On 14/05/2019 20:07, Mathew Newton wrote:
On Tuesday, 14 May 2019 15:42:12 UTC+1, Andrew wrote:

Did the builders use foil-backed plasterboard for the
upstairs ceilings (they should have).

No, just paper-backed plasterboard and a skim coat.


well that is part of the problem. Water vapour molecules will
migrate through the ceiling and into the loft space. The
breathable membrane should cope with this but seems not to.

Modern building regs make foil-backed plasterboard
mandatory for upstairs ceilings, for precisely this reason,
but older re-roofed properties probably still need some normal
vents even if breathable membranes are fitted.

Lofts that have 300mm of rockwool are much colder, so
condensation is more of a problem.


snip