Install the reflective foil faced version that is attached to the bottom of the rafters. Install soffit intake vents and a good ridge exhaust vent and be amazed at the effectiveness in keeping the heat out of the attic space. The shingle warranty will be retained with this type and voided with many other type radiant barriers. If the spray foam type is used it is not as effective and has to be vapor permeable to avoid structural damage over the years. Do not depend on the salesman of the radiant barrier to give you the best advice. This from 30 years in the shingle manufacturing business as the Technical Manager, FWIW, Ed.