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.
You will need the equivalent area of a 10mm gap along the eaves edge, you will also need high and low roof vents to ensure a flow of air through the loft. It may be cheaper to fit ridge vents in place of the high level vents.
stoneyboy, thanks for your advice. It looks as though quite a few vent tiles will be needed. The problem is mainly on one side of the property and I wondered whether the corners of the bricks on which the flattened ventilators sit could be relieved by 10mm to open up the ventilators. If I had UPVC soffits and facias fitted (which I was thinking of doing anyway) the "modified corners of the bricks wouldn't be visible. It looks a relatively straightforward thing to do but I'm not a tradesman and it may be much more difficult than I think.
Since you have had problems with the low level ventilation I would suggest you stick with tile vents, the only down side is that they are relatively expensive and they need fitting in the roof.
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