terry wrote:

Ben: It sounds as though your attic is not ventilated correctly?
Assuming your house is has insulated ceilings with an appropriate and
sealed vapour barrier on the warm side of the insulation ?
The condition could be serious and cause rot/mould.
Normally what is happening is that warm and therefore moist air from
the house, some of which will inevitably leak into the attic is
condensing on the cold nail tips.

The requirement here, for example, is that there be venting of 0.3% of
the floor area and that it be cross ventilation.
That means that for each 1000 square feeta of area there be a minimum
of 3 square feet of ventilation (not all in one place) but spread
around as soffit vents, vents in the eaves or even roof vents.
If you have insulated ceilings make sure the insulation does not get
wet from such drips. Wet insulation is not only useless it can cause
rot and bad ceilings.

Small note on the "square feet of ventilation." I believe that figure is the
unrestricted area of the ventilation port. If the ventilation ports are
screened, you should take the blockage of the screening material into
account. The finer the screen - say the same material used on a window
screen - the less open space available for ventilation.

So, then: If you have window screen type coverings, I'd double the minimum
size requirement. If using chicken wire, you could probably forget about a
fudge-factor.

If you get bogged down in the calculations, you can remember the fall-back
rule: You can't have too much soffit ventilation.