In article ,
The Natural Philosopher writes:

I think we know know why...the real discussion issue is 'what is the
simplest way to fix it?

Yes, but at two levels.

The first is what can I do quickly (like in half a day) to dry out
the loft and allow the rafters to start drying. I won't be able to do
a long-term fix until later in the year.

I might try something with the dehumidifier, such as having it upstairs
and discharging into the loft, but I need to do some measurements to
make sure it extracts enough moisture so it doesn't make the situation
worse, and I didn't have a humidity meter on me when I was there last
time. (Unfortunately it won't work in the loft as it's too cold for it.)

Second is what's a proper long term fix (which isn't too much work of
course;-) This requires proper diagnosis, since fixing the wrong
problem won't do any good.

It looks like moisture through the lath and plaster might be significant.
So, what's the best way to do this? I could probably roll back the
insulation to get to the laths (it runs between the joists, and then
another layer across the top of the joists).

Could I simply buy some of the thinner damp proof membrane and lay
this over the laths, or is there a better material? I have a large
roll of non-breathable sarking which might work for this.

Can it go up and over each ceiling joist? Cutting it to fit between
the joists would leave long unsealed stretches along the joist edges,
defeating the object of trying to seal. However, if it goes up and
over each joist, the joist tops will be sealed cold spots (with only
one layer of insulation over).

Another option is replacing the ceilings, but taking down two large
lath and plaster ceilings is not something I like the idea of, and
overboarding always seems to me like a bit of a bodge.

--
Andrew Gabriel
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