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About 2 days or so after its been raining I get a large damp patch appear on my concrete floor directly under the UPVC window of the bedroom in my 1960's bungalow (carpet removed due to decoration works) this damp patch is the exact width of the window and extends into the room about 6 feet. It looks like it starts from where the wall and floor meet under the window. after a few days of no rain it drys up totally, the walls and skirting boards never get damp.
The cavity has been filled with the white fluff insulation, if that makes any difference.
The bungalow is built up from the outside ground level by about 2 feet.

Any suggestions how to fix?

Possible causes I have come up with are:

Water coming under window and running down cavity, but I have a sill made of roofing tiles that extend up past the window, so water would have to run uphill??

Water working its way around the sides of the window through the slightly cracked render and running down the cavity, unlikely due to the damp spot looking like its starts from the centre of the window

Bridged cavity at floor level either by insulation or by something else (although why would this just start all of a sudden)

DPM failure

Hope someone can help, before I start making holes and a mess trying to find the cause, Would much rather sort the problem rather then just cover it up with a waterproof coating

Buy a damp meter and check the wall under the window, then do the same on the other walls and compare.

You may not realise but, the wind can blow rain uphill and those slates may not stop the water from entering the wall.

As you say the rain may be blowing in down the side of the window, just because something never happened it doesn't mean it won't.

Is the wall exposed to the prevailing weather?

If it is, then the rain may be forced through the wall by wind pressure, and may be crossing the cavity via the filling or the wall ties.

Unfortunately things were not always built well in the past and you may find it best to remove the inner wall to see what they did inside.

May I mention that with my bungalow whoever made the roof a hand made job, did it very well, the bricklaying, chimneys, damp proof course etc the work was very poor.

thanks for the reply perry,
yeah the wall does get the force of the weather, the outside render does need a bit of attention, its on the list of things to do but think its time it moved up that list abit.
I'll try everything else before taking the inner skin of wall down, had thought that i might have to tho.
I'll let you know

you can drill the wall with a 10mm masonry bit and hire a borescope fibre optic camera to see into the cavity.
Spray the outside wall with a hose to check where its penitrating.