On 13/05/2019 13:46, Mathew Newton wrote:
On Monday, 13 May 2019 08:17:26 UTC+1, charles wrote:

Is there an open water tank feeding a heating system?

No, we have a sealed system (and unvented cylinder for HW).


Which answers a question I asked.

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

Do you have downlighters in any of the upstairs rooms,
notably the bathroom ?. These will allow warm, moist
air to leak into the loft.

Ditto a badly fitted loft access hatch.

Then there is also the possibiluity that warm moist air
is getting into the cavity and the water vapour molecules
migrate up the cavity, into the soffitt space and then
back inside your loft.