Internal wall chemical DPC... did it work?

[confusedjames]

Hi folks,

I've recently had a chemical DPC installed on an internal wall where there was damp. I was told rising damp.

So chemical DPC installed. I was told to use a lime-sand plaster but used gypsum. I left the walls to dry for about a month and have now plastered over them only to find the same spots are damp where the rest of the plaster is now dry.

So, I read up a little bit. I understand that a lime-sand plaster is breathable so will allow moisture to escape (provided I also use a suitable breathable paint I guess).

My confusion is this... if the chemical DPC is functioning as it should, should I really need to use a breathable plaster? Surely once dry the wall should stay dry if the rising damp is stopped.

Also, if the DPC hasn't worked and I am still getting damp, wouldn't using a lime based plaster mask the damp by allowing it to escape? Obv moisture escaping the wall is good, but would this mean it would be hiding a failed DPC?

I'm a little confused about this so any ideas would be much appreciated. THanks in advance

 

[ree]

You've not read up enough. What you have coming thro the gypsum plaster is salts - brought to the brick face by the original damp.

Unless your house is extremely old or v. unusual you will have a DPC, and DPC's rarely fail. So you probably have some kind of bridging, or penetrating damp.

Remove the skirting and knock off plaster to about 1m high. Render up with a 3:sand to 1:lime mix, and a say limelite skim finish.

If you want to know more about your original damp then post interior and external pic, and research on here and the building forum for all the variables we would have to know if we were to help you. eg. solid floor ground levels, solid or cavity wall.

 

[jeds]

It's unlikely your original problem was rising damp. It was probably condensation dampness - which you are still getting. Gypsum plaster and dampness doesn't mix. My advice is hack it off and use the correct material.

 

[confusedjames]

Hi Guys,

Thanks for your responses.

Okay, so from what I can understand although the bricks might be dry to touch they can still be damp (apparently 20-30% dampness before they may be damp to touch?).

I had drilled into a couple of bricks (thin drill bit) earlier, before redoing the plaster and saw dry brick powder come out... no sign of damp - I was told this was a better test... is that right?

Below are links to some images of the wall

View media item 83827 View media item 83828 View media item 83829
I've tried to avoid bridging as I was aware of this but on close inspection maybe I haven't been conservative enough?

The wall is an internal wall. If it is condensation, would I be able to see condensation droplets on the wall? There is only the one wall shown but on the other side in the next door room there is also the same effect

 

[confusedjames]

On the other side of the wall... you can see where most of the plaster is drying but the point where the old damp originated is not /

View media item 83831
So ree, it's the salts coming through. Okay. I have read they will be hygroscopic and absorb and hold water, and gypsum is quite an absorbing material anyway...

so is it just the bricks need longer to dry out or is it that the existing salts in the bricks are causing the new plaster to absorb the salts which then attract and retain moisture?

I re-read your post. Just to be clear, the plaster you see is new plaster that I have put on after the DPC was injected. The walls had been exposed (no plaster) for a couple of weeks before and after the DPC was injects.

**EDIT** Another little update I re-read the DPC installers recommendations and it seems I was even more confused than I thought... they recommend a cement-sand render, not lime-sand. But I have read the cement-sand render is "...effectively impermeable to water, and will tend to trap moisture in the brickwork behind it...". This tallies with what I've read about cement renders. So... if they are recommending this, surely I would never know whether their new DPC actually worked (if indeed it is rising damp).

I've found this kit http://www.damp-meter-direct.co.uk/protimeter-salt-analysis.htm#.VDppkvldU1Z for analyzing salts in walls to proove rising damp by detecting common salts found in soil. Anyone used this before?

 

[ree]

I apologise for missing the point that this is an interior wall situation. But my advice ref re-rendering with S&L (or S&C&L) still stands. Do both sides of the wall(s). Sometimes, even with being on site, its extremely difficult to determine where the damp is coming from, and a knock-off and render is the best option.

In pic 4. search for the original DPC below the floor level in the first or second bed in view.

The new injected DPC (FWIW) is too high but the skirting should cover the unplastered course.

What i can see certainly looks like damp in the brickwork and not pure condensation (damp & condensation can sometimes be present together).

Is the crawl space under the floor deep enough for you to access?

Note: you will have to run new floor board(s) under the door threshold from joist to joist.

Note: the DSS box is too proud.