Damp external wall under lath and plaster.

23 Mar 2013
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United Kingdom
We have recently moved into an old Victorian property which has not been very well maintained. We are currently renovating the living room and planned to insulate the solid wall internally. We have removed the lath and plaster and the stud work that held the laths and found a few of the bricks are damp (they powder away when you rub them) and the plaster under the laths on the solid wall is crumbling off and cracked (and also the purple coat is powdering away as well).

The wall in question is an external wall and someone has raised the level of the concrete path (outside) alongside this external wall by about a foot, so is probably above the DPC (if there is one). The outside wall has been painted over, I am not sure if this is helpful or if the wall needs to breath to help dry out.

I would like to know what I need to do with this wall, before I proceed with the stud wall insulation. I can think of three options, but would very welcome more ideas or improvements for the best solution??

Pictures are in the Damp Living Room album.

Option 1.
Just attach the stud wall insulation to the wall and hope for the best.

Option 2.
Remove the plaster and attach the stud wall insulation to the bare brick.

Option 3.
Remove the plaster and apply a coat of under plaster and then the fix the stud wall insulation.
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Whatever you do internally will be ineffective unless you tackle the problems outside.

The wall is obviously remaining wet, probably due to the raised path, and also the external paint. With this arrangement, any dampness coming up from the ground (it's unlikely to have a dpc) will be unable to dry off externally and will come through inside instead.
We planned to render the outside. Would this allow the brick to breath if we were to remove the paint first?

Also, what would you suggest is best for the inside for now?
For whatever reason, the wall is obviously damp and you need to find out why, and then rectify that first.
There was plaster under the studwork, which suggests that in the distant past, someone may have tried to rectify a damp problem.
It would be easy to say put up some studding, insulate between with kingspan, and then foil-backed plasterboard.
But after a while, if you haven't rectified the problem, you will get the usual damp smell from behind the wall, and the work will be wasted.
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look at the other walls and determine where the dpc (if any, probably slate) would have been. Look under the floor and find where the air bricks were, they have doubtless been blocked up.

look for the drains, gullies and pipes in the area.

You do know you are going to to have to dig out that raised path, don't you?
We planned on digging it out, so that is good news.. I think

Thank you so much for your advice!

What should I do with the damp plaster that is on the damp brick.

1. Hack it off then fit the insulation straight onto the brick?

2. Hack it off then apply an undercoat of plaster before fitting the insulation?

3. Just dry it on the brick and fit the insulation to the old plaster patching up the large holes?

(all options we will dry off the wall with a dehumidifier and fit the insulation with an air gap that leads to under the suspended floor, as this is where the air bricks feed into.)

This is the path along side the house:

This is he condition of the bricks from the outside and the air brick which has been left open within the concrete path:
I see a downpipe and a wastepipe close to the damp wall. No surprise there.

I won't be surprised if you dig them out and find leaks.
I'd have thought that you'll need to fix the outside first (drop the path below the DPC, restore any blocked ventilation holes, fix any drain leaks etc) and then let the bricks dry out before you do much inside.

If you're insulating the inside wall then you may as well strip all the existing plaster off. Might help dry things out a bit quicker but I'd imagine it may take a while.
What people are saying here is that this is not a job which can be rushed.
At the risk of being repetitive, you need to sort out the damp problem first.
This means lowering the path, checking - and if necessary adding more - airbricks and getting the paint and plaster off the wall.
Then leave adequate time for the wall to dry out before attempting any reistatement; this could take some time.

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