Damp problems again!!!

[Stuart Noble]

different

It does help by making it slightly more flexible and ensuring that it bonds effectively in the first place. SBR allows an overall render thickness of

3/8", which wasn't considered possible before. Structural movement is generally small so it doesn't need to be that flexible to permit movement. There's also the chicken and egg aspect. Things that get wet swell so, if you can stop them getting wet in the first place, the movement is likely to be less. You can't control movement in the ground below but what you do above ground helps. I don't think anyone has the answer to these problems of maintaining Victorian houses without spoiling their appearance and at the same time making them fit to live in by today's standards. It doesn't help to adopt a brown eggs and bicycles stance and say we'll just use what the Victorians used. In the case of my current house, what they used was lime with brick dust, which sets just fine (and how else could you lay bricks with it?) but, it seems to me, hydraulic lime gives you the worst of both worlds, the brittleness of cement without its good properties. I really would like to hear comments from anybody that's actually used lime mortar for re-pointing. Was it hydraulic, how long did it need to be covered etc?

[Pete C]

What's wrong with that? These days I prefer to learn from others experience and expertise than by my own mistakes.

I've lived in a number of houses. I do have a bit of first hand experience of wood and rot, damp and dehumdification, steel and corrosion though...

Yes, SBR is an excellent bonding agent and waterproofer. Although renders based on Pliolite resins based in turn on SBR are often microporous, I don't think that mixing your own render using an SBR additive will guarantee it will be microporous. It seems that the amount of SBR in the render is important to get the best of both worlds.

cheers, Pete.

[Stuart Noble]

Ah, well that's where we differ. Although I listen to experts, and (occasionally) read instructions, I don't trust either, and there's no substitute for conducting your own experiments if you have a basic understanding of how different materials work.

This is a case in point. The basic resin gets developed for a particular application (this is classed as a synthetic rubber I believe) and the marketing guys start wondering how else it could be used. Good for paint because it doesn't have the tacky finish you associate with flexible coatings and looks the same as normal paint. Floor screeding is another success for SBR. As for its use in render, it's immediately obvious that, unlike pva, it has no affinity for cement. The SBR separates out after a few minutes and has to be constantly re-mixed, so it's a marriage of convenience rather than a properly formulated product. As I said, I think the microporous issue is a red herring. For a render consisting mainly of sand to not allow the passage of air would be some feat. Things are not so easily sealed to the point of being airtight. Somebody recently posted a list of surface coatings and their vapour blocking capabilities, and they were all pretty pathetic. For example, no amount of paint will stop a piece of wood shrinking to the dimensions dictated by the ambient conditions, even if the end grain appears to be sealed.

[bigcat]

waterproof.

thickness of

not with Vic houses

its standard practice

yes they do

It doesn't help to adopt

Victorians

[Pete C]

True, but sometimes it's not practical to conduct experiments, eg whether a wood preserver will perform well over 5 or 10 years.

I would disagree with this, any render allowing air to permeate freely from one side to the other would also be highly porous to water, defeating the object of using it. Also it would be liable to frost damage.

Render is also used extensively on straw bale buildings, where air infiltration through any tiny gaps or pinholes can cause condensation within the wall.

There are some big differences which is why wood stain is often used in preference to varnish or paint.

cheers, Pete.

[Mike]

I'm in the process of doing a large house a section at a time. I also use lime internally for plastering and mixed with Optiroc (LECA) for use as insulated flooring. Get through huge amounts of the stuff.

No - standard Buxton non-hydraulic lime. I never cover it at all, simply ensure the stone is well dampened beforehand then re-dampen the pointing where needed in days to come.

I have found the best approach is to put on more than is needed then once it is set hard wire brush it to the required size. However this only works with stone - brick would be stained by the lime.

[Mike]

The most common render for straw bales is lime and hence it is permeable to water.

[Pete C]

Indeed, there is a good study of vapour permeability and water absorption of different render here:

It also shows that a siloxane coating will hugely reduce water absorption through lime render without reducing vapour permeability.

cheers, Pete.

