In article ,
Roger Chapman writes:

Does this sound plausible?

Yes. (But see below).

Any bright ideas on fixing it, apart from rolling up the loft insulation
and blocking off the new air vents? It seems to me such a roof is
dependent on heat leaking from the house.

Are you sure that there is no leakage of air from the main part of the
house into the loft. Your increased ventilation may have the affect of
providing a pressure difference between house and loft and an influx of
warm humid air into a cold space will certainly cause condensation.

There are no holes punched through the ceiling. The loft hatch
and ceiling isn't going to be completely air-tight of course, but
it's no worse than it was for previous 25 years. Nothing has changed
in the house either regards producing moisture.

There will be some slight transfer of heat to the loft however deep you
make the insulation so I have my doubts about the roof managing to
radiate down to below ambient. Grass frosts typically occur below 3C but
the grass is almost isolated from the ground and even with the limited
heat coming up from the earth air temperature above bare ground is much
closer to zero before that attracts a frost coating.

With a similar problem in the past I have used EasyLog data loggers to
establish the extent of the problem but that was transient rather than
continuous through the winter. (EL-USB-lite IIRC).

The house has temperature logging, but not in the loft. So, I just
added a sensor in the loft and I'll compare it with the outside sensor.
Having come out of the loft and closed the hatch, the loft temperature
is rapidly dropping towards outside temperature, but too early to know
where it will settle yet.

It looks like we might get a couple of nights at 0C this week, but not
with clear skys, but I'll see what readings I get.

--
Andrew Gabriel
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