My loft is currently boarded, but the insulation under the boards isn't very good - just a bit of fibreglass, and most of that is missing. I wanted to improve on it, and I discussed it with a chap at Sheffield Insulations who told me that 100mm depth of Celotex/Kingspan between the joists would be as good as 270mm of fibreglass. (Yep, I know it's pricey stuff - but I don't mind shelling out if it's the easiest way of doing the job. The alternatives involve putting the boarding on props to take a greater depth of fibreglass underneath, or laying insulated boards under the hardboard - both of which seem like too much work.)
However, I've done a bit of reading, and heard warnings about doing this as moist air from the house would condense on it. The bloke at Sheffield Insulations reckoned this wouldn't be a problem as the bottom surface of the Kingspan/Celotex wouldn't get that cold, as it would be insulated from the cold roofspace.
Any insulation experts on here have any views? And what's the score on throwing away bags of old fibreglass insulation - will a council tip take it?