Waste of money and time. While all the radiant efficiencies are shown with lamps and thermometers the fact remains if you do not have sufficient ventilation and the product does not allow for moisture transfer you will collect moisture and overheat the space above this material.
While the focus is on keeping the heat out, it really should be about keeping the heat in. And that is done by air sealing. The result is much better performance with the insulation and much lower energy bills as well as increased comfort in the home.
Insulation traps air which keeps heat from moving through it. However fiberglass insulation in a poorly or incorrectly vented attic has the heat drawn out of it every time the air blows in the attic. This happens both by poorly designed vent systems that do not use baffles at the soffit areas, and by air leakage through stack effect coming up and out of the house.
You can check easily if your home has energy loss through stack effect. Simply go into the attic and pull up insulation around and over the walls located below. If you see dirty insulation that is the air movement with the insulation trapping the dust like a filter. All the radiant barriers in the world will not keep the heat loss in this area from happening.
Correct these issues and if need be add more insulation in the attic and you can forget about using these shiny metal barriers. Which once they get dirty loose their effectiveness. Have you ever seen how dusty the attic gets?