Insulating a dormer roof

MrWillyWonka

We live in a chalet style house which has 4 rooms upstairs all with a dormer roof, built in the 60's.

Most of the roof space is well insulated except the flat dormer roof. The joists are 100mm deep. Initially I was going to put down 50mm Celotex in between the joists which would leave a 50mm air gap. However after taking some photos on the flat roof it is apparent there is already some kind of insulation present. The photos suggest 25mm of mineral fibre on the underside of the roof. Since the dormer extends 2m outwards I can only imagine the insulation was either put there when the house was built or when (if) dormer roof was re-roofed.

Can anyone advise on whether Celotex is the right way to go now I've made the discovery.

Image - top is the insulation, bottom is the plasterboard for the room below.

andyhop

Either insulate below with kingspan or similar

You could also insulate from above if the felt or covering is in poor condition , insulate above the deck if you have the height

MrWillyWonka

andyhop wrote: »

Either insulate below with kingspan or similar

You could also insulate from above if the felt or covering is in poor condition , insulate above the deck if you have the height

Can you just clarify "below"? Do you mean to put the Kingspan/Celotex below the plasterboard or below the existing insulation. I'm just wary of any condensation issues.

As for the felt covering, the whole roof was re-felted just before we purchased the house last year otherwise a warm space would have been an option.