Picture courtesy of Ty Mawr Lime
The difference in breathability through a wall is really important. For the past 100 years we have worked with the notion of making the outside more waterproof to stop any water from getting into the wall. Builders, Councils, Individuals have all spent their monies on cement renders and thick waterproof paints thinking that this will stop any water ingress into buildings. But water has its wily ways. Cracks in the render, rising damp up through old walls, aged silicon sealant around windows and doors, blocked guttering and failed roof tiles have all allowed water to get in. Once in, the water is trapped by the render and paint and lo and behold we have peeling paint, blown render, damp internal walls and mould. So much for working against nature and water.
So if we turn the tables and try and work with nature and water, what is is the solution?
Well if we accept that our walls can get wet, then we have to be a bit clever and rediscover what techniques builders used for hundreds of years to keep the insides of houses dry. After all people have never wanted to live in damp homes.
The answer is actually quite simple. All you need to do is to make sure that the inside of the wall structure is less breathable than the outside. This means that any moisture in the wall will always be drawn towards the more porous side. The higher internal pressure also pushes any excessive humidity to the outside.
So how to achieve this.
We need to get a basic idea of how this works in practice. The simple thing is to look at the old buildings and see how they did it. The answer it turns out is really easy, but not only that, it is achieved by using local natural materials so it has a carbon saving as well.
The main elements of creating a breathable wall are:
1. using surface area (the higher the surface area – the more drying surface there is)
2. using a variety of porous materials in layers of increasing porosity to the external finish
So the basic equation is to have a really porous inside coating (like a claypaint or a limewash). This will allow the internal surface to do its thing with temporary peaks and troughs in humidity.
A smooth less porous plaster / render (like a lime based or clay plaster). It is best not to use gypsum here as gypsum will fail over time if it is exposed to lots of moisture
A thick wall (that you already have) – this porosity needs to be less than the external finish (and it generally already is as it will be a mix of stone and brick)
A highly breathable render on the external face (we would recommend a lime putty with limestone aggregate mix) with a reasonable rough surface finish
A final coat of lime wash. This is the most breathable element and will get wet very quickly, but will also dry very quickly.
If you want to keep a stone finish then use the lime putty mortar for the pointing
If you are just looking for a painted finish then use the lime wash or a silicate paint
If you have a very exposed wall then you may need to have a less porous finish on that wall. Silicate paints can provide this as they are tougher than limewash, whilst still being breathable
If in doubt about what to use then contact a reputable lime rendering company
This gradient of breathability has a range of benefits apart from keeping the inside dry.
It regulates humidity in the house, so if the walls can breathe the internal surface can absorb any temporary excess (when cooking, washing, showering etc.) and then slowly release it either back into the room as it’s humidity drops, or through the wall. A stable and lower relative humidity in a house has been shown to be much healthier for us, as bugs, moulds etc either prefer very dry or very wet conditions – not so good in the middle.
This ability to absorb excessive amounts of humidity also takes the pressure of high humidity off of other sections of the house, so condensation is reduced.
A dry breathable solid wall is around 38% more efficient than a wet solid wall so there are lower running costs to be gained once the capital work has been done.
Are there any downsides?
Well there is an aesthetic one. Limewash changes colour when damp and so you will be able to see your wall working. This might please some people as it is a very organic process that shows nature at work, but others might wish for a more consistent colour finish.
If you have used a hydraulic lime and sand render then your limewash will not last as long, so there is a cycle of maintenance that needs to be borne in mind. Limewash (depending on location, aspect, etc.) should last around 4-5 years before needing a further coat or two.
Timing is important. Not only does lime need to be applied during the warmer months (spring to autumn) it also needs a keen eye to ensure that the various layers bond well together. So an experienced tradesperson is required. This process takes longer than cement render as you are working with the materials and allowing them to do their curing at the rate determined by nature (weather, temperature, aspect, etc) and the underpinning substrate. So getting the render right might take a few days, or a couple of weeks. So patience is required, this is a way of working with your building and not against it and all buildings have their characters!
Let your building work for you. Remember that working with your main wall structure brings benefits to the wall itself, the health of the people inside and will ensure that the building has a long and stable life. Working with water is after all a lot easier than trying to work against it.