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)
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