Upvc window insulation

[gaz1]

your not dealing with wood in a roof so it should be the exact opposite

even if the board is being heated and the insulation is behind it it shouldnt swet and cause due drops or be dripping wet through as there is no cold air or warm air getting to the board like the metal roofs in winter that drip

all i can see is that you are suffering cold air and warm air on that board causing the whole issue because you hadnt sealed the window cill it started to act as a syphon expansion area more so if the sun was hitting that board

during the day the air would come out of that area sucking fresh air in and at night the eact opposite pulling cold air down into it when the room was being warmed up

 

[Kram]

Each time I look at it I am less certain about anything

The bottom corner is still damp. Everywhere else is dry but the upper left corner by the window too has a very small damp area, which hints it might be the roof!

The wall measures 295mm thick which I assume is made up of 102mm outer brick, cavity gap, ~140mm inner brick, and a bit for plaster board, assuming standards...

I have drilled a couple 10mm holes into the cavity. Both are thin plaster, where I was expecting a red brick across the cavity (I spent ages looking for my long drills and hammer drilll!).
Whe I find my bore camera I will have a look inside.

The top hole has some kind of fabric, dpm?, which now has a 10mm hole in it

The bottom hole has a wad of rolled thicker fabric which does not cross the hole.

@Onoff I'll get you to have a look, next time, if I ever get this casting done