and radiation). The best and cheapest way to insulate a residential structure is to add fiber glass insulation to your attic. The cheapest is blown in. Attaching radiant barriers under your roof deck or draping it under the roof deck is good but more expensive. Also it will cause the roof deck to absorb the heat more thus the shingles on your roof will be several degrees warmer. That will cause the life of the shingle to lessen.
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