How to sort this ?

Hi

I removed the plastic window sill and found that the slab beneath the UPVC window is soaking with droplets and mold. The gap is about 2 inches wide, and 4 inches or so deep. At the bottom is what looks like concrete (was expecting an open cavity).

The droplets are forming against the back of the window sill concrete slab. This is a north facing wall BTW.

Should i leave this, or fill it will insulation?

Thanks!

I can't say for sure due to the picture and the description. but it looks like the window installers haven't sealed the back of the window.
It's my guess that the water is coming in from under the window and becoming lodged at the back.

silicon sealant will do the job. just make sure you do the bottom, the sides, and the corners.

Putting insulation down there won't make any difference except give you wet insulation behind you window.

Another possibility is condensation. Reduction of moisture in the room will reduce the water and mould.
There should have been a cavity closer in there to avoid/reduce the possibility of condensation.
This link explains cavity closers:
http://www.kingspaninsulation.co.uk...de3-1c18e97d4fdc/Kooltherm-Cavity-Closer.aspx

After properly sealing the exterior edges of the frame, you could try to replicate a cavity closer with some insulation wrapped in a waterproof cover and fixed to the top edge of the inner skin, but I wouldn't guess its effectiveness.
I don't think it's possible to retrofit a cavity closer without removing and re-installing the windows. Although the windows don't need to be removed entirely, just moved to the exterior about 50mm, then replaced properly.

It is definitely condensation. This is a small window in a sheltered location, it rarely even gets wet when it rains! The concrete window sill is acting like a cold bridge.

I guess the thing at the bottom of the gap could be some sort of cavity closer?

All the cavities are open at the top as they enter the eaves/attic.

Guess i should try cramming some insulation in there (i can cut kingspan to fit).

Thanks
Mike

filled it with expanding foam and the sill is definitely less cold now.