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Vicki Cowan

I’d be interested if Gleb could comment also – the roof space problem Sally referred to is something we talked about in our conference call the other day.  It’s something I’ve heard about occurring in a number of situations – as Ian says as a result of the temperature of the cavity being lowered on cold nights causing condensation on the metal which then wets the insulation and this wicks through to the ceiling gib.  Sadly not unlikely, it does happen.

Another more common cause can also be damp subfloors -particularly with brick veneer where often the cavity was left open between the subfloor and the roof space – the water wicks all the way up to the roof space – and you see mould growing on the ceiling and top of the external walls – but its actually caused by a damp underfloor.

This can happen in other types of cladding situations also, where holes were drilled in the bottom and top plates at the time of construction to help green timber dry, or you can just get a stack effect where ground moisture can be drawn up through open paths in the wall cavity into the ceiling space.

So the ceiling can look OK – but the problem is actually one of rising damp from the subfloor.  Given about half of our subfloors are underventilated (BRANZ House Condition Survey), and that even a dry subfloor produces 45 litres of moisture a day (BRANZ data) – and more than 180 litres/day if there’s ponded water under the house, subfloors can often cause quite major ceiling and wall mould problems.  The trick is to look down not up for the cause!