Dampness around bay window

[Gees]

I have a 110 year old house and have recently noticed a damp area next to the woodwork on the inside of my bay window from about 2" above the skirting board to about 4 feet up the wall. It extends about 5" into the plaster running away from the woodwork. In other words a 5" vertical strip next to the woodwork. The bottom bit next to the skirting board is bone dry! I have lived here for 20 years and have not had this trouble before. I suspect a leak from above dripping down the cavity and somehow wetting the plaster!
Would cavity wall insulation ( to fill up the bay window voids)help?

 

[Gees]

Further investigation ( I pulled the plaster off ) has revealed what appears to be absolutely bone dry bricks underneath the damp area of plaster. The plaster was non stuck to the brickwork at all, it was al loose and dropped off in big bits.I can see no way for the plaster to have got to be damp.

Next question. I have not got a "dampness" Meter available but do have a very accurate digital multimeter ( a fluke 21 )

can someone tell me what sort of resistance reading should be between two points, lets say one inch appart.

I get from infinity at the best to 10 megohms at the worst, but dont know if the lower reading is good or bad???

 

[Gees]

Problem Solved... Stripped off all the damp plaster and rendered area with mortar containing damp resistant chemicals ( B&q ). Skim over the top and Now BONE DRY!!