flat roof condensation risk

[smoothound]

I am being told that the following roof construction offers no condensation risk because of the semi-permeable nature of the single ply membrane - and because of the fact that the vapour barrier means that there is no 'new' moisture access to the air.

but using free online cond-risk tools (and my common sense) - tell me otherwise -so i am concerned - any opinions please?

The flat roof (decent falls) is unventilated and a parapet enclosed type so ventilation is not possible.

Roof construction from inside - plaster skim, plasterboard, vapour barrier (penetrated by downlighters of the sealed bathroom type - these should retain the vapour barrier integrity) - 147mm stud/ferring frame stuffed solid with rockwool about 200mm+ on average, 18mm ply and finally alkorlit single ply membrane which is breatheable and laid onto thin fleece (not glued down)

the theory is that dew may form from the 'trapped' air in the frame cavity - but this would be transient since it would be re-absorbed into the air when sun warmed the roof or temperatures rose a bit.

 

[freddiemercurystwin]

Is this a new build roof or a refurbishment?

If a refurb this is typically what you would do if you were doing a garage conversion (and was retaining the roof deck/finish) and there is little you can do other than this.

Rockwool is a poor choice though as it can sag after time (even when its stuffed in in the first place) and thus a cavity beneath the deck can form which is obviously increasing the condensation risk.

 

[smoothound]

Thanks Mr F - mor eor less what I imagined - and yes I raised the sagging issue too

cheers

Al