Breathability of clay paints questioned.

[malcolm]

I was a bit worried so tried a test. A bit of water thrown at clay paint (year old Earthborn) over lime plaster resulted in a couple of drips falling to the floor and the rest seeping in. Water thrown at mystery emulsion paint over wallpaper balled and rolled off.

For science I tried throwing water at emulsion over lime and that behaved like the clay paint.

I'm going clay paint over lining paper in the latest room in order to preserve the 300 year old plaster.

 

[MikeG]

Great test!

I think they might have to be just a tad more sciency over at BRE (or whatever it's called these days).

 

[Abi Cott]

I know there are specific grades for "scrubability" and opacity. I got sent on a paint training course once. Paint science is quite in depth, something you realise if you're sat next to a paint scientist during a 4 course meal. I might try asking my employers paint supplier some questions. I have access to the tech guys.

 

[livias]

[Non scientist alert] Any idea what clay paint is actually made from? My guess says pigment for colour, clay to give it body and glue to make it stick to the wall, in which case the question is .. what sort of glue and how breathable is it? Glue does not sound like a breathable substance to me

 

[Pennyviz]

can't see any glue. Methylcellulose is a thickener,

https://earthbornpaints.co.uk/product/claypaint/

Penny

 

[Flyfisher]

I know there are specific grades for "scrubability" and opacity. I got sent on a paint training course once. Paint science is quite in depth, something you realise if you're sat next to a paint scientist during a 4 course meal. I might try asking my employers paint supplier some questions. I have access to the tech guys.

That would be very interesting.


As for testing by applying water, I'm not sure that would be applicable because liquid water and water vapour have rather different properties. For example, liquid water will 'ball and roll off' materials such as Goretex or Tyvek roofing membrane even though they are highly breathable.

 

[Abi Cott]

The water test is how we were taught to test for porosity of a substrate.

For paint to bond properly with a wall it needs to be porous but not too porous.

If the wall is too porous it sucks all the water out of the paint before the bonds have time to form. Insufficiently porous and the paint just forms a layer on the surface and peels off.

If the wall sucks all the water out of the sponge then it's a good idea to put on a coat of PVA is what we were taught. Not exactly what you want to do to your lime plaster lol. It was an interesting course but obviously has limited applicability to my current project

 

[Abi Cott]

I've spoken to a lovely Dutch man from Ostendorf.

He wasn't able to make any specific comments on clay paints. However he did give me some interesting comments.

In his expert opinion paint does not change it's breathability with time, though applying more coats will have this effect.

No paint containing synthetic resins is suitable for lime plaster in a listed building, including all the paints he makes. Some may be more breathable than others but none are breathable enough.

His recommendation would be Keim paints which use a waterglass binder. Keim is the name of the guy that invented the type of paint. More than one supplier produces it. The way this paint crystallises gives a durable hard wearing colourfast surface that is extremely vapour permeable.

He did comment on the limited colour range available in this product.

So not exactly an answer to the question, but interesting none the less. Particularly as he was recommending a product he doesn't make and saying not to buy any of the ones he does.

 

[Abi Cott]

I'm sure his recommendation will please MikeG. I remember him being a Keim fan.

 

[Flyfisher]

No paint containing synthetic resins is suitable for lime plaster in a listed building, including all the paints he makes. Some may be more breathable than others but none are breathable enough.

Oooooo . . . so close to an actual number! If only someone could give a reliable figure for 'breathable enough', we'd then have a yardstick by which to judge the many types of paints and coatings available.

His recommendation would be Keim paints . . .

Yet another one. The only negative thing I can recall reading about Keim paints (well, not Keim per se but the waterglass-based formulations in general) is that surface preparation is a bit more involved than for limewash. However, if the paint lasts 20+ years then it seems quite a small negative to me. I'm increasingly tempted to use it and have only a few more years to decide.

