damp under bay window.

20 Jan 2015
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United Kingdom
Whenever it rains the wall under my bay window gets very damp. First thing I did was check the sealant all around the windows. Seems to be "ok" i.e. no holes/gaps etc but I think it has never been renewed since the windows were put in 20/25 years ago.

I noticed the sealant that was in the gap between the exterior window sill and the frame was rock hard and cracked so I spent about 30 seconds removing that yesterday (it was that well adhered obviously) and replaced all that.

So my question is can old sealant be the cause of this i.e just because there aren't any visible gaps or cracks in the sealant it could still be the cause and I should rip it all out and replace.

Secondly (and I don't know if this is just a coincidence) the sill outside is concrete and back in the summer I noticed some of the paint on the sill was flaking and bubbling so I removed a large patch of it and then just forgot about it so there's currently a large patch of sill that's just bare concrete. Also lots of other little patches around the sill that are bare concrete. Could this be contributing i.e. do you actually need to paint a concrete sill or will this not have any bearing on the damp problem?

I don't have problems with condensation and I ventilate the rooms as often as possible when the weather is nice. I also have a dehumidifier running for another damp problem upstairs with another window (but that window definitely needs replacing). The walls are cavity if that helps and the outside is rendered with no signs of any holes/cracks etc.

The bay window above it in the bedroom (fingers crossed) is fine, no signs of damp there. Roof has been checked and is fine, gutters were cleaned at the end of the summer but weren't full. There's no other damp above the window just literally below which makes me think its more to do with the windows than roof etc.
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