Hi,

I've been retrofitting celotex to a room. It's gone well except for
one slight problem: the window reveal (I put 25mm celotex inside the
reveal). I'm worried that even if I use 9.5mm, rather than 12.5mm,
plasterboard on top of the celotex that it might overlap too much of
the side of the window frame. I'm not sure how much of a problem this
will be: any overlap will only be cosmetic, it's not going to block
the window or prevent it opening or anything that drastic but will it
look "odd"?

What would you do? Use 9.5mm PB? If there is an overlap (i) it will
probably be hidden by blinds/curtains/whatever SWMBO puts there or
(ii) if we do see it, we may grow used to it.

Alternatively is there something thinner than 9.5mm PB? I don't expect
there is a plasterboard any thinner because it would be too fragile to
transport but can you plaster on top of say hardboard? Does it need
any treatment first (primer? undercoat?) or just PVA?

Or can you PVA celotex and plaster directly on to that?

TIA