 

[Abi Cott]

I found this when I was searching. It's the most comprehensive discussion of sd values I've found

http://cornishlime.co.uk/information/breathable-paints-explained/

 

[Lithgae]

Could well be the case, though it's difficult to assess such figures in isolation. For example, for a paint (or any covering) to be usefully 'breathable' what figure is actually required?

What's required would have to be tied into what the paint is covering, and the vapour permeability of that component. On the face of it this is simple, but as I see it there are three problems.

The first is that manufacturers seem to quote different data, probably depending on what suits their marketing purposes, and that makes it difficult to make a comparison. A material might readily soak up liquid water through capillary action, but be less open to water vapour permeability. Even for Vapour Permeability F and B quote an Sd value others quote a u value, so unless someone is willing and able to do the mathematical conversions its difficult to compare.

The second issue is the materials they are covering. Presumably that's going to be a lime based plaster in most old houses, but lime plasters have different properties depending on the mix, and how the vapour travels through the wall will depend a lot on whether the wall is Granite, Whinstone, Sandstone etc.

The third issue is design. In a bathroom do I want the wall to be particularly breathable or do I want to slow the vapour diffusion and instead extract the excess. I once heard it said that building layers should become progressively less open from inside to outside. That makes sense to me, but only if the outside is drier than inside. A lot of the time that's not the case.

However, to keep it simple and just compare paint.

According to https://www.mikewye.co.uk/product/seciltek-silicate-paint-sp-01/ "Paints with Sd values less than 0.1 m are considered to be highly breathable."

According to https://www.organicnaturalpaint.co.uk/blogs/what-is-breathable-paint-and-why-does-it-matter/ its 0.02 to 0.5.

According to https://www.lime.org.uk/community/paint-advice-and-faqs/how-breathable-are-your-paints.html limewash is 0.01

Either way its seems that the F and B Estate emulsion would be fine, and the earthborn paint probably would be too, as they are 0.01 and 0.02 respectively.

I suspect we spend too much time worrying about the finer details of breathability. Plastic, synthetic paints that don't publish vapour permeability data, or are actively marketed as a vapour barrier will be a problem, natural products with lime/clay, or water based emulsions that have published values in the ranges above I think will be fine.

 

[Abi Cott]

Had a nice voicemail message from Colin at Earthborn. He sounded keen to discuss the subject. I'll give him a ring back tomorrow.

 

[MikeG]

Hopefully by tomorrow someone will have fixed the forum such that it doesn't take 5 minutes to load every page.

I remain sceptical of any claims of measured breathability, when these are figures produced by the manufacturer themselves rather than an independent body. If the BRE as-was produced these figures according to a standardised test then fine, but there are too many variables here. For the sake of argument, if you slap a microscopically thin layer of paint onto a substrate of your choice and measure its vapour resistance before it's even set you are going to get a very different result from 3 layers of paint on a standard substrate at a controlled temperature and humidity, and X days after the coating was applied. I'm not for one second saying Earthborne or other manufacturers are cheating, just that the lack of an independent and standardised testing regime leaves any claim, by any manufacturer, vulnerable to the charge of producing meaningless and incomparable results.

 

[Flyfisher]

All fair points. As you say, a real issue is the lack of standardised tests so although manufacturers do their best their results are not easily comparable.

I also agree with Lithgae's points about the properties of the materials being covered; ideally it's important to know the breathability of all the components of a wall in order to make an informed decision about the type of covering that would be suitable. Typically, no one does this because the information is not easily available (or even at all) or difficult to interpret, so we tend to rely on rules of thumb or 'received wisdom' . . . . a bit like the average builder I guess. :wink:

 

[Abi Cott]

I've just had a nice little chat with Colin from Earthborn.

He was able to tell me that the clay paint is manufactured by Ecotech in Germany.

It's vapour permeability is independently tested.

That once dry it is stable and therefore an alteration in vapour permeability over time doesn't make scientific sense.

