To add to what Swingman has mentioned here, my parents had the rediant barrier sprayed into their attic and had 8 more inches of insulation blown in. This was done last year in July. So far this year they have averaged a
20% savings in hours used on the months that they have been using the air conditioning.I can tell you right now that pay back will be a long process if you only consider your electricity savings to the cost of the improvement. I can tell you that you will probably save money on equipment repairs and or equipment replacement. The AC simply does not have to work as hard. My parents keep the thermostat at 76-77 degrees all summer long and the temperature stays there. Last year before the improvements the inside temp stayed at 79-81 degrees. Additionally there are no longer any "Hot Spots" in the house. Regardless of which room you are in the temp seems to be the same.
Last, I built a wood and Hardiplank store room about 5 years ago. I chose to use Radiant Barrier Decking over standard Decking. The cost was about $4 per sheet more. The store room has one door that stays shut and no windows. It has sofit vents only. The store room does not feel any hotter in side than the out side temperature. IMHO well worth the small additional expense as Christmas candles do not melt nor do I when I am inside the store room.