Chimney Breast Damp under gypsum and cement plaster, intend to renovate with wood fibre and lime - Advice needed

[Stickworm]

Hi

I'm planning on insulating the interior of one room of a building built around 1950 and converted into flats in the 1990's. Current plan is to remove the current plaster (which seems to be a gypsum or cement in places at a guess) and they apply a layer of Prooftherm followed by Steico wood fibre boards with lime plaster on top.

We had issues with the roof and chimney flashings causing damp that were fixed about 4 years ago by professionals. Since then one of the walls against the chimney breast has been suffering from major efflorescence, and was maxing out the moisture meter, I've removed the plaster on this wall. Now that the plaster is off there's a very obvious smell of damp and I can't tell if this is historic from the original leak and water being trapped or if there is still a problem with the chimney stack. Damp stop paint had been used on that wall previously to add to it's lack of breathability.

Is it essential to wait for the wall to dry completely before applying prooftherm/wood fibre boards considering as they themselves are breathable? I'm currently running a dehumidifier, fan and the heating in there (I've potentially picked the wrong time of year for all this). Obviously it's important that I'm confident any leaks have stopped first, I assume it's possible that water could have been trapped in there that long? Condensation used to form on that wall specifically before the plaster was removed. I'll use some Rempro salt neutraliser on the bricks, would this also be required on the stone?

The mortar between the bricks is quite crumbly in places, I've included a video of this. Could this be problematic?

Thank you for any help or advice.

 

[Cubist]

I think the answer to your key question is; Yes you can get on with the insulation project AND you should not wait until those walls are thoroughly dry. And here's my reasoning: -

Right now outdoor humidity levels should be lower than those indoors and, as the winter months proceed and temperatures drop, outdoor levels of humidity will continue to fall. As a consequence any moisture still trapped in the masonry, will be drawn toward the exterior in an effort to equalise its moisture level with the wider environment. However, if you leave the masonry indoors exposed this same result MAY ONLY be achieved IF the indoor humidity level remains above that found outdoors. BUT should the indoor level fall below that found outdoors the process can reverse and moisture can then be drawn into the walls from outdoors.

Therefore, you may want to consider switching off that dehumidifier as right now it is not helping your aims.

 

[Kearn]

Dare I ask what prooftherm is…?!?

If you search the threads, there are quite a flurry of similar queries lately on woodfibre boards, so you should find some more info.

Intimately, to insulate, they don’t want air gaps under the substrate, which is presumably what the product above is. However, this just needs to be lime.

The challenge with fitting the boards and plastering, is that IF there is still a leak, you will not see it for some time, damp fibre won’t insulate, and eventually you have to reverse it all to fix an issue.

There’s no need to waste money on ‘salt neutraliser’ it will continue to come out of the brick/stone as it needs to… just brush/vacuum. It shows that you’ve had a sustained leak and wet fabric…

 

[Stickworm]

Hi Cubist, thanks for that it's useful info.

Kearn, Prooftherm should come up with a google search (not sure if I'm meat to post links to products here) it's a perlite lime mix breathable insulating render. Less expensive than something such as diathonite thermactive which has similar properties. Yes I very much need to work out if there is still a leak. If there is it's hugely reduced from the original issue we had, I just wasn't expecting it to still be so damp behind the plaster when it was removed, but I guess it could have just been trapped there all this time.

 

[Cubist]

Hi Stikworm, the problem you're facing with apparent damp in that wall may in part be due to the prevailing weather conditions. If the exterior of the wall is subject to frequent and driven rainfall this would significantly inhibit its ability to expel any water that may have penetrated and taken up residence in the masonry from any leaks. However, that water from the leaks AND any accumulating from driven rain would also be subject to the effects of gravity and would be much more evident in the lower sections of the wall. In fair weather conditions you could crudely estimate that the wall would dry out at the rate of 1inch per month, so 9 months for a 9inch wall. At this time of year that pace might slow somewhat but provided you maintain the balance between relative indoor and outdoor i.e. always lower outdoors, then it will dry to the outdoor ambient level.