Cement Render, Extension, Mould, Solutions needed

[Frank1990]

Hi all -- really looking for some sound advice for an annoyingly persistent problem!

Situation.
We've had an ongoing issue with mouldy/damp patches in our bedroom since we moved in 3 years ago. They are around a plug socket and mid wall. It is an old property but the bedroom in question is a newer extension, with no cavity and 2 of the walls are external. It has a cracked cement render around the two external walls. In keeping with some of the other things in the flat, the builders who did this extension have done a shoddy job and is not within building regs.

There are some small strips of exposed concrete from installing new windows.

Cause?
We've had several professionals in to offer advice and have all given different theories and solutions. A damp company said that the patches weren't consistent with rising damp and that it is likely to be from condensation. Others have recommended a DPC. Some have suggested sealing cracks and waterproofing render.

Solutions?
We have painted the room in anti mould paint to try and combat some of the mould problems and it has certainly helped, but we've found that there are still patches of silt/salt and mould in corners. The paint had cracked in some areas of the wall too.

I scraped away recent patches right down to the plaster (found no mist coat...). I applied a damp seal, 2 layers of joint compound and re-painted.

We installed an ultravent air brick to encourage airflow.

We bought a hydrometer to measure humidity. Whilst opening the windows definitely helps, humidity increases a lot when it rains, which makes me think it might be more than airflow/condensation issues.

We haven't dealt with the root cause of the problem yet though.

Next steps...
This is where I'd like some advice!
I'm considering filling the cracks on render and applying an waterproof membrane?
Should we get the cement render completely replaced? With what?
Should we get a DPC even if it v unlikely to be rising damp?
We have applied for external insulation through the greener home grant scheme - it hasn't happened yet, but questioning whether this will offer any solutions?

 

[^woody^]

That internal condition is indicative of penetrating damp, and in that location could be rising damp too, or both.

Cracked render lets water in and holds it in, and it may well run down the wall and settle on the DPC to give the appearance of rising dampness.

The render needs renewing as it's difficult to successfully repair it.

 

[KenGMac]

Agree with "woody" above.

Because of the visual indications that is why the Specialists you have appointed cannot agree on a single definitive causation

Could I suggest you investigate the external render a bit further? using a short pole or similar, tap the render around the areas of the cracks, if it sounds "Hollow" then that is an indication that the render has / is de-bonding from the brick, indeed there is some visual evidence in the images posted that one area of render has risen slightly above the level of the render on the other side of the crack???

As a very, very temporary "fix" until the render situation can be addressed, and hopefully reduce the damp penetration is to use a thin scraper and gently clear the crack, use the corner of the scraper. then using some clear mastic, with a very very fine nozzle size run a bead of mastic into the crack, do NOT smear the mastic over the surface of the render, this can seal the crack over winter, reduce the volume of rain entering in behind the render and may??? reduce the volume of water that is trapped from causing more internal damage.

Ken.