Insulating Roof in Loft

[leeweedon]

I'm planning to insulate the roof in my loft using Celotex foam.

There is no membrane behind the roof tiles. Do I need to put a layer of anti damp in? Or just put the Celotex between the rafters then plasterboard over the top?

 

[tony1851]

I just put the Celotex between the rafters then plasterboard over the top?

But leave a reasonable gap between the top-side of the board, and the underside of the roof covering - say 1" min.
Most important - make sure there is sufficient ventilation above the insulation. If you can see lots of little gaps between the tiles, that should help ventilation.