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?
I have examined the state of the newly applied sealant and it all looks good
and continuous.
There is a generous drip overhang on the outside sill.
Under the sill and all round the frame is rendered and painted - no obvious
gaps.

Can the seals between the glass and uPVC frame be defective?
Any other suggestions? Unlikely to be roof related as this is new tiled
version.

I notice that the window sill has a lip just under the main frame which
presumably stops water tracking back underneath.
If water gets past the glass seals, where will it go? This lip looks like
it will stop water getting out that drains "through" the frame.
There are milled slots in the bottom of the frame (under the opening window)
which appear to act as drains. These look as though they will discharge
water on the inside of the concealed sill lip.

Can anyone follow this lot?

Any suggestions for making room water tight appreciated.

TIA

Phil