Condensation in old dormer flat roof. Options?

[Restoration man]

I had a leaking flat zinc roof on a dormer in summer which I fixed. I left it open in the bedroom so I could check it when it pours with rain which happens here all summer and it's been perfect.

I decided to stick a sheet of ply on the ceiling as a temporary measure for winter and insulated between the timbers with sheeps wool which I've been using in the house.

The ply sheet was about 6 inch to small but fine as a temporary measure.

I took the sheet off today and under the wool is soaking. I'm presuming this is from room condensation.

What are my options when I permanently refit this. Can I insulate or should I leave it empty? There is sarking board at the sides of the dormer but the roof void isnt really ventilated and there is no soffit etc in which to put vents.

I'm planning to finish the room side with timber t and g linings.

Thanks

 

[foxhole]

Celotex

 

[Restoration man]

If I use Celotex do I leave a gap between the top of the Celotex and under side of the flat roof?

Or fit the celotex hard to the underside of the deck?

Rememering I'm struggling to ventilate the roof space. The joists are only 80mm deep.

I was thinking of drilling 6 x 10mm holes in the main timber near the pitched roof to allow air movement from the sealed flat roof void to the joining slate roof voids. The thought being the pitched roof is slated and breathable through the sarking gaps where as the flat roof is unbreathable and unventilated due to zinc covering?

Cheers

 

[CJRatch]

80mm celotex between joists pushed up tight. Foil tape any joins and foam any gaps along joist edges. Board with foil backed plasterboard and skim. This will stop warm moist air hitting the cold underside of your roof. Pointless leaving a gap if you have no ventilation.