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Applying Coloured Render to Thermalite Blocks

Coloured render is a popular choice for many homeowners, but on a Thermalite substrate finding the ideal materials to create the coloured render effect can be tricky. Renowned for being one of the most popular high performance building blocks, Thermalite offer excellent thermal performance, breathability and moisture resistance. The only thing about Thermalite blocks is that they create an extremely soft substrate and therefore can be tricky to render. Ideally, Thermalite blocks need finishing with a material that matches them in softness and flexibility in order to prevent cracking.

We have seen so many cases where an inappropriate render has been applied to a high performance block substrate, and we know that many builders experience problems when faced with high performance blockwork, as knowing the right quantities of materials can be extremely tricky and a bit of a balancing act. Luckily, here at EWI Store we have come up with the perfect solution for how to apply coloured render onto Thermalite blocks.

All About that Basecoat: Coloured Render on Thermalite Blocks

Our Lightweight Basecoat is ideal for use with Thermalite blocks because (as the name suggests) it’s incredibly lightweight and therefore works well in conjunction with the Thermalite, providing a stable base for the coloured render. Because the Lightweight Basecoat contains lime and perlite, it has the breathability and flexibility of the lime while also maintaining the strength of the perlite; it’s therefore ideal for a soft and lightweight substrate such as Thermalite, because the Lightweight Basecoat will resist the common issue of cracking and render failure.

When using the Lightweight Basecoat with Thermalite blocks, you can apply a two-coat or three-coat system.

Two-coat system:

The two-coat system consists of the Lightweight Basecoat, which is applied in two passes. Rather than priming the substrate, two thirds of the total basecoat thickness is spray applied first (approx 12mm thick), then left to ‘pull back’ and dry slightly. After this, the final one third is applied to take the basecoat up to its total thickness. Once this has set for 24-48 hours, a tightcoat is applied; this is essentially another thin layer of the Lightweight Basecoat which is sponge or rubbed up to achieve the required texture. This is then left to set before it is primed using a render primer and then the coloured render of your choice is applied (Silicone, Silicone Silicate, Mineral or Monocouche).

Three-coat system:

Most recommended for the UK is the three-coat system. This is because of the weather conditions that we experience here – the two coat system most likely would not be able to withstand the harsher conditions.

The application of the Lightweight basecoat is the same for the three-coat system as it is for the two; the basecoat is applied in two passes. The key difference is that after the basecoat has been left to set, the Premium Adhesive is applied and Fibreglass mesh is embedded within it. This will give the system the strength and crack resistance that is required to be able to hold up against the harsher climate. After the Premium Adhesive, a render primer and then the coloured render of your choice is applied.

The Best Coloured Render for Thermalite Blocks?

When choosing a coloured render, there is a vast array of different brands and different renders that all offer a variety of benefits. Silicone Render is a well known coloured render, offering breathability and vapour permeability. Silicone Render is a thin coat coloured render and, as the name suggests, is applied in an extremely thin layer which means it is highly flexible (see below for a video demonstrating just how flexible it really is!).

Flexibility is an important quality to look for when choosing a render for Thermalite blocks, as the blocks are so soft that they very easily expand and contract during heating and cooling. A hard and unforgiving render such as sand and cement would only crack with the movements of the blocks. Check out our blog post ‘Sand and Cement Render on High Performance Blocks’ for more information!

We recommend using Silicone for rendering Thermalite blocks, as it will compliment the substrate with its vapour permeability and will also prevent water from getting behind the system and into the blockwork. It’s important for Thermalite blocks not to get wet, because during the process of drying they can very easily crack due to their softness; Silicone Render creates an impregnable shield against water ingress.

And there you have it – how to apply coloured render to Thermalite blocks. The process is very simple and the materials really save going through the experience of a failed render. For any further questions about using coloured render on Thermalite blocks, call up our technical team or leave a comment below! We’re always happy to give our free expert advice.

Applying render to a range of substrates…

Recap of Materials for Applying Coloured Render to Thermalite

We upload a new blog post every Tuesday and Thursday, so stay tuned for more content!


2 thoughts on “Applying Coloured Render to Thermalite Blocks

  1. What guarantees are given for the colour match over a given time period?
    I have white thermo blocks on my extension that need rendering. Would you suggest a three part silica render to cover this?

    1. Hi Russ, would always suggest Silicone Render as part of a render-only system. If you give our sales team a call, they can advise on the guarantees. Thanks

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