Insulation for cold flat Roof without air ventilation

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Hi, help please on protecting a newly built cold roof whilst trying to add insulation and under boarding.

We have a newly built extension utility area to the side of our house. It is constructed with a cold flat roof with small pitched ends that have flat back parapet style to the mid flat roof. The total length is about 10m (the mid section flat roof makes up about 8m). A spray foam has been applied to the deck underside (a roofing felt style barrier between deck & foam).

Due to the construction of the flat roof beams running side to side and with the way doors have been fitted each end it would be near on impossible to add any air ventilation.

I would like to finish off internally by boarding the underside but have concerns about the terrors of condensation and damp problems. Due to the lack of air ventilation my intention is to fit a tightly sealed damp proof membrane as a vapour barrier then simply plasterboard I would keep all the electrics and lighting below the p/board so not breaking the barrier).

Does anyone see a problem with my intentions (YouTube videos of rotting flat roof decks has worried me).

I am not being restricted or governed by building regulations but just the need of doing the best & right thing.
 
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A vapour barrier alone won't prevent condensation and mould risk, if there is still an air void.
 
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OK, thanks for the reply, I was hoping with the area being treated like a garage and not being heated then in an old fashioned way that a sealed barrier would give enough protection.

Working on the barrier not being suitable and the fact I can't add any outside ventilation I will accept the spray foam to be all the insulation I have so now will just like plasterboard the ceiling for cosmetic reasons. To prevent the damp and condensation problem would it be acceptable to have a number of vents coming straight down into the new extension area, I'm assuming this would allow warm area to rise above the p/board.
 
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Any air in a void is at risk of creating condensation on a cool surface. The principles are that you remove the cold surface (via insulation) or ventilate the void to reduce moisture build up. The ventilation is not with the same warm moist air from the room.

If you can't ventilate the void, then the only option is to completely fill it with insulation, remove an voids and then use a vapour check layer to keep air out
 
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