Best way to deal with damp wall under Victorian sash

[nishnish]

Hi,
I've pulled the radiator off a wall under an original Victorian square bay sash.

Untitled

• nishnish
• 19 Nov 2011
• 1

Untitled

• nishnish
• 19 Nov 2011
• 1

The plaster behind is damp. Smells a lot since being disturbed and is pretty loose in a lot of places.

Untitled

• nishnish
• 19 Nov 2011
• 1

I'm wondering how best to deal with this.
The room will at some point be completely redone to modern standards with 25x50mm battens plus 50mm of celotex and plasterboard.
However, currently I just want to fix this area under the window to make the room habitable.
I was planning on removing all the plaster under the windows, then battens with plasterboard as a quick fix.
This will need some kind of vapour control to prevent further room moisture hitting this wall?
The wall outside is painted which probably contributes to this damp?
The window frame above is also rotten.
(It's turned into two questions in one, so I apologise. However, I think they are related enough to be in one post.)
We are having the windows replaced with uPVC over the next couple of months. The fitters recommended keeping the inner part of the wood, "to retain original features."
I'm not so sure. The frame on the outside is rotten at the bottom and needs attention. Inside they are in ok condition facing the room, but not mortared in well.
The screwdriver is 8in long:

Untitled

• nishnish
• 19 Nov 2011
• 1

And has popped out of the front:

Untitled

• nishnish
• 19 Nov 2011
• 1

Untitled

• nishnish
• 19 Nov 2011
• 1

You can see they haven't been painted in a long time and the only seal was some crappy mortar.
I knew the outside was rotten, but didn't realise it was that bad. Doesn't show in pic, but it looks like wet rot along with woodworm.
So in short, would you bother keeping those sash linings, considering their condition? Will they be prohibitively expensive to fix?
Will the quick fix of battens and plasterboard with vapour control be enough to prevent the damp returning?
Thanks for making it here

 

[JohnD]

what is the damp wall made of, under the window?

you have to cut out all the rotten stuff, and treat what remains with preservative.

is there a dpc? Is water penetrating round the frame (could be condensation)?

is earth or paving raised against the outside wall?

does the gutter/downpipe leak?

 

[nishnish]

I've added a larger scale pic of outside.

what is the damp wall made of, under the window?

Brick, painted. It steps out at the bottom, with some kind of render on the front part of it.

you have to cut out all the rotten stuff, and treat what remains with preservative.

That could be quite a bit of work? Easier to simply remove the lot? Well, I'd like to keep it, but not if it's gonna cost a lot. If kept, there would also be modification required to the inside to accommodate 50mm of celotex at some point.

is there a dpc? Is water penetrating round the frame (could be condensation)?

No DPC. The paint has decayed along with the mortar 'seal'. Basically bare wood exposed to the elements.

is earth or paving raised against the outside wall?

Not especially. There are air bricks there ventilating the cellar underneath.

does the gutter/downpipe leak?

Don't think so.
Thanks.

 

[nishnish]

A little more prodding with a screwdriver, reveals this:

Untitled

• nishnish
• 19 Nov 2011
• 1

Untitled

• nishnish
• 19 Nov 2011
• 1

Those big chunks simply came away in my hand. The missing mortar around the bricks in the middle was not scraped out by me. There was just nothing there behind the sand/brick-bit/dust combination which was filling the void. Those holes go back 7in. This is likely a cause of damp plaster? Just a pinch of lime mortar and air between plaster and outside.
It's clear that the whole area needs re-pointing/re-building. I won't do it all at once of course I don't fancy those big stones being unsupported.
What's a good way to rebuild the finish?
There is about 25mm gap where the wall surface should be and the bricks. This seems pretty constant from top of skirting to edge of sash lining:

Untitled

• nishnish
• 19 Nov 2011
• 1

You can see another area of wood worm with dust at top of pic.
The gap between the wall surface and the stone is about 70-80mm. The Victorians had filled the space left by the stone with little chunks of brick and mortar; tut tut. Do I need bother filling that gap with mortar or the like?
Splitting lots of bricks the correct thickness would be a right pain. Would thick porous tiles be a suitable material to use?
Or will some celotex do?
Thanks.