Stephen Tuckey
Sounds like a condensation problem.Never easy to get rid of.Certainly treating with bleach or mould remover will give temporary solution. But you need to get to the root cause of problem which is caused by warm moisture laden area cooling down and condensing on cold surface where air circulation is at its lowest ie in corners or/and behind large objects of furniture. Letting more cold air into room is unlikely to help,if a bathroom is nearbye keeping door shut until steam clears could ! The most practical thing is to raise surface temp by insulating in cavity walls,if no cavity then try thermal wall paper liner. Other things to help Raise air temp. Increase ventilation to that area Reduce moisture content of air Avoid flash heating,keep temp more constant Steve Tuckey of Swiftsure
