Insulated plasterboard over existing dot and dab?

17 Feb 2010
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United Kingdom
I have dot and dabbed an external wall and chimney that I thought was dry but it is showing some damp patches where the dabs are. I think it's probably due to condensation as I covered the wall with 2 coats of SBR slurry mix before dabbing as a precaution against penetrating damp. I realise now that this was a mistake but rather than rip it all down and batten, can I just stick insulated boards over the existing ones to avoid the cold spots? Everything is flat and level so it would be far easier than going back to brick and battening / insulating etc.
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tireetim, good evening.

Several potential things?
Is the Flue Vented at room level and at the chimney pot? [but the pot vent needs to be rain protected]

You could simply introduce a barrier between the "damp spot" plasterboard and anything else you care to retro fit? BUT the dampness will be on-going under / behind the newly applied [whatever]

If the area of chimney breast is not too large, my preferred option would be to strip back the area on the chimney breast and using treated timber then Insulated Plasterboard to effect a repair.

Lets not forget, flues contain some nasty contaminants, and the big one is Sulphate, you may find that the dark marks are indeed the effects of Sulphate attack, what happens is that any fossil fuel Coal, Wood, Peat Etc. produce Sulphate, this chemical adheres to the inner surfaces of the Flue. when moisture comes into contact with this chemical, a very mild form of Sulphuric Acid is formed, this acid then attacks especially Lime Mortar and dark stains form.

You have probably seen on the Gables of older properties the dark stain mark from the Fireplace to the chimney head, a direct result of Sulphate attack.

How thick was the p/board that you dabbed to the wall? You might have got away with dot and dabbing Insulated plaster board ( poly-backed)on that area. I had damp walls in my old house and I put a scratch coat over the stone work and dot and dabbed poly backed plaster board on and it has been ok now for 15 years.
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Thanks Ken and Roy.
The chimney was capped and vented and I thought dry as it was back to brick for a few months and I have been monitoring it. It's also the same on the external walls either side of the chimney so I don't think the issue is penetrating damp but more condensation forming on cold spots where the dabs are. It is just plain 12.5mm plasterboard, not insulated as it is on a cavity wall. I wondered if putting insulated board over the top would prevent the damp in the board behind as there would be no air gap. To be honest it's going to be £200 plus of materials to find out so I'm thinking just go back to brick and batten, fill gap with rock wool, then plastic sheet and standard plasterboard on top? Never used poly backed board but it looks expensive, especially for the one with a vapour barrier built in. I'm just a bit surprised / annoyed as I had a few quotes from builders and none of them suggested anything other then what I've done. Seems that dot and dab on an external wall (with standard plasterboards at least) is a definite no no.
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Roy, just out of interest, why did you scratch coat the bricks before dot and dabbing? Just to level?
We scratched the brick work with a water proofer in the mix, if it wasn't a cavity wall it just gives a little more damp protection from just a brick wall' . And also I did scratch my stonewalls (with a few coats) to bring it out a bit straighter before I dot and dabbed them.. And if we were dot and dabbing around corners we used to put some 6" D P C. up the corners so the boards butted up to each other and no damp from the walls tracked back to the board corners...
Thanks Roy. I've got a lot to learn. Will be ripping it all off tomorrow and starting again :(

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