Chris Plastow
This is condensation only, not rain water penetration. The humidity within the property is too high and or the wall is too cold. A de humidifier is an immediate answer but expensive to run. Insulating the wall is good but won't help the windows. Installing a HRV150-12jour fan should fix it. Google it. It's all about ventilation! Clean off the mould immediately or it will get into everything and smell very bad!