[Andrew Gabriel]

That's a very sweeping statement for two substances with extremely different properties. It's possible to make use of the various different properties (visocity and surface tension are two which spring to mind and are often utilised) to make a material behave very differently to water and air permiability. Surface tension in particular is a very useful property which can be used to either encourage water permiability via capilarity, or to discourage it by use of a positive maniscous as often done by silicone based treatments.

[Stuart Noble]

This is the whole point. Water vapour will travel through virtually anything. Droplets of water are easily repelled by a surface which is a) vertical and b) not unduly porous.

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[Stuart Noble]

Or Thomsons Waterseal to get technical. The same stuff that's used for dpc injection, only painted on your wall. I don't think that would go down too well on the periodproperty site. Fortunately it isn't a "coating" because siloxanes don't form a continuous film. They are like tiny pellets of grease which are reasonably effective until the pressure of rainwater dislodges them.

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[Pete C]

Hi,

AFIAK Thompsons water seal is silicone based, which is quite different to silanes and siloxanes.

There's a bit of discussion on chemical DPC for period properties here:

As far as Thompsons water seal goes, anything that's pushed heavily to the DIY market is unlikely to contain a lot of expensive ingredients ;)

cheers, Pete.

[Pete C]

I _think_ you got mixed up between air and water vapour, if you had said

'As I said, I think the microporous issue is a red herring. For a render consisting mainly of sand to not allow the passage of water vapour would be some feat. Things are not so easily sealed to the point of being a vapour barrier'

that would have made a lot more sense to me, though I would still disagree.

Anyway IMVHO a lime render with a siloxane treatment will be much more vapour permeable than a plain cement render, if you feel that they are more or less the same or the latter will always have enough vapour permeability then we'll have to agree to disagree.

cheers. Pete.

[Stuart Noble]

Doesn't say what's in it on the tin, but I guess it's the same as trade water repellants, which certainly are siloxanes. Silicone is not used on masonry because of its film forming properties. The idea with siloxanes is that the particles remain separate, and can be moved about within the wall by water pressure.

Who says siloxanes are expensive? Being the basis of dpc injections, I wouldn't think so.

[Stuart Noble]

I don't think there is a difference in this respect. Air at 80% relative humidity will travel through anything that isn't airtight and take its moisture with it.

How come sand and 20% cement turns into this airtight barrier? IMO neither option will offer any hindrance whatever to vapour. If rain water

*droplets* get behind render then the bulk water cannot get out again, but that has nothing to do with vapour.

To come full circle, the advantage of the lime is, of course, that it remains flexible, but I have my doubts about the effectiveness of anything sprayed on to something as absorbent as lime. Try varnishing a loaf of bread! What might be interesting is incorporating the siloxanes (the water based type) into the mortar. Hmm.

[Mike]

The only safe thing to pu on lime is limewash :-)

[Pete C]

Looks like I was too generous about it, the following page for 'Thompsons Water Seal Ultra Waterproofer' which references the MSDS at the bottom is revealing:

'Paraffin 008002-74-2 2.9% Solvent naphtha, petroleum, medium aliphatic 064742-88-7 7-8%'

ie paraffin wax in a base of white spirit. No silicones, silanes or siloxanes mentioned at all.

For reference, here is a page from the same database for another product that /does/ contain silicones and siloxanes, being mentioned in the MSDS though they comprise < 1% of the product:

cheers, Pete.

[Pete C]

I would have thought a lime/sand render is pretty airtight. Can anyone else comment?

Well siloxane based DPC liquid can be used successfully on genuine cases of rising damp involving lime mortar, as discussed on the PP thread mentioned, so I expect siloxanes can be used sucessfully on lime render too.

Also this company list a waterproofer as being suitable for render, limestone/sand brick and limestone which is very absorbent, so I expect it would be fine for lime render too. I'd expect they can advise in any case:

[Mike]

Depends on the lime:sand ratio and the mix of sands used. Getting this right is the key to a good lime render.

[Stuart Noble]

Naptha's a lot faster than white spirit, but 2.9% paraffin wax seems like a lot of solvent for very little solids. Not exactly environmentally friendly.

used to have some quite detailed specs about their products.