Paint over render

[MDE]

About 4 years ago I got a builder to remove the cement render from my barn (rubble stone walls) as it was flaking off anyway. I asked him to render it using lime as I thought that was the right thing to do. Little did I realise that most builders don't understand lime and it now transpires that he used a lime and cement mix. I don't know what proportions he used, but there must be a reasonable amount of lime as the render has whitened with age. Unsurprisingly, there are now a few cracks. Also the wall is a bit damp. I think the main reason for the damp is that the render is quite absorbent - it is easily wetted by the rain but dries out well, too. Some of the damp is, I think, caused by condensation on the cold walls (bare stone inside). I think the solution is to reduce the capillary action of the render without reducing breathability. Two solutions have been suggested to me and I would appreciate comments or further ideas:
1) Paint it with silicate paint
2) Apply limewash with pva (first coat) and add linseed oil to improve water run-off
I know that some may say "knock off the render and replace it with lime render", but this isn't really an option as it is stuck fast - removing it would damage the wall. Also, I think it is a fairly breathable mix - the problem is that it is excessively hard, hence the cracks.
As regards the condensation, I presume the best solution is to lime plaster the inside of the wall.

 

[MDE]

I see that 64 people have read this, but nobody has commented. Is that because the question is
(a) Too stupid?
(b) Too difficult?
(c) Too boring?
(d) Other?

 

[Tom]

I suspect people don't want to get into the usual ping-pong of messages, each having their own ideas on what's right or wrong.

It amazes me how many people specify what materials must be used but then allow their builders to bring non-specified materials on site!! I guess it depends on how much you project-manage your own site or how much you allow someone else to do what they like. If *you* demand certain materials, in my opinion, it's up to *you* to ensure they're used. Otherwise you end up in the position MDE has found themselves in; non-specified materials used, looking for a quick fix.

Just my opinion, of course, but I'm ready for the barage of abuse for airing my views!!

Jaqy

 

[Mark]

well/

the wall is damp ? where/how ground levels prevailing weather roof/guttering etc?

What is the barn used for ? what floor? what ventilation

pva in limewash ? No. Just limewash please.

 

[charlie-ia]

i vote b)
sorry!

i also like "ping pong of views" - some of us dont have enough knowledge to make a firm descision on whats right or wrong yet, and as everyones case is slightly different the discussion is often as useful as a straight, specific answer.

 

[MdB]

Limewash + PVA doesn't sound a good idea. The PVA will stop the breathability properties of the limewash. It will also trap moisture behind it.

Limewash with 1% max linseed oil will help rainwater run off the surface of the wall and may fill in some of the small cracks if you are lucky. I guess it is likely to crack further though with time. This means you are likely to have further moisture ingress which you don't want to trap so don't do anything which will stop the moisture from evaporating from the wall.

 

[MDE]

I totally agree with DWB. Unfortunately I wasn't able to be on site when the work was done. I won't make that mistake again!

Mark, the wall is damp on the inside. The roof has been replaced and is fine, with guttering on all but the gable ends. The floor is earth and cobbles. There was a hayloft at the south-west end, the floor of which has rotted, together with most of the joists (particularly at the ends). The main beam for the floor is still sound and is set into the gable end wall and rests on an internal wall at the other end. The end that is in the gable end wall is slightly damp and a bit soft, but the beam is pretty thick and is therefore surviving so far.
Presently the barn is used for storage, but I want to re-instate the hayloft for use as a fruit store etc and to use some of the barn as a workshop.
Someone in the trade suggested adding PVA to a thick limewash and just using that to fill the cracks, then covering the render with plain limewash (ie no PVA, but possibly some linseed). Elsewhere on this site, someone has suggested that mending cement render with lime is a bad idea as it draws damp underneath the render: the PVA seems to be intended to stop that.
FWIW, it seems to me that the main problem with cement render on a rubble stone wall is cracking, not breathability. With a lime render the water is only drawn in a short distance, then evaporates; any cracks are self-healing. With cement render, the water goes in a long way via the cracks that inevitably form (the render being too hard and not having any self-healing properties) and is then trapped behind the render. The only statistics on breathability I can find show little difference between cement and lime renders. I guess this is a bit controversial.

 

[Nemesis]

I think at times messages get lost - it all depends on many factors, such as who is online at a certain time, how much arguing over other topics is going on.... they move down the page and that's that!

I do at times look for those little lost messages and bump them back up, I plead in mitigation... don't feel unloved.

I wonder what lime mix it was? It may have been mostly lime with a bit of cement added - many still think this a good idea, but it isn't and weakens the mix while making it less 'flexible' (as far as any render is flexible).

Well, It's not always possible to watch over builders all the time, and you can specify til the cows come home but some still think they know best and ignore you. It's hard at times to tell what's been done with mortar mix until it's too late.

Limewash may help cover the cracks, but NO PVA!!!

Condensation may have nothing to do with the render of course.

 

[MDE]

I think that the trick is to post messages when the weather is rotten.
I have just chipped a bit of the render off and dropped it in some vinegar - it is fizzing gently. Comparing the level of fizz with some one-year old lime render (semi-hydraulic), it seems about the same. I don't know if this is a fair test or if more sophisticated analysis is available, but it would seem to indicate quite a bit of lime in the render.
Sounds like the answer is limewash (no PVA). I suppose it is still a good idea to add some linseed oil to reduce the capillarity, as it is an exposed position.

