Phil Addison wrote in message . ..
My daughter's house has had a patch of damp appearing under the upstairs
windows since she bought the house 5 years ago from a builder (I did


big snip

I think you might be barking up the wrong tree here Phil.

It's almost certain that your problems are due to the outside
rendering and the poor detailing/workmanship of the window.

If you have a strong mix on the external wall (3:1) or in that region,
then it's virtually waterproof. (BRE say mix strength this will hold
back hydrostatic pressure). Any water that gets behind it has
nowhere to go to get out but through your internal walls.

You say that you have hairline cracks and also that the wall is on an
exposed elevation. Put these two together and add it to the poor
detailing of the cill – has he for instance just plonked it between
the reveals or is it inset. Is there a good seal to it, is there a
dpc beneath it, are you sure the drip mould works?

The question of the internal rendering is a red herring. It could be
plaster or render, as long as it isn't repeat isn't waterproof then
it's probably doing its job. If it were waterproof, the water would
slowly but surely go further and further down your wall until a big
rust red mushroom popped up between the floor joists.
Check the render, check the detail, keep the water out and you'll be
OK.
Easy when it's said like that in't it

Regards

Patrick