Eamonns job is exactly what I was describing ,except the 1" Kingspan would have more than double the insulation properties of polystyrene which was all that was readily available until recently,Btw ,spray- on would expand to about 3" and is very unfinished looking.
There is another product available which would do for the ceiling which is like foil backed bubble wrap,similar stuff as is ued behind radiators on external walls ,for a small roof area it wouldn't be too expensive and that will definitely cure condensation .It's only about 6mm thick.
Eugene
  