 

[Mark]

the wall is damp on the inside. The roof has been replaced and is fine, with guttering on all but the gable ends. The floor is earth and cobbles.

That would suggest to me that water is getting through the render. Assuming that is that the damp isnt concentrated at particular places - which you dont mention.

Presently the barn is used for storage, but I want to re-instate the hayloft for use as a fruit store etc and to use some of the barn as a workshop.

So nothing that generates moisture inside the barn. And as a barn I can assume decent ventialtion ?

Someone in the trade suggested adding PVA to a thick limewash and just using that to fill the cracks, then covering the render with plain limewash (ie no PVA, but possibly some linseed). Elsewhere on this site, someone has suggested that mending cement render with lime is a bad idea as it draws damp underneath the render: the PVA seems to be intended to stop that.

Bad idea. You might as well fill with commercial filler. how big are these cracks? I would fill with lime mortar and then limewash.

FWIW, it seems to me that the main problem with cement render on a rubble stone wall is cracking, not breathability. With a lime render the water is only drawn in a short distance, then evaporates; any cracks are self-healing. With cement render, the water goes in a long way via the cracks that inevitably form (the render being too hard and not having any self-healing properties) and is then trapped behind the render.

I think the two issues go together certainly.

The only statistics on breathability I can find show little difference between cement and lime renders. I guess this is a bit controversial.

I would say very controversial - where did you get these figures?

 

[MdB]

Lime in vinegar will fizz. Cement in vinegar may fizz. No fizz means no lime but fizzing doesn't tell you much.

 

[MDE]

Thanks for all the comments. The figures on breathability are taken from the paper by Neil May of NBT "Breathability: The Key to Building Performance" which gives typical vapour resistivity figures for cement plaster of 100MNs/gm and for lime plaster 75MNs/gm with ranges of 75-200 and 45-200 respectively. Interestingly, both these figures are higher (ie less breathable) than gypsum plaster or foamed concrete. He also gives figures for capillarity, indicating that silicate paint has much lower capillarity than limewash for similar breathability (but no figures on limewash with added oil).
See http://www.greensteps.co.uk/tmp/assets/1163178050906.pdf

 

[Nemesis]

Page ten - the caveat at the top is interesting - and the lime quoted is hydraulic.

I haven't time to read it all, but that rings a few alarm bells.

 

[MdB]

Please do read the article. It has some interesting points in it. Thanks for the link MDE.

 

[Nemesis]

Haven't time - but read the caveat and also consider that lime is many things, can't all be lumped together.

 

[MdB]

Yes the cavaet is important for that table of figures but show me some contradictory figures. In fact show me any figures at all. It is the first time I've seen figures comparing different materials and even though they may be inaccurate it certainly gives a feel for what the differences between materials are. If someone now claims that lime is 50x more breathable than cement then I can ask for their evidence as this would appear to be rather unlikely. If however, someone claims that lime wash is 50x more breathable than exterior emulsion then this I can now believe.

The rest of the document is very much worth reading as well. It makes some good points on modern construction methods as well as modifications to old constructions. It has made me think on the best methods of specifying my new front wall. It would take the time to read it.

 

[Nemesis]

I suggest that you contact SPAB technical helpline and ask for technical specs for lime v cement, and I suspect EH has them too.

And please don't lump all limes together - hydraulic and non-hydraulic have very different properties. As indeed that paper states.

EH has carried out considerable research on lime and its properties. If it thought using cement was better (and it used to...) then I think it would continue to use it.

Also - while gypsum may lap up water, it also tends to fail where it has been subjected to it.

The final conclusion is about right though.


However, over long term it has found that cement has caused damage.

I am sure that someone like Mike Wye , or Ty Mawr, or the other good lime people can give technical specs too.

Lime has other benefits of course:

http://www.vcrestorations.co.uk/benefits.htm

http://www.vcrestorations.co.uk/symptoms.html

http://www.vcrestorations.co.uk/symptoms2.htm

http://www.vcrestorations.co.uk/intro.htm


In fact the whole site states the obvious and is illustrated with useful examples.


Modern construction methods are a different matter too - we are talking here period buildings.

 

[Penners]

Lime in vinegar will fizz. Cement in vinegar may fizz. No fizz means no lime but fizzing doesn't tell you much.

I thought OPC (unless buffered) was definitely on the alkaline side, and so would fizz in acid, like lime.

I wouldn't take fizzing as any kind of reliable indication that there is a high proportion of lime in the mix.

 

[Nemesis]

Cement can be pretty white too of course.

 

[MDE]

The render sample that I put in vinegar has now completely disintegrated, leaving just sand in the bottom of the glass. Would this happen to a cement render?

Re the merits of lime versus cement. It seems to me that the main problem with cement on an old building is that the (in my case rubble stone) walls move but the cement doesn't so it forms cracks which suck in the water a long way. Even if the render were breathable, this would take a long time to get out: usually it freezes behind the render and over time the render detaches from the wall. A lime render is more flexible and any hairline cracks self-heal, so the water soaks in slowly and evenly and then evaporates out in the same manner, so that (hopefully) it never fully penetrates the render. If lime render is more vapour-permeable than cement render, then this would help, but I'm not sure it's the major factor. I think that the whole issue is quite complex and, while it is clear that lime is better, I really would like to understand exactly why. Aside from the paper I've referred to, there does seem to be a dearth of good technical research that covers all the relevant aspects of materials performance.