If you are looking into cooling your attic with a radiant barrier, you might be overwhelmed by all the choices and varying claims to energy savings. The primary products available are: foil-backed OSB decking (underside of roof), foil that is tacked on to rafters and paint. As a rule, the best radiant barrier products have a reflectivity of .9 or 90% and .1 emissivity or less.
Foil-backed roof decking is the best form of radiant barrier. Clean foil has a reflectivity of 97%; the best paints can achieve reflectivity of 77% (or 23% emissivity) under controlled conditions. However, for most people re-roofing and decking your house is simply cost prohibitive, not to mention wasteful. This is a great solution for new construction or substantial remodels.
The next best option is the radiant barrier coating (aka paint). The best radiant barrier paint products will have emissivity of .25 or less. I prefer the Low/Mit products.
Although the paint products aren’t as effective as the foil products, they can help to reduce your overall heat gain. The difference in attic temperature, even with coatings, can be substantial, as observed with thermal imaging. As a test, I had an attic sprayed with Low/Mit and left an area un-coated. The coated areas were 20 degrees cooler than the area that was not coated. Amazing!