Shed_Fiddler fired this volley in
. au:

Ideas ???

Well, start with basics. The beam itself is becoming cooler than the
dewpoint of the air.

Since that beam is close to a door, one can construe that it's close to a
wall... and perhaps _itself_ not insulated from the outside air
temperature at night.

If I were looking for the problem, I'd be looking for a thermal path to
that beam from outside. Perhaps the cladding is screwed right to the
beam? Perhaps the eaves are venting up directly across the top of that
beam (between roof and insulation sheets), and once the air has removed
the heat from that beam, it's warm enough and diffuse enough not to cool
other beams it passes on the way to the peak? Or perhaps once having
passed that beam, the air is mixed with other inside air, and thus is not
so cool anymore.

There are all sorts of ways to imagine that happening, but the simple
fact is that the one beam is getting cooled to below the dew point, while
the others are not.

LLoyd