Rendering

[aartsetuyn]

I have recently moved into a new home and have no past experience of all this stuff. The home buyers valuation noted that i had penetrating damp due to the rendering on the exterier walls being porous. This has led to the internal walls in the upstairs bathroom and bedroom being damp. The advice was to strip back all the render and replace with new mix and non porous sand and cement layer. Its quite a costly job and i just wondered whether it would be possible just to simply paint the exterior with a non porous paint. Or is it too late will the damp then remain trapped in the bricks and rendor or will it dry out in time? I guess i was just lloking for a cheeper option any ideas?

[aartsetuyn]

I have recently moved into a new home and have no past experience of all this stuff. The home buyers valuation noted that i had penetrating damp due to the rendering on the exterier walls being porous. This has led to the internal walls in the upstairs bathroom and bedroom being damp. The advice was to strip back all the render and replace with new mix and non porous sand and cement layer. Its quite a costly job and i just wondered whether it would be possible just to simply paint the exterior with a non porous paint. Or is it too late will the damp then remain trapped in the bricks and rendor or will it dry out in time? I guess i was just lloking for a cheeper option any ideas?

[Stuart Noble]

The cheap option is to paint the render with a pliolite based paint from Johnstones or one of the other trade outlets. The paint itself isn't cheap, but magnolia and white is often on special offer. The walls should dry out to the inside over time. Ordinary masonry paint would almost certainly be a waste of time

[Phil L]

At the risk of sounding obnoxious, your valuer is talking bollocks, no amount of porosity will cause damp in a wall, if the rendering was not there at all, the bricks would not allow damp to get across to the inner leaf, the only reason they are is because the rendering has come away from the brickwork and this gap is holding the water, so no, there is no paint on the market that can help in this situation, only having it re-rendered can stop it now.

[meow2222]

This is the result of using cement render that is seen again and again. Cracks form, water gets in, but it cant evaporate out enough, so the wall slowly gets wetter and wetter. It it goes on long enough the wall may get wet enough for freeze thaw cycles to start breaking the brick faces up. If the unwise owner rewaterproofs, the cycle continues and further brick deterioration occurs.

The advice youve been given is no use, nor is painting. While these attempts are popular they are basically counterproductive.

The dampness level in a wall is an equilibrium between water getting in and damp evaporating out. The only way a wet wall will dry is if you let it evaporate dry. The best thing is thus to remove render to let it dry off. Attempts to 'waterproof' walls are precisely what has caused your present situation, there is never long term sustained waterproofing, so water ends up getting in, but cant evaporate. Going with a finish that allows evaporation is the effective solution.

Paint wont help the wall evaporate its damp. Only removing render will let it dry out. For a refinish, lime render allows evaporation, cement doesnt, so lime it. Equally avoid evaporation-proof paints such as masonry paint or pliolite.

Some of this is alien to some here, if you want more explanation of how this works try

[Mark]

If it is a cavity wall you are again talking B******S As the OP did not state the type of construction, you are just jumping to conclusions and spouting the usual line from your only source of knowledge. periodproperty.

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[meow2222]

Are you saying the same basic principle of water in versus evaporation out does not apply to cavity walls?

NT

[Mark]

Open your eyes and look at how many houses have been built over the last _100 years_ with a cement render finish. Your "basic principle of water evaporation" is nonsense when applied to a cavity wall house. Too much theory and zero practical knowledge yet again.

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[meow2222]

I am aware not every house has problems, and why. I thought you had something useful to add.

NT

[The Natural Philosopher]

Well: In that he identified that you had not, he had ;)

[Phil L]

He's right in one respect inthat sand/cement sets way too hard and is therefore susceptible to cracking...this could be because the modern approach is wrong WRT mixing, it's commonly accepted that 'strong is good' when the opposite is actually true, the house I am in was built with a mix of 10 sand, 1 cement and 1 lime....while the pointed surface is porous, the inner is rock hard, exactly as it needs to be, and the house is 60 years old.

[Mark]

when

All types of external render whatever the ingredients and mix will crack given sufficient movement, the only thing meow is correct on is if water gets into a defect in the render and freezes it will lead to more cracks in the render. Maintenance of the external surface would prevent this. The point I was objecting too was the notion that damp remains trapped in the bricks and render and will not dry out, in the external cavity wall this would be almost impossible in this country. This is the very reason why cavity wall construction often with a cement render was first used in coastal towns a 100 years ago, to keep the damp out.

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