Solving a damp problem...

[spooky_b329]

I've decorating/renovating a c.1895 terraced house, solid wall construction.

In the rear upstairs bedroom we have two damp patches which have resulting in the plaster blowing, about 20cm diameter, one under each end of the window sill, above the ends of the radiator. Window is double glazed and does not get condensation on the glass so I'm assuming its penetrative. Outside everything looks fine, (rendered and painted with Sandtex paint I think) no cracks or missing sealer around window, only thing I noticed was under the exterior window sill, some of the paint seems to have lifted, its still secure but I'm I expect a few jabs from a screwdriver would see it flaking off.

Whats the best way to solve this, is there a clear sealer I can use on just the affected area or will I have to remove the flaky paint and repaint (if I can find the same colour).

Also, how long is it likely to take to dry out once the source of the problem is sorted...plaster has been bare for 4 weeks now (before it had thick textured wallpaper hiding all these nasty bits) and still damp.

Thanks for any help

 

[Balenza]

which have resulting in the plaster blowing

When you say plaster "blowing" , what do you mean ?.

 

[spooky_b329]

Its bubbled away from the wall, hitting it with your fist results in it falling off the wall. I removed the blown part but the dampness extends into the 'good' plaster slightly. This is just the finishing plaster, the render or filling plaster underneath is still solid.

 

[spooky_b329]

Only the left side has blown, missing plaster on right hand side was just loose, not damp.

 

[Balenza]

I think you need to tackle this problem from the outside.
If the windows are wooden then perhaps they are decayed slightly and letting the water seep in and accumulate until a damp patch appears inside.
It could also be a combination of both failure of the window frame and the rendering around the frame and ultimately the seal allowing water ingress in which case you need to investigate and ascertain what has actually failed.

 

[spooky_b329]

Thanks for the reply.

The windows are Anglian uPVC double glazed units, less than 5 years old. Outside sill is made from sloping tile and all seems in good condition, plenty of sealant around all joins.

I 'spose that only leaves the paint, if its lifted slightly I suspect it might be allowing water to seep through and sit against the wall and soak in. I don't know what condition the wall is underneath but the finish is quite neat so can't be that bad!