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,
Owain writes
On 20 Dec, 20:33, Onetap wrote:
* BSRIA (I think it was) published a research paper on condensation
the underside of steel roofs. It was a common occurence in 1970s
buildings. The metal roofs radiate a lot of energy into the sky on a
clear night, causing them to cool far below the ambient air
temperature.

This is intriguing. Can a *black body sink* extract energy in this way?

There's some form of, I presume spray-on, anti-condensation treatment
available for metal shipping containers.

It is possible to buy roofing sheet with the anti-condensation finish
pre-applied. I've not seen any but believe it is a *felt* which retains
the condensate until it is evaporated by the next bit of sunshine. This
is said to be the way cement fibre roofing works. Retro-fitting might
not be practical.

I doubt Andrew has funds or authority to renew the entire roof. One
issue will be that the roof sheets may be secured to purlins rather than
rafters so attaching membrane to the underside will tend to run water
down to the timber. I have heard that trapping a layer of relatively
static air can work to minimise condensation but at the risk of
encouraging rot in the wood:-(

More ventilation? The moisture content needs to be below the level at
which *dew* forms or zero in a frost!

regards



--
Tim Lamb