flat roof extension - about to insulate after alterations.

[gollum]

I have a flat roof extension done around 1975. The ceiling joists are close to 7 inches. There is rockwool between the joists. and normal plasterboard which I want to just repair.

I have removed an internal wall under the flat roof which divided the kitchen and living area where there were no visible problems on the ceiling.

What I want to ask is if I want to avoid any condensation problems how is the easiest way around it. Can I use rockwool where the wall used to be and patch up the plasterboard . ? How can I do this as I have heard about risks of condensation.

Does this condensation happen visibly on the ceiling or internally to the flat roof void once the warm air reaches the coldness of the decking underside .?


Thanks for your advice

 

[gollum]

Let me add to this that there are no ventilation grills a round the flat roof.
So I gather from reading some other posts I have an option of making it all one . ie no air gap but if i do this i need to install a membrane. What are traditional ways to install a membrane? Are we talking about foil backed plasterboard ?

I mean I just want to understand how condensation happens if I completely fill the void with say space blanket. Does the warm air permeate the plasterboard and move through the space blanket to the decking underside which i presume is the coldest part and then subsequently dump moisture due to temp drop ? And the only way to stop this is a membrane so the warm air does not permeate the plasterboard ?


Thanks again.

 

[Alastairreid]

http://www.flatroofs.co.uk/#/flat-roof-insulation/4532391027

 

[gollum]

Hi ,
Thanks for the link.
The closest match for my roof would be the warm deck but this link explains solid insulation going over the top of the existing roof in effect making a new roof.
What I simply want to know is can I just fill my existing void with space blanket leaving no air gap. Also I believe I will have to install a vapour barrier and how to do that . Is it with specially backed plasterboard. ?

Tks