Just adding to my attic insulation, and I notice that there is no vapour barrier under the existing stuff which has been there for c.25 years. There seems to have been no deterioration in the 25 year old stuff.
Should I now add a barrier or just leave things be ?
It's not required. The amount of moisture which will come through a ceiling (barring outright holes which should be filled) is negligable compared with the ability of even a small amount of loft ventilation to to disperse it. Moisture in lofts is caused by holes in the ceiling (e.g. downlighters, around pipes, etc), uncovered loft tanks, leaks in the roof, all combined with very poor loft ventilation.
Go down the pub
Is the house 25 years old? Does it have ventilation at the eaves. We bought a new house in 1979 which did not have adequate ventilation of the roof. For some reason or other, I had need to go in the loft during frosty weather and found the roof glistening with frozen moisture! If I had stayed there, I would have drilled holes in the soffits to ventilate it. We bought another (new) house in 1985 which has adequate ventilation at the eves and is OK. It was a known problem with houses in the 70's, I seem to recollect.
Mine was similar, 1978. It was the fashion of the 70s to seal lofts, resulting in condensation. I've taken out the bricks on top of the wall plate used to seal under the felt and fitted soffit ventilators, and I no longer get the drips from the felt.
Mine has no obvious source of ventilation, but I'm yet to see any condensation in it.
Does anyone know what a high humidity % would be for a loft so I can measure it?
Steve
In the cols frosty winters around 1980, water dripped from the felt, ruining papers stored up there. It is a low pitched roof. I haven't had any noticable problems since the ventilation was added.
I never measured, but I'd be worried at anything above 80%.
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