On Saturday, December 7, 2013 5:46:49 PM UTC, Big Les Wade wrote:
posted



In my

case I had a combination of 100mm of Rockwell and 100mm of Celotex

plus boards.



When I put this forward in my building plans the local authority

building control people wouldn't accept it, on the grounds that

different layers of materials with different insulation properties would

cause condensation.


I guess if there was wool under celotex and no vapour barrier, you could get condensation on the underside of the celotex, and if wool gets wet it loses its insulation properties, but this would then be no worse than just the celotex.
My BCO was OK with rockwool above celotex in a roof.
In a loft with just wool, moisture can percolate through the wool and into the air above.
Simon.