Damp in old houses

Damp in old houses is often misunderstood or oversimplified. The two most common solutions are very basic, the modern which is to cover it all over and hope it goes away and the so called traditional, which is replace everything with the installers preferred lime mortar/plaster, which alarmingly is often the same mix. Neither of these is particularly professional. Both have their merits I suppose but it is like trying to build a cabinet with a sledge hammer or paint the Cistine chapel with a roller. Often the cause is never prevented or fixed correctly and little to nothing is actually done to improve the state of the building units. In more than a few cases the solutions are compromised by the lack of awareness of the specifier and installer. 

The most confused aspect of lime seems to be its sacrificial nature; this is often referred to as a means of buffering the building units to ensure their longevity. I struggle to understand how this is even remotely logical, there are still soft sections of mortar in some very, very old walls. Thousands of years. And I would ask what people think will happen if the mortar is stronger than the building units...? This is because of thermal dilation; stiff mortars like cement move more and can't repair themselves like lime. The increase in strength is almost coincidental and it wouldn't matter if the mortar was stronger than the building units if it had a similar rate of expansion and contraction or the building had expansion joints. The strength issue is relative to the original mortar vs the new because if the front of the wall moves at one rate and the back at a different rate then it will often find a weak point in an arise and break their if the binder of the new mortar is strong. Hence why people think its a strength issue vs the building units. If you want to be specific; its adhesive strength vs tensile strength...I think. You're new mortar is incredibly well bonded and acts like a big mesh over the face of the wall, this mesh expands and contracts, the back of the wall is a different mesh. The building units are literally torn apart as those two forces fight one another. I get why people think its strength and it may seem pedantic but my issue is that people are making the argument that a NHL 3,5 mortar can be paid over an air lime with 1.5 Nmm/2 at most. It can't and not only that but it will have less freelime in it so will draw water back into the wall as the original mortar has a greater capillary action. 

Lime dries, more lime dries quicker. Lime dries by pulling water to the driest section by capillary action. This can be in the wall too. 

The real issue is salt, it has always been salt and it always will be salt. So many aspects of masonry are for the sole purpose of preventing salt from accumulating in building units. Water brings the salt in. Salt is everywhere. Modern buildings are strong enough to resist the effects of salt and thermal dilation is mitigated by expansion joints. In old buildings the rate of thermal dilation is lower and is managed by limes increased flexibility and micro fissures forming in the lime and then being repaired by carbon from the atmosphere being deposited in those cracks by moisture movement through the mortar. It is then converted to calcite to fill those micro fissures. The salt is borne by the water and then it's deposited on the surface of the mortar joint or render as the water is removed by convection. 

Once lime is full of salt it can become unbreathable and then it needs replacing. That's why it's called sacrificial. Red Mortar will last far longer does this very well.  

So many of the whole house lime replacements fail to acknowledge this eg. concrete floors and gypsum plaster causing damp in wall bases are removed and hot lime plaster is put on the walls and limecrete is installed in the floors. Well...that damp is because cement has a lot of salt in it and the wall bases are very porous, specifically designed to absorb moisture and salt in fact. So when the original concrete dried out, the salt water was absorbed by the wall bases. They dry out on the surface because of how these walls work (see below). The salt then forms new unbreathable minerals as it combines with the freelime, the same freelime mentioned  before in relation to drying action and the repair of cracks when combined with carbon from the atmosphere. This is basically the same thing happening with the wrong ingredients, in the wrong place; the surface of the joint or render where we want our moisture to be released. This is identifiable by spalled and crumbly mortar interspersed with sections of very hard discoloured mortar. The spalled crumbly mortar is down to trapped water behind the newly formed mineral crust freezing. 

To slightly digress, you can remove salt from masonry with a lime poultice as that's what it is, essentially. So if you have a wall in the condition mentioned above and then replaster/repoint it with a hot lime mix, it will poultice the salt into itself and then become compromised. So what often happens is that one 'lime specialist' will do this and then it appears not to work so people hire another who replaces it and then it works because the salt is removed by the first persons plaster. 

If you dry out a salt laden wall without removing enough salt then it will expand when it dries and cause the same type of damage that freezing does. 

The limecrete installed will be made from NHL5 and in some truly terrible cases, recycled aggregates. Now I'm going to say this anyway but it should be obvious, recycled aggregates are chock full of salt. The most stupid and ignorant thing I have come across recently, that or hempcrete, which is just another sort of stupid. That water has to go somewhere and it often finds itself into the wall bases which are already full of hygroscopic salt from the last lot of concrete. NHL is often full of salt too and there are much more stable, strong binders that can be used. My preference would be a pozzalonic hot mix. 

So it is often more sensible to cut out the edge of the concrete which is 'creating pressure and driving water up the walls', rather than introducing more salt. The good option is to use a proper lime concrete instead of the incredibly basic and ignorant limecrete. There a host of bespoke solutions that can be utilised like clay lining, rammed earth floors, pervious concrete, sulfite resistant cement, suspended floors, opus signinum etc. 

Because I'm not a twat, I won't just say, "poultice everywhere at great expense". You should but considering how reluctant people are to pay for anything which isn't shiny and on display I thought I would provide an alternative. Use a brick dust base coat before plastering as this will prevent the salt from being drawn out of the wall into the new plaster. It will also, hopefully, prevent the wall from drying completely and therefore preventing the salt from causing damage as it dries. But this shouldn't be an issue unless you have exposed masonry. Which is just another sort of stupid, but that's a story for another day. 

Moisture in walls, it is not as simple as you think. The two most overlooked issues are; firstly it moves into the wall as vapour and leaves it as liquid, then a vapour at the surface. But the leaving is an imperfect process, so it leaves behind some water which collects in the pores. Once enough water is amassed in the pores again it can then flow out by capillary action. Secondly when there is a lot of it, it drains downwards. Which is a rarer problem with most houses but important with cellars. This is also the issue with chemically injected DPC's, they prevent downwards draining. you can see the physical action of how moisture in walls works when you apply a new lime rich hydraulic plaster to a damp wall. It will get dry at first but still feel a little damp. Then it will weep with droplets on it's surface, dry off and then do it again.  

A note on rising damp; its a symptom not a cause. The cause is often salt or downwards draining within the wall and an impervious layer like a chemical injected damp course. So lime built houses can manage almost any amount of water when maintained well and designed with it in mind. If you create impervious layers with salt mineralisation or unsuitable solutions you will almost certainly have problems that look a lot like rising damp. And to be far, if a wall is salt laden and there is water below it, it will absorb moisture upwards but it is rare. Because the wall would have to not be damp, which is unlikely as the salts are hygroscopic so attract vapour into them. 

Just saying something doesn't exist and then applying a 'lime panacea' wholesale is not a professional solution. 

https://historicengland.org.uk/content/docs/research/water-in-permeable-building-materials/

https://www.youtube.com/watch?v=loM22NK4JdE&list=PLmJDknXr4_-JbgqQA5-kFdFkD4DL34XM4&index=9&t=2s

https://www.buildingconservation.com/articles/high-exposure-mortars/high-exposure-mortars.htm#:~:text=The%20combination%20of%20heat%20and,be%20beneficial%2C%20enhancing%20its%20durability.