External render finishes to solid walled buildings (esp. brick & stone)
Many of our homes are rendered, but most are not rendered using the right materials. Older houses (mostly made with solid walls of stone and brick) used breathable materials and lime mortars to bind them together. This system allows the walls to take moisture from the inside to the outside thanks to a number of factors:
1. The outer walls have a higher surface area (rougher texture) and this naturally means that they dry quicker and hence pull moisture through from the inside.
2. Houses have higher pressure inside than out, so air pressure also helps to push moisture through from the inside.
3. The materials used on the inside generally are less breathable than those on the outside, thus creating a gradient of breathability through the wall that attracts moisture to the outer part of the wall.
These characteristics mean that these walls will naturally keep the inner walls dry whilst allowing the outer walls to take the brunt of the weather. The outer walls will get soaked and absorb moisture, but will also dry out quickly as well. With the depth of these walls being between 22cm and 100cm this is no problem.
Over time construction practices, training and materials have changed and the pressure is now about using materials that are quick, convenient, easy and profitable. Material choice for most builders is not about choosing the right one through research, but about what they know. Our older terraces and buildings therefore get the same treatment as a more modern cavity walled house. This means the use of sand and cement for re-rendering.
The basic underpinning principle to this choice is that by using a water-proof render you keep the water out. Rising damp is then ‘cured’ by the installation of a chemical damp proofing course. Internal moisture is removed by mechanical ventilation and it is hoped that there is no water ingress from above (through leaky roofs, blocked gutters, worn felt etc.)
Unfortunately if any of these elements are compromised (cracks in the render, failure of the damp proof course etc) then water can get in, but not out again. So what happens? The walls get wet and wet walls = much higher thermal loss = cold spots = condensation = mould. Internal plaster gets soaked and paint starts to flake and peel off. So just by using the wrong materials we can change the house from a dry and active building into one that is cold, wet and in danger of structural damage.
Remedies
The remedy that is right for you will depend on a range of factors, including whether your home has already been rendered, what state of repair it is in, how exposed the house is to the elements and your financial ability to get it sorted.
1. With existing uncompromised cement render
If the render is not compromised and you have no problems with rising damp or water ingress from above and your home is well ventilated and heated then you are probably OK and need to do nothing (except hope that it stays this way)
2. With existing compromised cement render (but not ‘blown’)
If your render is cracked (and the really small cracks are the worst) and you have found evidence of damp in the house then you have two real choices:
a. repair the existing render and accept that the walls will be wet and cold for a while
b. bite the bullet and replace the render (see below for recommendation on type of render)
3. With existing ‘blown’ render that needs replacing
Once the render has blown is really is only doing harm to the building by trapping water. Hacking off the render should be easy, in fact you might find too much coming off at once, so be careful! Make good any poor bricks / stone etc and re-render using the following render:
Eco Home Centre recommends the following basic render mix:
Lime Putty (derived from limestone rather than chalk) mixed with a breathable aggregate (limestone dust for example).
This mix should be pumped onto the walls (to get more air into it – lime requires carbon dioxide to cure / set) and you might need to look at putting a hairy scratch coat on first or a webbed membrane.
This is the most breathable lime render you can get and so will act as a poultice to dry out your walls and hence keep them dry and efficient.
Top coats for lime based / breathable renders
The final finish is also very important. If you are using a lime putty render then you should use a limewash finish. This is effectively pure lime putty and so is even more porous than the render. This acts as a great ‘wearing layer’ for the render. Being so porous it will get saturated really quickly and will then shed water off of the surface when it is raining. When it stops raining it will pull the moisture through from the render and allow it to evaporate away. Limewash does need to be reapplied every so often (around 4-6 years) depending on exposure, but it is cheap.
For lime renders that are made from hydraulic lime or by using sand (not recommended by us) then you can use a silicate paint. These are really hard wearing and provide an excellent long lasting finish. Silicate paints are expensive, but provide value for money through being so long lived.
For guidance on builders / contractors who we feel understand these issues and are experienced in using and applying lime putty stone dust aggregate renders please contact the Eco Home Centre.
For more advice and products please call the Eco Home Centre on 02920373094.
Disclaimer
Rounded Developments Enterprises Ltd have endeavoured to ensure that the information contained in this report is accurate. However, Rounded Developments Enterprises Ltd. accepts no liability for the use of this information.
Statement of Vested Interest
Rounded Developments Enterprises Ltd are a well-recognised supplier of a range of sustainable building products and as such have a commercial interest in some of the recommendations contained within the report. In some cases, cost estimates have been given on the basis of current quotations for similar equipment supplied by Rounded Developments Enterprises Ltd, and may not be the only equipment available. However, it is our opinion that the study offers an appropriate level of detail in view of the resources available and information provided. The authors have no expectation of any order being placed with them and would welcome questioning of the choice and costs of any equipment.
