That'll do!

It has only slowly dawned on me since following threads on this subject that a "rubble" wall meant literally that - two reasonably sturdy skins, with the internal void just filled up with rubble tipped in. I suppose I'd assumed that the word rubble was a stonemason's disparaging term for random undressed stone picked because it happened to be roughly the right dimensions for the space, with a generous bedding of sloppy mortar to fill the gaps.

I'd also assumed that large stones that were long enough to go right through the wall were prized because they would add strength and cohesion to the wall. The best face would be on the outer wall, and the inner face was only approximately level because except in the most primitive of cottages it would be generously built up with mortar and render to achieve a nice flat smooth surface, for papering or painting.
That is how our house (c 1880) is constructed. The internal mortar is not particularly strong - the wall could be demolished by hand alone stone by stone starting at the top - but there are no gaps and it certainly doesn't just slump or run out if you pull out some stones.

I can see that having stones running right through the wall creates both thermal bridges and easy tracks for water to run past, so I agree it would have been logical to render the outside with something impermeable. However, that was not originally intended, unless the Victorians went to great trouble to do attractive brickwork around doors and windows and then immediately cover it over with render or corrugated iron sheeting. I don't see much vanity there!
So the design is inherently flawed, which it seems to me can only be mitigated to some extent by a compromise, hence the invisible "render" provided by some kind of damp-proofing solution.

An alternative would be to batten, insulate and board the inside of the walls. That requires a great deal of effort, extending the wooden panelling in the angled window embrasures, removing and refitting the surrounding mouldings, lengthening the top moulding, extending the wooden cills and remaking the moulding, and moving the skirting board inwards and refitting against the new boarding.
That would still leave the wall below floorboard level. Conventionally this would be a void between floor and ceiling below, in our house there are no ceilings, just the underside of the exposed joists, socketed into the walls with irregular rectangular gaps, some not very straight because the wood has warped or was never straight. I'm not sure that sealing a joist into a damp wall is a good thing.

So the whole thing has to be a massive compromise between aesthetics and comfort, complicated by the recent introduction of thermal efficiency into the equation. :)

Sounds tricky. I have only once lived in a house with stone walls and water came in there. It used to drip from the top of a window and the crack wasn't where you would expect. It took a hosepipe to find it. Stone is really tricky to seal.

I think Wales might be quite wet. I've heard of someone else in Wales with a stone built house applying a silicone spray product to the outside over a lime pointed wall. He shouts about it at every opportunity as it seemed to work for him.

If you were to try a spray on waterproofing product it would be good to chose a mild one that was still a bit breathable. I've spent the last few years removing waterproof stuff as while they might work initially they tend to have little failures after a few years so water can get in but can't get out again so you have the same problem with internal damp just worse.

Photos would be useful should the internet have developed to the extent that we can post them on here.

Coming a bit late to this, but an interesting thread.

I'm a little bit of a lime sceptic myself - but it seems to me, there are some situations where it's fairly clear that it's probably a good idea, some where it's clear it's probably a bad idea, and lots where it's very hard to say.

I think the argument for using lime mortar is strong where the masonry is soft/porous - old brickwork or sandstone for example. As far as I can see, it's absolutely true that cement mortars can do damage to soft masonry - the evidence is everywhere, in old brick walls and old sandstone walls where preferential erosion is clearly visible around cement mortar.

If there's no danger of the masonry eroding in preference of the mortar - granite say - then the argument for lime seems weaker.

A question which remains unresolved for me is how cement/lime mix mortars perform, long term, compared with "pure" lime mortars. I did a thread on this some time ago... I'm not sure if any better level of evidence has appeared since then:

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=8623

Yanntoe - moving to your case specifically - it's a bit difficult to fully understand the situation without drawings or photos, but am I correct in understanding that you've identified that the problem is at the window opening reveals because these are the locations where there's a direct connection from inside to out via a mortar joint, because at these locations stones run the full thickness of the wall, unlike the general situation where there's a rubble filled partial cavity separating inside from out?

I'd agree that's essentially a design weakness, and one that was probably "dealt with" in the original construction by disguising it with internal panelling and/or simply accepting more damp than we consider reasonable nowadays.

Approaches I might consider are:

1) If the stones that for the reveals are sufficiently hard, that there's not a worry that cement render will lead to erosion, then just point these locations with a non porous cement mortar. The rest of the general wall can still use a lime mortar.

2) Is there some way of introducing a kind of vertical DPC in line with the windows? In other words, outside of this line, it's lime mortar that you accept will absorb some water - however, from this line inwards, you use something else. I don't know if that would just be a change to cement, or digging out a significant depth of mortar over a 5-10cm length and replacing it with some kind of impervious sealant.

If the through-stones are also of a soft material, and might be transmitting damp themselves, there's also the option of cutting a vertical slot, all the way up, in line with the window, and inserting a strip of DPC and/or sealant. This is an approach I've used for joining into existing, old brickwork walls, to minimise the risk of damp travelling from inside to out, whether through the mortar joints or the bricks themselves. Of course, you'd have to satisfy yourself that doing something like this didn't cause any structural concerns.

