In article , TheScullster
writes
Hi all

As part of an extension re-furb we had 2 new windows fitted.
I noticed that at the corner where the internal sill meets the window
(actually there's a 3mm gap which needs filling), there were signs of damp.
A "quadrant" shape water stain was expanding from the corner up the new
plaster about 50mm radius.
The builder returned and ran sealant down each side of the window - for good
measure, I ran sealant along the top.
I have since painted over the plaster, but note that the window sill is
swollen in the corner.

The question is:

How can water be getting in?

[snip]

My guess would be is that it is water getting in the seals, draining out
of the drainage holes in the frame, then running off the ends of the
sill as they haven't been sealed up.

Had a similar thing myself in a window. Fixing involved taking the frame
out and sealing up the gap between sill and frame at the end.
--
Phil Hughes