In message , The Natural
Philosopher writes

I am interested in the purpose of the permeable paper?

So was I.

It seems to be that water will run down the outside of it, but wind
will not go through it, and moisture can, in time, percolate through it
from inside the walls.

Right.

I.e. it ensures that there will be no build up of moisture inside the
walls, whilst providing a showerproof coating.

My walls are

- 12x12 oak frame infilled with 7x3 studwork. On a double brick cavity
wall with rockwool battens inside.


Then layers from the outside:-

Render over mesh, onto 1" battens (airgap behind the render, open at
the base to allow any possible water ingress to drip out I suppose.
Then building paper, then 12" waterproof ply. Then the studwork, with
rockwool filled cavity to foil backed 12mm plasterboard laid over the
softwood studs and TO the oak frame.

When my farmhouse was *re-modelled* the specification was ....feather
edge or cement render on expanded metal over 25mm foil backed insulation
between battens. Then a vapour barrier over the original render (they
had to grit blast off the paint to restore permeability). The 4" stud
work and lath and plaster were left alone. I doubt it would meet current
regulations but it is !"£$%^& warmer than it was:-) I claim to have
originated the tale of the hot water bottle that froze on the bedroom
floor.

In an ideal life I'd have used celotex for even better insulation.

This construction entirely satisfied 2000 building regulations.

The structural strength given by the ply cladding is amazing.

12"?

The only drawback is you can feel the whole frame tremble when someone
slams a (very heavy solid oak) exterior door :-)

Oh. I have been considering an internal steel frame to strengthen a 2
storey barn and wonder if this might be an issue.

The exterior could have as easily have had weatherboards instead of the
render/mesh. There is no structural strength in the render - its purely
there to keep he driving rain off.

Huh! I am rebuilding a redundant cowshed and have found that the
internal render was actually supporting the roof. Around WWII the
ministry of agriculture decreed that dairies should have a white,
washable internal surface. This was most easily achieved by nailing up
mesh, slapping on a coat of the hardest render possible and giving it a
coat of whitewash. What nobody foresaw was the effect of shutting off
ventilation from untreated timber. The Oak studs and sole plate are more
or less intact but the wall plate (pine) has simply disappeared.

regards

--
Tim Lamb