Dealing with Hygroscopic damp.

katipip

I'm looking for advice regarding the products that I should use to treat hygroscopic damp in an internal chimney breast wall.

Our house is a 1930's mid terrace with a fireplace in the back room. We have lived here for 25 years and have never used the fireplace to burn wood/coal. Instead we've had a variety of gas fires in place with (until 2 years ago) a back boiler in situ. Two years ago the boiler was removed and a new one installed elsewhere.

Since then we have found damp patches on the wallpaper (gradually rising up the wall from skirting board level). These patches register very strongly on a damp probe. Inside the chimney breast the same bare brick wall is also giving incredibly high damp readings.

Several damp companies have quoted for hygroscopic damp treatment which, in summary, consists of removing plaster to height of 1m, installing a chemical damp proof course by means of low pressure injection and then replastering with renovation plaster. We have been told this will take 2 half days and then we can redecorate approx 6 months later.

Trouble is the companies who have quoted are not returning our calls so we assume they are not really interested in the work.

My next plan is for us to do it ourselves but need help with what's the best damp cream to use to inject the walls. Also what is the best plaster to use - any recommendations?

All advice welcome.
Thanks in advance.

DRP

I'm not a professional, but I doubt you will need a chemical DPC for hygroscopic damp, as it is just the salts in the existing plaster that are causing the problem.

I'd just hack it all off, let everything dry out, then re-render/plaster.

katipip

DRP wrote: »

I'm not a professional, but I doubt you will need a chemical DPC for hygroscopic damp, as it is just the salts in the existing plaster that are causing the problem.

I'd just hack it all off, let everything dry out, then re-render/plaster.

But then surely the salts in the chimney breast will start bleeding into the new plaster and the problem will reoccur.

I want to create a barrier to stop the salts in the chimney doing any more damage.

I've seen this salt neutraliser http://www.dampness-info.co.uk/tanking%20shop_diydoc.htm#Saltneutraliser but I'm not sure if it's sufficient on its own.

Mr.Generous

use permagard cream, easy to do it yourself.

https://www.permagard.co.uk/aida-damp-proofing-cream-kit-10-x-310ml?gclid=CKvr28-f5MsCFVRsGwodBQcIew

When you re-plaster you have to use cement, plastering sand and waterproof admix. If you use normal plaster it will draw moisture like a sponge.

Doozergirl

I presume the chimney breast is still ventilted at the bottom and at least partially capped at the top?

katipip

Doozergirl wrote: »

I presume the chimney breast is still ventilted at the bottom and at least partially capped at the top?

The top of the chimney is partially capped - when the back boiler was removed I made the mistake of sealing off the bottom end of the flue to prevent drafts (I now know that this was a mistake).

I've removed my improvised flue seal (tin foil secured with an elastic band!!!!) and reinstated full ventilation.

I suspect that my draft excluding actions aggravated the situation.

Mr.Generous

If its rising damp the capping off and draft proofing wont make any difference. 1930's houses were built with bitumen felt or slate damp course, both fail routinely over time.

Plastering with cement is not straightforward, you need to do a scratch coat then a fine finish, if your going to try yourself do some research. We do all the prep then get an expert to do the plastering. When I asked him how he does it so well he said "Its easy, just keep practicing for about 20 years."

blackshirtuk

Just had a similar problem, we remedied it by hacking off affected plaster, then used tanking slurry on the affected area, the let dry

It then had to be rendered with sand cement and waterproofer, then let dry, and finally plastered.

Happy to say it worked. We had already removed the chimney above roof level to remedy the problem!

http://www.wickes.co.uk/KA-Tanking-Slurry-Grey-12-5kg/p/139057?CAWELAID=120135120001185080&CAGPSPN=pla&utm_source=google&utm_medium=cpc&utm_term=&utm_campaign=Google%20Shopping%20-%20Building%20Materials&tmcampid=&tmad=c&tmplaceref=&tmcustom=mkwid|s_dc|pcrid|43364522812|kword||match||plid|&gclid=CjwKEAjwrOO3BRCX55-L9_WojHoSJAAPxcSPIlob_plEXOKlcqdJ3eQpS2mRVyHDlPqrM88c3hPVKBoC8XDw_wcB

DRP

katipip wrote: »

But then surely the salts in the chimney breast will start bleeding into the new plaster and the problem will reoccur.

I want to create a barrier to stop the salts in the chimney doing any more damage.

I've seen this salt neutraliser http://www.dampness-info.co.uk/tanking%20shop_diydoc.htm#Saltneutraliser but I'm not sure if it's sufficient on its own.

Yes you're probably right. All I can say is that it worked for us.

We had visible damp in the unused chimney breast plaster, coming through the wallpaper. We hacked off the old stuff to brick (which looked dry, although the covering plaster had been very damp), then scratch coat, then plaster. A year later - still no issues.

MX5huggy

I had 2 chimneys with what I believed was hydroscopic damp in the first floor bedrooms. Chimney A, was taken down below roof level both flues were sealed at fireplace level (ground floor and 1st floor fire places) then filled with vermiculite insulation. I didn't expect this to fix the damp but it did. I did it as the external chimney needed rebuilding as it was unused I removed it for a lot less. The filling and sealing is reversible if it causes additional problems.

Chimney B is still used for the GF fireplace with a wood stove so the first floor flue was again sealed and filled the ground floor one was lined with a flexible stainless flue liner and filled with vermiculite between that and the original chimney. The external chimney was rebuilt as it was falling down slowly. Then in the first floor bedroom I covered the damp with damp proof membrane for floors held up with battens between the battens I put Celotex to raise the dew point then plaster board and skim. It's a semi temporary measure it's worked so far and avoided the mess of hacking off.

Grenage

Doozergirl probably has the right of it, and that's it's just condensation from an unventilated stack. Keep it ventilated and you will avoid the problem.

Putting crap on the wall to cover up the problem is not a solution.