What are Thermoplan blocks?

With the increased popularity in clay blocks, we receive enquiries regarding the best method/materials for applying render to Thermoplan blocks, so we’re here to answer the question! First, a little bit about Thermoplan blocks…

Thermoplan blocks are made of fired aerated clay and are assembled together with mortar that is applied by a roller. The blocks themselves contain air pockets that run vertically through the blocks, allowing for ventilation within the walls and enabling a dry building structure. There are many benefits to using Thermoplan blocks, some of which include:

  • Quick and easy construction time
  • Excellent thermal performance
  • Comfortable living environment
  • Breathability
  • High indoor air quality
  • Eco-friendly
  • High compressive strength
  • Excellent fire protection

Applying render to Thermoplan: Basecoat layer

Because clay blocks are a very dry material, they are also highly absorbent. This means that great care has to be taken when choosing materials; clay blocks a a lightweight structure, so sand and cement renders will crack almost immediately.

The Lightweight Basecoat is the ideal solution for Thermoplan blocks. You can choose to use either the two-coat render system, or the three-coat render system for clay blockwork, although in the UK we generally recommend that the three-coat system will stand up against the weather best.

Two-coat system:

The two-coat system consists of the application of the Lightweight Basecoat, then a tightcoat and then a render.

In a typical render system on an ordinary substrate, a substrate primer would be necessary to limit the absorptive capacity of the substrate. However, when rendering a Thermoplan block substrate, to limit the absorptive capacity of the substrate you do not need to prime. Instead, you spray apply a first pass of the Lightweight basecoat at approximately two thirds of the thickness it should be applied at – around 12mm. You leave this to ‘pull back’ and dry slightly before applying the final one third to take the basecoat up to its total thickness.

The Lightweight Basecoat needs to be left for a period 0f 24-48 hours to dry, then the tightcoat is applied. This consists of another thin layer of the Lightweight Basecoat; it is applied to the dried basecoat and is sponged or rubbed up to achieve the required texture. Once the tightcoat has been applied, it can be primed and painted or primed and rendered using a ready-mix render (e.g. Silicone or Silicone Silicate). You can also use Mineral Render (and then paint with Silicone Paint) or you can use Monocouche render. The main thing here is that the tightcoat must be primed if you are using a ready-mix render such as Silicone or Silicone Silicate, because otherwise blooming may occur across the lime basecoat.

Three-coat system:

A three-coat system is the standard system that is recommended for use in the UK. The three coat system involves applying the Lightweight Basecoat in two passes, as outlined above. Once the Lightweight Basecoat has cured (after 24-48 hours), a layer of the Premium Adhesive with Fibreglass Mesh embedded is then applied on top. The Premium Adhesive is then primed using a render primer, before the render of your choice is applied; whether it’s Silicone, Silicone Silicate, Mineral or Monocouche.

The reason that the Premium Adhesive is applied as an extra layer is because it is far stronger than the Lightweight Basecoat, and when applied with a Fibreglass Mesh embedded within the adhesive it provides the required tensile strength to be able to withstand harsh weather conditions without cracking.

Applying Silicone Render to Thermoplan Blocks

Silicone render is a thin coat, breathable render. It’s the perfect choice if you want to achieve a long lasting render facade for a Thermoplan block substrate, as it will resist cracking while also creating a beautifully coloured finish.

An alternative to the Silicone render is the Silicone Silicate render, which is also breathable and flexible. Both renders come in a variety of grain sizes for different textured effects!

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