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Originally Posted by creeky
What Bernie said. I've not seen any real science that says radiant barrier is particularly effective as an insulation on planet earth. If you're doing an attic top up already maybe just save the time and money and go straight to the insulation.
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Radiant barriers are not insulation. They limit the heat radiated. The science is well proven in every field that uses hot and cold stuff. Cars use radiant barriers in underhood/exhaust applications. Aircraft use them in many places. Everything that has anything to do with space uses radiant barriers. Fire protection blankets are basically lie those cheap thin aluminum coated mylar ones.
It reduces radiated heat where as insulation reduces conducted heat. As for attics it does make a noticeable difference in attics usually attached to the roof decking.
There are three forms of heat transfer. Conduction convection and radiation.
Conduction is slowed by insulation.
Convection is somewhat slowed by air sealing and having areas too small for convection to happen.
Radiation is slowed by reflective materials like aluminum or better yet gold which is what some hyper cars use to fight radiated heat.
It works in houses but it's best to do it when you have the roof replaced than to go through the misery of stapling up rolls of mylar in the attic. Galvalum roofing also helps.