/This is classic condensation in the flue, due to lack of ventilation.
Is the flue still used? /Q
Yes occasionally /What's the construction, and does it back onto an outside wall? /Q
Brick, and yes it's on a gable end.
/What's the state of chimney flaunching?\q
Pass but top of stack was rebuilt approx 5 yes ago.
/> The flue behind the affected areas is lined and backfilled with vermiculite insulation.
Urgh - don't know how you'd ventilate that now. I suspect the vermiculite is partially waterlogged, and it can hold very large amounts of water./Q
Possibly though it's summer and why do I only have a few patches showing on the gfloor breast ?
Jim K