When the right type of radiant barrier is installed and installed properly, the effects are amazing! That being said, a home should be air sealed properly as well as the ac and duct system. Ventilation of the attic too, plays a big part in the BIG PICTURE! The longer the hot or cold air stays in the attic, the more time it has to sneak its way into your home! All these systems working together properly and then adding radiant barrier, is like Icing On The Cake!