On Mon, 28 Apr 2008 22:42:56 +0100, a particular chimpanzee, The
Natural Philosopher randomly hit the keyboard and produced:

Permeable membrane is not needed in a 'cold roof'

Its there to vent 'warm roof' spaces where the joists are covered and
there is very little movement past them as they are insulated, with a
small air gap.

A "warm roof" is insulated above the rafters and therefore doesn't
need venting (the rafters are held above the dewpoint). Any other
roof is a "cold roof" despite the advertising of the breather membrane
manufacturers.

The void between the insulation and the covering in a cold roof must
be ventilated in some way. If vented through gaps at the eaves and
the ridge any membrane can be used (slater's felt or breather
membrane), otherwise breathable membrane must be used in accordance
with the BBA certificate (taut & counterbattens over or draped, laps
taped or open, etc).
--
Hugo Nebula
"If no-one on the internet wants a piece of this,
just how far from the pack have you strayed?"