Black tarry substance on bedroom wall

[Allied]

My bedroom (outside wall) always has a lot of condensation and wallpaper never sticks under the window. Doesn't matter how well ventilated. At some point years ago, a 'friend' painted a black 'tarry' substance on the interior of that wall to stop the damp, but it just grows white mold on it. How can I get rid of the black substance and stop the condensation?

[buildersmate]

The black tarry substance will be some bitumen-based compound that your friend midguidedly thought would act as a block to passage of water. To get rid of it you will need a spirit-based compound - start with white spirit, though it will be a messy job.
Although the wall may be damp because some water is penetrating from the ground below, condensation is caused because the wall is cold and to solve this you need to find a way of insulating it. A product like this does it on solid walls - it forms a thermal barrier sandwich on the wall.
http://www.celotex.co...asonry-walls-External
(see the picture on the RHS that shows the construction) but unless you are handy you will need the help of a builder to do it for you.

[johnk]

Take a look under the windowsill outside and see if there are any holes or cracks allowing water in and check the drip grooves on the underside of the sill to make sure it's there or something else is to stop water running towards the wall

[johnk]

Also take a look at your gutters, make sure the aren't blocked and do drop towards the downspout. Also see if they leak, best done in heavy rain or put a hose in there

[The Builder]

The only thing I have to add to what the others have said, is to go with BM's link on insulating. Quite honestly, I wouldn't even bother to remove the bitumen. Screw 50x50 treated battens to the wall, cut Celotex between the battens, cover with thin vapour check, plasterboard and skim.
If you "dry joint" the board, (filling the joints and spongeing them over), then you won't even have to skim.

[Allied]

Thank you so much for your answers. The bedroom window is the width of the wall and has a large radiator below it, so I don't think the problem is too cold. Can't see anywhere that water is getting in, but the house is pebbledashed. Also would covering the bitumen compound with celotex be a problem bearing in mind that white mold grows on the bitumen?

[Allied]

I have just looked at the celotex site and realised it goes on the exterior of the walls - can't do that as the house is pebbledashed - Any ideas for interior treatment other that getting rid of the compound?

[The Builder]

No nooo ........... I'm talking about Celotex on the inside of an outside wall. Insulation will always improve things.......... provided there's a certain amount of background ventilation as well.
The only white mould I can think of (assuming it's not Dry Rot) is Cellar Fungus, which usually shows presence of spores as well.
Otherwise, it's "salt migration" ....... usually found on exterior brickwork.
If the bitumen is porous (maybe not thick enough), then damp from outside could easily migrate through the wall to the inside surface.
Dry-lining with insulation and vapour check is a standard procedure, but only one of many.
Lime plaster would solve your problem, but give no insulation benefit.

[Allied]

Thank you so much 'The Builder' for making things clearer. I will do as you suggested in your first reply to me. Thanks to the rest of you as well.