DIY Doctor

Ventilating the loft - sofit round venting discs

Postby Stu Magoo » Tue Dec 16, 2008 10:04 pm

I have recently moved into a house which has a slight problem with condensation in the loft space. The house is detached and was built in 1976. The original wooden facsia and sofits appear to have been replaced a couple of years ago with white upvc which is sealed and has no ventilation. This was highlighted in our building survey as the likely cause of the condensation. The sofits are tongue and groove effect and I can cut out holes in these to insert 70mm round venting discs. The house is rectangular in shape about 12.5 Meters X 8.2 Meters with a conventional tiled roof. I just have a couple of questions which I'm hoping someone can answer.
1) Should I insert these discs on all four sides or just the two longest sides in order to get a cross flow of air ? - Is it possible to over ventilate the loft ?
2) How far apart should I space the venting discs ?

I would be greatful for any advice
Stu Magoo
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DIY asbestos test kits

Postby stoneyboy » Wed Dec 17, 2008 10:38 pm

Stu Magoo,
Fit the soffit vents along the two long sides. The problem will be finding where the rafter ends are. Aim to fit a vent between every 2nd or 3rd rafter. Don't forget that with the age of your house it is likely that the original soffits you will have to cut through will probably be asbestos cement board. You may find fitting an eaves vent to be a safer solution.
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Postby eljaybee » Thu Dec 18, 2008 4:57 pm

It all depends upon a couple of factors.

How is your roof insulated and what is the pitche of the roof? If traditional ceiling with insulation at this level and roof is over 15 degree pitch then you need to equivalent of a continuous 10mm wide strip at the eaves. If a flat roof or pitched roof but the insulation follows the pitche of the roof then you need equivalent to a continuous 25mm wide strip at the eaves.

What is the free flow area of the circular vents? for example the redland circular vent has a free flow area of 2100mm2

Thus for a 12.5m long eaves you need the free flow vent in a traditional ceiling insulation pitched roof of 125000mm2 thus you will need 60 circular vents equally spaced. ie at approx 200mm centres.
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