We are installing lining on a cathedral ceiling. It is a colorbond roof and sisalation has been installed under the roof.
Bradford have a high performance bulk insulation batt which is 90mm and gives an R rating of 2.7 which we are going to use (unless someone knows of anything better).
If the space is sufficient we are considering adding a foilboard layer as well. (we know there has to be air each side of the board)
My question is, since we already have a reflective sheet in the sisalation, will this make a second reflective insulation ineffective? ie. would istalling the foilboard actually have any effect, or just be waste of money?