On Dec 19, 10:21 pm, (Andrew Gabriel)
wrote:
In article ,
andrew writes:

This cold weather has really shown up a problem with an unheated converted
agricultural implement shed. The building is used to store generators,
lights and small tools and has had a plasterboard ceiling added, to stop
dust and bird crap spoiling the kit.

The gable ends are ventilated and the roof is simple 4 by 2 rafters with
corrugated iron, the corrugations vent at the eaves and the ridge capping.

This roof is plainly a good radiator on cold nights and vapour from ambient
air is condensing, and sometimes freezing, on the inside then dripping onto
the plasterboard below.

I'm considering stapling tyvek in between the rafters or even full length
under them but worry about the effect this will have on old untreated roof
timbers?

Probably not good.

This is a hard one, and it's not simply a question of ventilating
which could even make it worse. As you correctly identify, the roof
is likely to be colder than the (outside) air. Ventilating more
(outside) air under it is just going to create more condensation.
The only circumstance in which ventilation would help is if you
are generating significant moisture inside the building which is
getting into the roof (and this could simply be a moist floor which
is staying warmer at night).

You really need a layer of air-tight insulation on the underside
of the metal roof, with no air gap or circulation inbetween.
That's not viable with roofing timbers in contact with the metal.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

could this be one of the only actually beneficial applications for
that damned Ren-u-therm spray-on urethane crap?

JimK