The Natural Philosopher wrote:
Andy Dingley wrote:
On 8 Mar, 21:34, The Natural Philosopher wrote:

In modern houses, (post the ark basically) there should be a waterproof
outer coating, and the indside breaths to lower any moisture, and you
have ventilation to exhaust it from

If we were talking about "modern" houses, we either wouldn't be having
this conversation, or we'd be asking how to sue the builder for
getting it wrong. "Modern" in this sense is really quite modern, and
much newer than where many (most?) of us live. It's certainly far more
recent than the Ark.

When I said post the Ark, I sort of had a reason.

Meows stuff applies to soft brick or timber houses with lime mortar and
no DPC.

Once you move to portland cement, the mortar is more or less waterproof
compared with lime, and the bricks have to be strong enough to be used
with it.

It was about the same time that relatively hard brick and portland
cement came in that DPC's were also becoming used, and tee design of
houses shifted from allowing water to get out, to stopping it getting in.

Use of open fires also tended to remove internal moisture adequately.


Given a proper overhanging roof and drip boards around windows, such
houses are perfectly dry in the absence of strong driving rain or other
persistent soaking.

Us of a impermeable coating on such a house is either decorative, there
to act as a barrier in the presence of driving rain, or possibly to
prevent spalling when frost hits a slightly prmeapble surface of poor
brick. Its often applied where frost damage has already happened.

Needless to say if the ingress of water is elsewhere than via the
coating or cracks in it, the coating merely makes the symptoms worse.

Hence th myth that applying such coatings makes the PROBLEM worse. It
doesn't. Its simply makes it more obvious.

I repeat, in a post 1900 style house, control of damp by structural
breathing is the wrong approach. The structure isn't permeable enough
with or without coating.

You need to fix the leaks in the impermeable surfaces..and generally
those are failed guttering, timberwork, lead valleys and the like. Or
bridged DPC's.

Condensation inside is curd by ventilation, insulation, and heating.


Even modern bricks are not impermeable, and cement mortar used in
brickwork ditto.

The house wall being discussed is non-cavity, so the wall:
a) conducts water/damp through it
b) has no cavity surface from which to evaporate damp
c) is much more likely to experience transient surface condensation
d) due to all the above has a much higher risk of water problems than
a modern cavity wall
e) Has warm humid air on the inside and often cold air outside, which
routinely is _below_ the dew point of the interior air.

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.

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.

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.


NT