Posted By: YanntoeThanks for the suggestion lineweight.
Basically, I've pretty much done what you say which is to insert an SBR/ mortar mix into the joints around the reveal, and then re-lime point the external part with NHL 5.
I've then used a breathable water repellent, Stormdry, on the wall.

So far, this has worked, but we're only 2 months into the wet season so far so we'll reserve judgement until the Sprng. There are still areas which need to be done, so once the weather allows , I'll finish the job.

It would be interesting to know whether the water repellent is necessary, or whether the focused approach to the reveals would have solved the problem by itself.

I'm almost certain that the water repellent is the key.
This is the third attempt at getting the wall dry and whilst I have "grouted" the area around the windows, it is clear from the water marks in the wall that water is arriving at the location from above. Basically the lime pointing in the wall becomes saturated and eventually where it arrives at the reveals the water wicks through the wall.
If one could grout the whole wall then I'm sure that it would work without the Stormdry, however this is essentially the same as pointing the whole wall in waterproof cement mortar. The lime would be purely decorative.

DJH is right though, water penetration is only an issue on the wall which faces the weather and the corners at either end. The rest of the house seems to cope.

However, I did read the research document suggested above - all of it, and the clear conclusion was that exposed lime walls get wet right through and rendering is the only thing that deals with this.

A modern breathable repellent plus judicious use of cement mortar grouting seems like the last possible action before rendering.

So, fingers crossed.

NB we do get an average rainfall of 200mm per month and most of this arrives from the west. There's no way that this can evaporate from a west facing wall during the winter, so if it is absorbed by the lime pointing it will accumulate in the wall. If there is a bridge at any point in the wall, water will penetrate and become visible on internal plasterwork. So reveals are a real issue.

Not really lineweight,
As in many stonebuilt houses, the reveals are angled which means that where the window sits, the wall is only one stone thick. At this point the lime mortar on the outside is continuous with the inside plasterwork. Moreover, there are effectively "through stones " at the reveal rather than an outer stone , rubble and then an internal stone Leaf of stonework. So here joints run all the way through the wall. Therefore if enough water arrives at any of the window margins, either from direct rainfall or seeping down from above, it will eventually get into the inside if evaporation doesn't exceed wetting.
And in the West of Cumbria evaporation has no chance for at least 7 months of the year, or maybe more!

Firegirl - Have you thought of standard EPS EWI as an option? I have used it on a couple of stone/rubble built houses without any problems and with great advantage to the thermal efficiency of the buildings.

Cement render is definitely wrong. That's the starting point. It should come off.

Lime render might be part of the solution but in a high wind situation I think the only solution is a two-stage system where there is a (ideally a balanced*) rainscreen followed by a drainage plane and then the airtight structure.

So lime render on the outside of stone might be a good drainage plane. Outside of that could be a timber rainscreen. Or you could add external insulation finished either with a render or a membrane, and outside of that a timber rainscreen. Instead of a timber rainscreen you could use render boards and render, or a separate stone wall etc. Conceivably you might get away with growing plants as a screen.

* Do a search if you don't know what a balanced rainscreen is, or ask here.

Posted By: Peter_in_Hungary

An interesting report, from my quick reading it appears that most of the failures are given to failures caused by faulty construction / application rather than inherent faults in the system.

That applies to all ETICS installations and isn't necessarily limited to this study. I think the purpose of this part of the paper is to outline the common failures found in ETICS. Here is a link to another paper discussing failures in southern Europe mirroring the same issues https://www.apfac.pt/congresso2012/comunicacoes/Paper%20104_2012.pdf

thin film render performs better than thick film render (thick film= 15mm or more)

Hmm, not sure how you can arrive at that conclusion given that the paper says:

"A small share of the knowledge base in the SINTEF Building Defect Archive is associated with
thick-render systems (render thickness > 15 mm). As a result of poor previous experience with cracking,
such systems are not currently used in Norway."

So >15mm thickness renders are not the subject of this paper.

EPS is a better option than mineral wool in driving rain conditions

I don't come to that conclusion regarding what is said re EPS, rather that EPS is preferrable as a substrate for the render due to how it intereacts with the render during application.

"The ageing properties of systems with rendering on EPS are generally found to perform better than
those with rendering on mineral wool. The difference is revealed both in building-defect experience
and through accelerated climatic ageing. The main reasons for the difference can be associated
with the difference in substructure. Whereas EPS absorbs very little water, mineral wool can absorb
large quantities of water. This means that more water is drawn out of the fresh rendering mortar
before/during setting. Too little water in the mortar during the hardening process can lead to a more
porous render. Furthermore, installing mineral wool boards with plane surface is more challenging
than the installation for EPS. Accordingly, mineral wool board may make an unfavourable substrate
for the rendering because of the high risk of cracking during the hardening process of the mortar.
In addition, mineral wool can absorb more moisture than EPS, causing slow drying of the ETICS and
presenting a higher risk for frost damage of the rendering. The U-value is also affected negatively by
the moisture content of the insulation."

