On Feb 27, 10:16*pm, andrew wrote:
Roger Chapman wrote:
Temperature is only half the equation. Do you have a dehumidifer to put
up there for a bit to see how much water you can extract at say 80%
humidity?

The hypothesis is that the roof is a good radiator and insulated from the
building and ground. Air passes through the roof space. As the air cools
faster than the surroundings barring those that are good radiators and not
able to conduct heat from the ground ( cars and windscreens being examples)
it approaches saturation. This saturated air passes into the roof space and
out the other vents but on the way it contacts the cold roof underside and
reaches its dewpoint. It's a small matter if there is only one air change
in the night, maybe 10 grams per m3 which would evaporate next day, but if
the space is well ventilated...

I've seen Andrew G propose his theory before, and it's a shame in a
way that it may have proven true in practice.

It doesn't seem like letting heat into the loft is seen as a good
thing. It doesn't seem like blocking up all the ventilation is seen as
a good thing. So how else can the loft surfaces be prevented from
acting like a car and its windscreen (and attracting condensation and
frost)?

Or, to put it another way - how come all modern houses (lots of
insulation, lots of ventilation) don't face this problem?

Cheers,
David.