Amazing facts about lime render

You might be wondering what on earth is lime render?

Believe it or not, lime render is a type of plaster, traditionally used to coat internal walls. Nowadays most internal walls in homes are made of gypsum plaster. If you’ve ever seen inside a house before it has been painted, you will have seen gypsum plaster – it’s an unmistakeable dark pink when it’s put on, and dries lighter.

First, let me tell you something of the fascinating history of lime. Lime render is also sometimes known as lime mortar or lime plaster, and is made from burning limestone. The substance you are left with after burning gets harder with age. The earliest known use of lime in construction is about 4000 B.C., so it has been around for a long time. The Romans used lime in buildings all over their empire. They also invented underfloor heating, which is another thing I am using in this build. So the Romans still have a big influence all these centuries later, and lime render has been thoroughly tried and tested and shown to last a very long time.

Making lime render isn’t simple. It has to be mixed in exact proportions with sand. Lime and sand mortars take time to harden, and don’t harden at all if under water. However, those clever Romans even invented a way of adding other ingredients to make the lime usable underwater, otherwise we wouldn’t have all those amazing aqueducts they made.

Advantages of lime render

So if lime render is tricky to make, why are we going back to using it today? It has several advantages over gypsum plaster.

  • It works extremely well with an airtight membrane. (For my build this is a major advantage, and if you missed my post about the airtight membrane, you catch up here.)
  • It is breathable.
  • Reduces likelihood of damp or mould.
  • Lime render is a more traditional option than gypsum plaster, making it a better choice for an old house.
  • It is long lasting.
  • It can be used on the inside of a building (in place of gypsum plaster) or on the outside (for example instead of concrete.)
  • Easily recycled.

Advantages of using lime render with an airtight membrane

Let’s take a closer look at that first advantage – at why lime render and an airtight membrane work so well together.

It might seem bizarre to use a product that is breathable along with one which is airtight, but this isn’t as contradictory as it seems. In his article A Guide to Airtightness, eco expert Tim Pullen explains that breathability is not primarily to do with air. Airtightness means that there is minimal leakage of cold or hot air into or out of the house through cracks in the building materials, gaps between joins or where different materials meet – for example around windows or doors. As Pullen explains, UK airtightness regulations are fairly lax, and renovating to eco standards means going way beyond those standards.

Breathability, on the other hand, simply means the walls of a house take in moisture, but will also release it so that it dries out again. A building material with the ability to release any water vapour is called “hydroscopic.” When creating an airtight building it is essential to have breathable walls, otherwise you’d have a very damp house. Breathable walls reduce the growth of mould and dust mites, both of which contribute to allergies.

Years ago, before we had cement, all buildings would have been breathable and would have been plastered on the inside with lime render. In the UK everyone changed to using cement as it was much easier to use. However, lime is very recyclable. Old lime that has fallen off a wall can be used again as aggregate and mixed in with new lime. Old cement can also be recycled, but at a greater cost.

How the lime render and airtight membrane work together in our build

Lime render also acts as an airtight barrier itself. For our build, in areas of the house where there is lime render, such as on the inside of all the external walls, we can smooth the render over the edges of the airtight membrane. Together this forms a continuous airtight barrier. To attach the airtight membrane to the plaster successfully, we have to use a tape fastened to the edge of the membrane, which has a mesh on it. The mesh is what gives some grip and makes sure the join is secure. The lime plaster will then form part of the continuous airtight lining to the house.

As you might imagine, this is a complex process that required quite a bit of planning, preparation and care, and that needs a post in itself! Stay tuned for the next exciting instalment of how we used secret strategies to improve the breathability of my home!




Unexpected complications when rebuilding an old house


Having done the ripping down and strengthening up of the building by putting in metal beams, we arrived at the stage of taking off the main roof.


Lots happens at once during an eco-renovation

The builders took all the tiles off the main roof, and stored them in the front garden as they will go back onto the new roof. Whilst this was going on there was another team putting in the new joists for the attic bedroom floor, the old ones having been totally removed. You can see in this photo that there is a bit of plastic sheeting poking out over the tops of the walls. This is the start of the airtight layer that will eventually line the whole of the inside of the house. It will be continuous, with the sheets joined by a special tape that makes it completely airtight so no cold can get inside.


Working out what straight is!

One issue that came up was the front wall of the cottage, which isn’t straight at all. Sandy, the architect, had a meeting with the builder to decide where straight is, as far as putting the roof back on goes. They looked at the plan in the photo below and decided where straight would be. This was quite a complicated task because they had to follow the roofline of the neighbouring house and to take into account the line of the back wall. At one point, it looked as if the pitch of the roof might have to be at a different angle at the back from at the front. In the end, they were able to make the straight line though back wall parallel to the straight line at the front and so the pitch is even.


Sandy took a line which is going to be the straight roof line, and where the wall bends in, the eaves of the house will overhang a bit more at one side. There has to be something covering the insulation etc when you look up at the underneath of the overhang, every detail from every angle has to be thought of. But it’s all a bit wonky, it’s an old house after all!


Luckily the back wall is fairly straight. Having said that, it might be straight horizontally but it is uneven vertically, which caused some challenges you can read about in The Main Start of the Eco Build – Unbuilding and Ripping Down.

An old man who used to live in the village told me that he could remember when the house used to be one storey, and the people who owned it took off the original thatched roof and made the walls higher. The walls of the original cottage have two layers – an inner and outer shell. When the new wall was added, it was only one layer thick, and surprisingly it was the inner layer they extended! This creates a shelf on the outside where the two walls meet. The shelf was of mortar and open to the elements. Because mortar is porous, water could seep in.


Between the layers, the original builders put rubble and waste building material, some as fine as dust. Over the years, this settled, but with the porous shelf this settling increased, since rainwater soaked through and washed it down. This meant much of the wall had no insulation and it was no wonder I was so cold during winter. It also meant that when the Society for the Protection of Ancient Buildings (SPAB) came to check the cottage for damp, they found it – but in an unusual place. It was very damp just below the shelf, so water had seeped in. The dampest place on an old wall is normally close to the ground where water gathers, and this can soak up the wall.


To protect the cottage from further seepage, the simplest solution is to make the shelf watertight, most likely by fully covering it with a sheet of lead. This lead covering is a job for later on in the build, and meanwhile the builders took the old roof off, saved the tiles and started work on creating the new roof. You can read all about that in my next post.