It also contains no resins or synthetics that could alter permeability and has been tested as being as vapour permeable as lime wash.

I've sent him a link to this thread so he might pop in and say hello.

 

[Lithgae]

I suspect we worry a bit too much about how breathable a paint is. I can't imagine the 18th century builder worrying about how breathable the materials they were using were. As has been mentioned before, the number of layers, how thick it goes on etc, will all impact the reality in practice. Paints will sit in a range and I suspect that all paints that are marketed as breathable, with published figures to back it up, will probably be fine, and will probably be more breathable than the stone and lime mortar underneath them. As mentioned in one of the links I posted, that's probably going to be paints under around 0.1 Sd.

For me I'm still not sure why I would want to spend 40% more on clay paint over F and B Estate emulsion which is more breathable. Does anyone know if there are any other characteristics of clay paint that make it more desirable?

 

[Lithgae]

I suspect we worry a bit too much about how breathable a paint is. I can't imagine the 18th century builder worrying about just how breathable the materials they were using were. As has been mentioned before, the number of layers, how thick it goes on etc, will all impact the reality in practice. Paints will sit in a range and I suspect that all paints that are marketed as breathable, with published figures to back it up, will probably be fine, and will probably be more breathable than the stone and lime mortar underneath them. As mentioned in one of the links I posted, that's probably going to be paints under around 0.1 Sd.

For me I'm still not sure why I would want to spend 40% more on clay paint over F and B Estate emulsion which is more breathable. Does anyone know if there are any other characteristics of clay paint that make it more desirable?

 

[Flyfisher]

I suspect we worry a bit too much about how breathable a paint is. I can't imagine the 18th century builder worrying about just how breathable the materials they were using were.

I think you're right about the 18th century builder but it's probably only because they were very limited in their choices of materials. Lime and limewash would have been the main available products so that's what was used - the fact they happened to be breathable and therefore very suitable for the buildings of the day was probably just a happy accident.

Today, we have all manner of material technologies to choose from, which not only makes things more complicated it also makes inappropriate choices more likely. There's nothing wrong with modern materials, only their inappropriate application.

I'd suggest that the importance of breathability in most period properties is really just an accident of history rather than by design and the plain fact is that many (most?) such properties were inherently damp. However, a combination of better ventilation (or poorer insulation depending on your point of view) and more tolerance of harsher living conditions meant that such properties were simply accepted as being what they were.

But let's be honest. We might all love the character and romance of period properites but few of us, if any, are actually prepared to put up with authentic 17th, 18th or even 19th century living conditions. Well, not unless we had a full complement of staff to make daily living reasonably tolerable I suppose. No, we want to live in an old, characterful period property with low ceilings, wonky roofs and stone floors but at the same time we want the heating to come on automatically in the morning so we awake to a hot shower in a luxurious indoor bathroom . . . which is where all the conflicting demands on properties and modern materials begin.

 

[Lithgae]

Flyfisher, I completely agree, but I'm not suggesting that we slap any old modern plastic emulsion on the inside walls or cover the external walls with cement. I'm just saying that neither earthborn claypaint or F and B Estate Emulsion are original materials, but if breathability or probably more correctly vapour permeability is what we are after, both are 'good enough'!

I've got a wall where the original plaster had to be taken off before it fell off and I have used EPS insulation and gypsum plasterboard in its place. I know some people will react in horror at this, but EPS is breathable and so is gypsum, and if I cover it with a breathable paint the entire structure will remain breathable. I am pretty sure said 18th century builder would have used it if it was available at the time. Don't get me wrong I have a certain amount of sadness at losing the old plaster and I have gone to lengths to preserve the lath and plaster on the other walls and ceiling, but sometimes we have to be practical, and as you say we want to live in warm convenient houses.

P.S. I don't agree that these old houses were damp, an open fire is a pretty effective extractor and air would have been changed too often for damp to set in. It's only when the fires were blocked up and the windows sealed that the damp sets in.