Dave Liquorice wrote

If you use celotex or similar you need to arrange ventilation between
it and the ceiling to avoid possible damp problems. Glass fibre is
breathable.

Dave, in "normal" conditions of up to say 75% RH and a comfortable temperature,
vapour will always condense first on the cold side of the insulation, not on the
warm side. That's why "cold" roofs need to be ventilated. If there is a vapour
barrier it has to be on the warm side. Abnormal conditions would be something
like a steam bath in an igloo!

Peter