Mystery water seeping into wall somewhere. Builder and plumber not sure why?

Dear IMH, this sounds at first read like a classic case of interstitial condensation. This is increasingly becoming a major problem as roofs have membrane installed, people have double glazing and insulation added to their buildings, without adding adequate ventilation to the roof space. In basic terms the warm moist air ( from showers, cooking and even breathing ) rises through the building to the roof where it cannot penetrate the 'Breathing' membrane. The moisture condenses at the dew point ( this varies through the day/night and seasonally). The condensation runs down the underside of the membrane in til it reaches a purlin or wall plate wher it then finds the easiest path down. It will reappear at random points in a wall where an obstruction to the cavity provides a bridge. The inner wall being generally dryer draws this moisture through capillary action to the warm room. Voila, running water in an otherwise dry wall. The answer most commonly is to force ventilation into the roof space by installing air bricks in a gable end or rotary ventilators to the ridge. I hope this is helpful. Dear IMH, soffit venting is another issue, it is generally lower than the wall plate on which the roof beams sit, and the soffit is below the dew point. Both these factors are crucial in detecting interstitial condensation. Furthermore, if you have generous insulation laid to the roof space this will drastically reduce the air circulation unless you have left a gap of 200mm at each rafter end. This unfortunately would render the insulation less effective. The introduction of air bricks to the gable ends is specifically so that the air movement is both unrestricted and occurs 'outside' the insulation blanket. As for the 20 year delay, this is not unusual, you need to bear in mind that every day you contribute to the total mass is water vapour contained in your building. The rate of vapour release is then determined by the structure, i.e., render with modern mortar or a breathing lime mortar, any masonry paint that has been used, even down to the type of tiles on the roof and the interbalance with your heating system. All these factors we now know change the balance of moisture retention in a property. A simple test, is have you had any cabinets or wardrobes against the walls that now show signs of damp ? And did the backs of the units show signs of mould when moved ? Good luck Carl

condensation, poor ventilation.