I went exploring my mouldy solid wall this afternoon and couldn't resist hacking off the blown plaster. I was very surprised to discover this black bituminous cardboard underneath, corrugated very much like a radiator, and plastered over. Not perhaps surprisingly, the bricks underneath felt pretty damp (it's a sw facing wall).

I just wondered what the black cardboard was & why it was there? My gut feeling is to hack it all off, leave it for a couple of months to dry out (hey, brick feature wall, anyone?;-), and internally insulate. I've only got room for 25mm of insulation, but it'll prob be better than cardboard (although I did wonder about the airspaces?)

Thoughts on black stuff & putative plan much appreciated :-)

Tania

Ps anyone who replied to my posts on mould; thanks :-) I'm ventilating, heating more & dehumidifying .

probably an attempt to cure damp by ventilating the wall and separating the wall from the room used to see it a lot in basements especially in London, are there any ventilators in the skirting board or to under the floor?

Thanks Fostertom. The name was on the tip of my brain!

The ridges are a key for plaster. There's another similar product out there, too.

and to provide channels for air movement/water drainage behind the finished plaster

As already said it's main use was for tanking of basements, many early conversions /renovations in Bath used this method.

My Dad covered a garden shed in it, it survives to this day, just.

Figure out why the wall is wet before looking for a solution.

If it's a single brick wall in an exposed location companies like Celotex/Kingspan typically recommend installing a membrane before insulation. See their sdvice on garage conversions.

Ah! I may have discovered at least one reason why the walls is damp. The render on the other side has been cracked for a while & I was assured that it was fine. However, it looks like its now cracked along the "score lines" (it has a mock block effect), to the point where there a thin line of moss growing out. That's not good, is it?!
Given my intention to build an extension over this bit of wall in the next coupld of years, I'm really not sure what to do next; membrane & iwi? I don't feeling leaving as is is a great option (I've only hacked a small patch off).

Thanks :-))

Posted By: nbisharaAh! I may have discovered at least one reason why the walls is damp. The render on the other side has been cracked for a while & I was assured that it was fine. However, it looks like its now cracked along the "score lines" (it has a mock block effect), to the point where there a thin line of moss growing out. That's not good, is it?!
Given my intention to build an extension over this bit of wall in the next coupld of years, I'm really not sure what to do next; membrane & iwi? I don't feeling leaving as is is a great option (I've only hacked a small patch off).

Not sure that sticking polystrene on the outside is the best thing to do here at this stage as you are talking about a lot of work which then needs undone at a later date, the water at this stage will be getting in along the render cracks (especially if the cracks are big enough to be growing moss) and tracking down and soaking the inside of the wall. One way or another the render needs to come off where it has cracked and be re-rendered.

Heavy gauge plastic sheet fixed to wall above problem area with large seam of mastic and battened or even chased into the wall with mastic bead ? I suppose it could depend on how soon you intend to start the extension build, if it's soon seems a waste to prepare and re-render

How much driving rain etc. does the wall get? (location? relative exposure? prevailing wind? sun? height?) Any leaky (or missing) gutters etc?

A piccy might make things clearer.