I can identify where this is applies specifically to wind driven rain as it may lead to less defects, thus improving its performance in the relevant climates.

And the more exposed the situation the more care is needed with proper installation especially at sills and ledges.

I'd most definitely agree to that, but given the specifics of the paper:

"Driving Rain Challenges
According to Köhler [7], most moisture defects in Sweden are found in buildings on the west
coast, from Skåne to northwards of Gothenburg. There is a clear connection between driving rain loads
and the extent of leakages. The same pattern can be seen from a survey of Norway, yet the extent of
mould damage is small. The greatest moisture defect is found in areas exposed to considerable driving
rain loads.."


"5. Conclusions
Many of the ETICS perform satisfactorily in parts of Norway; it is often a good and effective
way with which to insulate. However, this survey has shown that such façade systems are vulnerable
in areas with heavy driving rain loads. The technique provides only single-stage protection against
wind and precipitation. Hence, the consequences of cracked and weakened areas in the render are
considerable. Results from this survey and previous research shows that façade systems with ETICS do
not dry effectively after being wetted by rain exposure; therefore, these systems are highly vulnerable
to moisture that penetrates into the insulation layer"

From what I derive from this paper, if I was living in an area in the UK subject to considerable amounts of wind driven rain, I would most definitely be looking at using a 2-stage system as insurance against this, especially if there's local history demonstrating its use for the protection of the building fabric.

Posted By: Peter_in_Hungary
And as numerous posts and threads on this forum it is clear that builders lack either the understanding or the will to do a job appropriate to the method being used.

Yes, as you say, this may indeed be the most significant risk.

Update as requested by fire girl above.
Well it's been a wet and windy winter and we have now turned the corner in terms of getting some sun on the west wall as the equinox approaches!
And the news is good!
Most of the problematic areas are now dry.
The Conservatory Is dry and the West Facing window reveal is dry.
The North reveal is probably dry but I'm not 100% sure. It's certainly hugely improved, but until redecorated I can't be absolutely certain.
There is one small patch with some discoloration on the inside paintwork, but I'm pretty sure this is due to condensation on a " through" stone. The plaster is hollow at this point so I'll repair it this summer and see if it remedies the issue. I'm optimistic!
All in all, judicious use of water proof cement in the problem areas. (Probably less than 1% of the exposed walls) plus Stormdry over the whole problem area has worked.
Cosmetically it has darkened the stonework slightly, but a casual observer would not notice this. It's very easy to apply.
I'm not sure how long it will last but could be redone from a ladder in a few years' time if needs be.
The Superdry is pretty expensive but goes further than expected so all in all probably good value.
Hope that's of some use, certainly I would employ this technique on any exposed lime pointed wall, particularly where there are angled window reveals.

Posted By: graham_cbrukhot lime

Are you sure you mean hot lime? Otherwise known as quicklime? CaO?

If you don't want to use NHL, for reasons I can sympathise with, I think it would be more normal to use a mortar made from lime putty, perhaps a pre-made product. Somewhat simpler to deal with than slaking quicklime!

I haven't used it, but a quick look at the Remmers Funcosil FC data sheet suggests it is probably OK, but I would certainly do a test on a smallish area somewhere first, as they advise.

Posted By: djh

Posted By: graham_cbrukhot lime

Are you sure you mean hot lime? Otherwise known as quicklime? CaO?

If you don't want to use NHL, for reasons I can sympathise with, I think it would be more normal to use a mortar made from lime putty, perhaps a pre-made product. Somewhat simpler to deal with than slaking quicklime!

Well to be precise I meant hot lime mortar. We used it hot (mixed on site) in the insulating plaster but plan to use it cold (pre-mixed) for the pointing as we're waiting till the frosts have passed (if they ever do - apparently we've just had the frostiest April on record). I'm led to believe that there is then less benefit in using it hot and it's obviously quicker and less labour intensive not to have to mix on site.

Hot lime (mortar) is increasingly believed to be superior to NHLs but it's also more absorbent, hence my interest in trying it out with the StormDry additive...

(Good article here: https://www.buildingconservation.com/articles/hot-mixed-mortars/hot-mixed-mortars.htm)

Hi Graham,
Yes - our window reveals also had a slate "dampcourse" as did the space between the joists in the 1st floor where they meet the walls. Is that a local thing I wonder?

I guess you also get all the weather from the West on one of your walls - Our West wall was originally rendered, but none of the others. Knowing what I know now, I would probably have re-rendered it rather than trying to replace the cement pointing of the 1990's but never mind.

I'd be interested to hear how the Hot Lime mortar goes - the NHL I started using was NHL3.5 but I've now replaced it with NHL 5, on the problem wall - as the stone is basically indestructible it shouldn't be a problem. NHL 5 isn't as nice to work with as 3.5 and does tend to shrink back from the stonework unless you are very careful, so in hindsight, given that even the NHL5 gets wet, I should have used NHL3.5 and applied the Stormdry to that.
Are you doing the "Hot Liming" yourself or using someone like Stephen Grindrod?
Good luck - looks like we're at the end of frosts now and might even pass the "above5 degree" test over the coming days.