Hole in wall underneath window

Quite extaordinary, if there is no inner skin damp will be able to travel directly onto the back of what ever has been used to create an inner skin, this sounds like studding as it is explained. I would advise a Bricklayer with experience to review what you have exposed and advise you on the best corrective methods. Remember damp can be quite insidious, revealing itself only after the problem is well established, smell, the appearance of mould,(and their spoors) together with rot attack are usually sure signs and can be expensive if left unattended to. Even so I really do reccomend a serious survey from your builder be arranged as soon as may-be. Kindest Regards, Truebuild.