Plastering/insulating over a wall with potential damp issues

5 Oct 2017
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United Kingdom

I have a question (no doubt asked in various formats in many other threads) about some work we are planning on getting done.

Our living room (old building, 1800s) has a single skin wall and there are signs of damp near floor level- small patches, but there. The flooring was lino done on top of concrete, with plastering all the way down to floor level. We are looking to have the walls plastered in some way.

Basically, it's a case of contrasting opinions from a builder and a plasterer. The builder suggests dot and dab and then using Celotex 100mm insulating board on the walls- he says the damp should not penetrate into the board and shouldn't cause any major problems, but does note it won't stop any potential small patches of damp in the future, including in the flooring. The plasterer is not a fan of dot and dab on a single skin wall, especially where damp is a potential issue, and has suggested timber re-framing the relevant walls, then inserting insulation as we wish. Would the timber be susceptible to damp (even if treated) or the insulation used?

I wondered if anyone had some thoughts on the above? My gut told me that dot and dab was more of a 'quick fix' and may still lead to damp issues, but the builder was insistent this would not lead to worsening of the problem, and he has a good reputation locally (with good references!). Of course, 'solving' the damp issue would be ideal, but both plasterer and builder have the attitudes that injections or sealing the walls is ineffective/troublesome. I'm realistic in my expectations from an old building!

Essentially: dot and dab with insulation board vs re-framing the walls.

Happy to provide more details if I can/if useful.


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I wouldn't bother doing anything until I fully understood the damp issue.
Old houses like this were built a certain way so modern 'solutions' could create more problems, like they may have already done.
Is the outside ground level high? ie have successive driveways / patios been built up too high?
Hi there! Thanks for the reply.

The outside of the property where there is damp doesn't have a high level- it is roadside and the wall in question is next to a drain which sees a lot of rain flow near to the wall (down the street) and some pooling in really heavy rain. There is a layer of stone/cobbling between the wall and the water flow to the drain, but we presumed damp may always be an issue when heavy rainfall lets the water stand for a while near to the property. The room is prone to a lot of condensation on the inside walls as well, so don't know if that's a factor?
Have a read of the plastering wiki -
Damp and Condensation
Managing Damp in Old Building

It may offer some suggestions.
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Thanks for the reply- I'm convinced our issue is condensation rather than any external source and it has been improving greatly over time (compared to when we first moved in) just through improved ventilation/dehumidification.
I'm not a professional, but in terms of the internal wall plastering, I've used lime hemp plaster, with the thinking that it will help to insulate the internal walls, and also avoid the issues of wood or gypsum reacting to damp.
The base of a single skin wall with tarmac on the other side will be very cold due to thermal bridging. If you get an ir thermometer you can confirm that it's cooler.
You can check the humidity and temperature in the room to work out whether it's below the dew point at the base of the wall, or whether it's cold because it's evaporating damp.
If it is condensation you need to either reduce the humidity (more ventilation, heat, less damp clothes etc) or increase the wall temperature (continuous insulation with no air gaps as you'll get condensation behind)
I think the insulation will be fine dot and dab in the condensation case, but if it's penetrating damp it won't do the wall any good.
If it's coming through then try to prevent any water pooling, and shelter the wall too if possible. It needs to dry out quicker than it gets wet
We had a similar issue in our house.

Upstairs bay window wall is single skin brick and was wet at the lower half during the cold winter months. I took everything down to brick, put battens up, filled with insulation board, plaster board over the top and then skimmed. Since then, the wall is room temperature to the touch and doesn't allow anywhere for the warm, moist air to condensate.

The only trouble we have now is that we get even more condensation on the windows...
The only trouble we have now is that we get even more condensation on the windows...
It's got to go somewhere unfortunately! :unsure:
One tip if triple glazing is not your thing, is to open the windows for half an hour every morning. Also look at improving the general ventilation.
It's got to go somewhere unfortunately! :unsure:
One tip if triple glazing is not your thing, is to open the windows for half an hour every morning. Also look at improving the general ventilation.

Yes I think we've just pushed it up higher.

We do have trickle vents but I find them a bit pathetic. Triple glazing is definitely on the cards for one day, but in the meantime I keep the windows ajar during the day and locked on the latch.

'Her indoors' complained too much in the winter months when they were open so I had to keep them closed in the mornings :sick:

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