John You can put some numbers on what you need using Manual J8. The demo version of the software for this by Wrightsoft will give you some idea of what is going on. An example from my house bedroom is that with R28 insulation on the ceiling and a radiant barrier under the tiles the cooling requirements are 163 cfm. With no radiant barrier this rises to 170 cfm. With no insulation at all the figure is 338 cfm! i.e. the most important thing is the glass wool and not the radiant barrier.
Geoffrey