hi, I am getting a flat roof in an extension replaced and upgrading the insulation which is probably non-existent. I've had 2 builders look at it. They are both suggesting a PVC roof but their insulation methods are different. One proposes using 2 layers of 60mm Kingspan foil faced insulation boards - the 2 layers laid in opposite directions to minimise gaps. The boards go above the joists (I think) and then there is a sheet of plywood followed by PVC. The other proposes laying 200mm of rockwool on the existing ceiling and then plywood/PVC layers similar to above. He says the rockwool is better than the rigid boards as it can be pushed into every area. How would these 2 options compare in terms of effectiveness of insulation and cost - I don't have prices from them yet. The man proposing Kingspan says the combination of the 2 60mm layers would have a U value of 0.18.