e) Has warm humid air on the inside and often cold air outside, which
routinely is _below_ the dew point of the interior air.

I would venture to suggest that, with central heating, most homes have
warm, dry air on the inside, especially in winter.

And now, its being proposed to add a vapour barrier *on the outside*
of this wall. That simply isnt an effective solution. To use that
approach is to misunderstand how water is handled by such a wall.

No surface treatment is ever a "vapour" barrier. It isn't like covering
your house in cling film, it simply acts as a barrier to water droplets,
which is a completely different issue. Take a piece of new wood and
allow the rain to pour down on it for a couple of weeks. On planing you
will find that only the surface is wet. However, that doesn't mean
moisture isn't going in and out in the form of vapour, and there is no
way to prevent that. There are figures somewhere detailing the vapour
permeability of various surface coatings, and IIRC they are way up the
scale.

SPAB's advice re avoiding impermeable coatings is not aimed at huge
rambling residences, its aimed at walls that dont conform to modern
construction standards, ie dpc & cavity. SPAB's advice re lime is a
different topic aimed at not exactly the same types of wall.

It might help if SPAB defined "impermeable" in terms of coatings. AFAIK
there is no such thing short of full scale encapsulation. The siloxane
type products outlined earlier in this thread don't form a film at all,
so describing them as a "coating" is very wide of the mark.

And far from SPAB's advice being 'obsession', they actually are the
experts on this, and have learnt this stuff from both a huge amount of
experience and understanding the theory behind it. For someone to
claim they know better than spab simply because thats how it used to
be popularly done and they dont see what the problem is, is dubious at
best.

I don't mind listening to experts, even the self-appointed ones, but I
stop when their arguments cease to make sense.