I've had condensation in the well insulated loft of my house each year since buying it new 12 years ago. This year it was very severe and resulted in extensive mould/mildew. The problem was investigated twice by NHBC surveyors during the 10 year guarantee period and on each occasion their written report stated the roof space ventilation complied with requirements and condensation was normal during cold periods. A further very recent inspection by a local builder revealed defective construction of the roof has caused rafter ventilator trays to be crushed and completely sealed between the top of the brickwork and the plywood panel supporting the leading edge of the tiles, i.e. there is no gap to allow air flow into the roof space. About 80% of the ventilators on the South facing front of the building are affected in this way and approximately 20% on the North facing side. The condensation is predominantly on the the North side of the roof when it occurs. I've now been advised the only solution is to fit roof vent tiles on the South facing side, but I need advice regarding how many and where they should be fitted - low down the roof to attempt to achieve the originally intended crossflow or high level to allow the rising warmer air to escape. Any help will be greatly appreciated.