damp problem in bedroom - would a humidity reading confirm damp isn’t caused by condensation

There are 3 types of damp in properties - Rising, Penetrating and Condensation. With the first 2 types moisture must pass through the underlying ground and/or brick/sand/plaster before it arrives on the internal wall surface and will consequently always leave a watermark (tea/coffee coloured stain) and/or a white mineral deposit. Condensation is different as it originates in the air and does not leave a watermark or mineral deposit. It is therefore simple to visually distinguish between these types and therefore tell if your problem is humidity related. Also, Rising will never be seen above the ground floor (because of gravity). Please note whilst it is possible the cause of the cold spots on your internal wall is water ingress caused by failing pointing and/or missing tiles outside it is more likely (and common) that the outside walls are simply the coldest surfaces in your bedroom and the cause of the mould is humidity related. A damp meter used in the vicinity of the mould growth will indicate the extent of dampness inside the wall and a hygrometer will tell you the relative humidity level but please note the level is always changing as temperatures rise and fall. Best time to take humidity readings (to give the most accurate lows during the danger period for mould growth) is late at night after heating has been turned off.