Breathability focus: painting exterior lime render
13th September 2021
When it comes to painting the exterior walls of your property, a paint that is durable, weather resistant, user-friendly and unlikely to fade will be top priorities for many homeowners. But when painting exterior lime render, there’s a few other points to consider too.
Exterior lime render
Lime render is a traditional building material that allows the walls of a building to breathe. Due to this high level of breathability, lime render helps retain flexibility and reduce the risk of decay; which is a particularly important factor for older buildings.
Modern cement render, by contrast, isn’t breathable and tends to ‘seal’ the walls. Although it is often used to repair previously lime rendered buildings, if used on old, traditionally built properties, cement render can cause moisture to become trapped, leading to a variety of problems.
The Society for the Protection of Ancient Buildings (SPAB) offers lots of advice and information on the treatment of lime rendered buildings and the importance of breathability, especially on older buildings. They explain:
‘In the context of building materials… the generic term ‘breathable’ is used to describe an important property of building fabric: the extent to which building materials are able to transmit moisture. Breathable materials should be sufficiently permeable to enable absorbed and surface moisture to evaporate during ambient drying conditions. Loss of moisture from the material needs to occur rapidly enough to ensure that there is no accumulation of moisture which can lead, over time, to significant deterioration of building fabric, development of mould growth, infestation, or other form of decay. A breathable material, by definition, cannot be impermeable and should not trap moisture.’
Painting exterior lime render
Whilst both lime and cement based renders have their own advantages and disadvantages, it’s especially important to use a breathable finish on lime rendered exterior walls. Using conventional ‘plastic’ based masonry paints on lime substrates could potentially cause longer term issues by trapping damp, and even causing damage to the fabric of the building itself.
Limewash is the traditional finish for exterior lime render. It has an authentic, chalky finish and is highly breathable. It doesn’t seal the surface and instead lets moisture pass easily through it.
However, it requires many coats and is labour intensive to apply. The colour range can also be more limited due to its natural ingredients. Finally, limewash usually requires regular upkeep – recoating every few years – which isn’t always practical for many homeowners.
Earthborn Silicate Masonry Paint is a highly breathable protective finish suitable for lime render, recommended by lots of lime specialists. It’s a two-part system consisting of paint and primer and works by physically knitting into the substrate, forming an extremely long lasting, highly breathable yet protective finish.
How to apply Earthborn Silicate Masonry Paint to exterior lime render
We’re often asked about the best way to paint exterior lime render, so we’ve put together a few pointers which we hope will help:
- Earthborn Silicate Primer and Silicate Masonry Paint should be applied to bare, unpainted masonry substrates such as lime render. It’s also suitable for use on brick, stone, pebbledash and concrete.
- The surface should be clean, dry, sound and stable and the temperature needs to be a minimum of 8°C day and night when painting and drying.
- If painting onto new lime render, allow enough time to for the lime to cure as well as dry before applying any paint. Your lime provider should be able to advise, but as a rule of thumb we recommend 4 weeks’ curing time for every 5mm thickness of render. Painting before this time can prevent the lime from curing properly so it’s important not to paint too early.
- Whatever the age of your lime rendered property, it’s imperative to use a breathable finish so as not to seal the walls and to allow the lime, and the building, to continue to breathe.
Following these simple guidelines will result in a super long lasting, highly durable and breathable finish for your lime rendered property.
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