Tag Archives: lightweight basecoat

sand and cement render high performance blocks

Sand and Cement Render on High Performance Blocks

Cracked Sand and Cement Render on AAC Blocks

We recently came across a case where a customer had sand and cement render installed on their home which was built with high performance blocks. High performance blockwork, such as autoclaved aerated concrete (AAC), is a lightweight concrete building material. These blocks are known as high performance because they offer insulating capabilities, fire and mould resistance and are also highly breathable.

The issue with using sand and cement render with this kind of blockwork is that sand and cement render is very old fashioned and lacks any sort of breathability and flexibility. The contrast between the two materials is that by using sand and cement render you are essentially using a low-performance render on a high performance substrate. The result is therefore excessive cracks in the render, as this customer unfortunately discovered. (see photos below!)

Hairline cracks in sand and cement render around doors and windows:

An array of cracks within the sand and cement render:

Render for High Performance Blocks

Because high performance blockwork is so lightweight and breathable, using a really solid and hard material such as sand and cement is not suitable. Instead, you need a lightweight and breathable material – such as our Lightweight Basecoat. This is because the Lightweight Basecoat contains lime and perlite, so it’s strong but most importantly flexible and breathable; therefore it is completely ideal for high performance blocks because as the soft blockwork moves with the building, so will the basecoat.

Once you’ve applied the Lightweight Basecoat to the high performance blocks, leave it to set for 24-48 hours and then prime the basecoat using our SiSi Render Primer. This will ensure a strong adhesion of the Silicone Render to the basecoat.

Thin Coat Renders for High Performance Blocks

Silicone Render is ideal for use on lightweight, high-performance blockwork instead of a sand and cement render (check out our blog on ‘Sand and Cement Render vs. Thin Coat Render’ for more information!). It’s a thin coat render and is therefore extremely breathable, flexible and crack resistant. This is the ideal render if you are looking for something that will maintain a clean and fresh appearance for years to come because Silicone Render is self cleaning.

An alternative to the Silicone Render is our Silicone Silicate render. This is very similar, except it’s a hybrid-silicone render; it therefore lacks the same level of self cleaning but the presence of silicone within the formula means that Silicone Silicate is also very breathable.

In some parts of the UK, poor weather means that installing a thin coat render is challenging. Our Mineral Render is extremely fast drying and offers the same breathability and flexbility as the Silicone or Silicone Silicate Renders. Mineral Render only needs painting with Silicone Paint afterwards to seal it in and protect it against weathering.

Paint Finishes on High Performance Blocks

Many people opt for a sand and cement render so that it can be painted directly after application and setting. As we know, when using high performance blockwork sand and cement render will crack, and your perfect painted finish will result in hairline cracks.

If a paint finish is your desired outcome, then using Silicone Paint on top of our Lightweight Basecoat is a brilliant alternative to sand and cement render on high performance blocks. Using Silicone Paint can achieve a smooth finish that is very easy to refresh and re-paint as time goes on.

Before you can apply the Silicone Paint to the substrate, you will need to do a ‘tight coat’ of the Lightweight Basecoat. This is where you apply a thin layer which is then either rubbed up or sponged to give the desired finish before painting.

Silicone Paint is the most ideal paint for use on high performance blockwork because it is hydrophobic and breathable, and will therefore work in harmony with the breathable high performance blocks. 

You can also check out our blog post on how to render ICF for an idea of how you would go about rendering different high performance substrates!

Recap – Suitable Render Products for High Performance Blocks:

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EWI-269, lime basecoat, lime render

Why use a Lightweight Basecoat?

We are excited to introduce one of our new products for our lime based range, the EWI-269 Lightweight Basecoat! Our Lightweight Basecoat is intended for use in render-only systems as a finish on properties with high-performance blocks such as Ytong, Poroton and Porotherm. The main intention of the Lightweight Basecoat is that it is a one-coat, breathable basecoat, used to level the substrate before being finished with a coloured render.

In this blog, we are going to properly introduce our new product to you and talk a little bit about what it’s for, what it does and why you should use it!

Why use a Lightweight Render System with AAC blocks?

Take for example AAC blocks (autoclaved aerated concrete) such as Ytong. These are produced from autoclaved lime, sand, cement, water and a rising agent. AAC blocks are high performance because of their thermal and acoustic insulation capabilities. They are more environmentally friendly and offer higher fire resistance than the common bricks and mortar.

The reason you would want to use a Lightweight Basecoat with AAC blocks is because these blocks tend to be a lot softer. AAC contains 60%-80% air, indicating a high insulating capacity and a softer substrate. If you were to apply a completely solid cement mortar over the soft AAC blocks, there would be an increased chance of cracking due to incompatibilities in the strengths of the cement mortar and the AAC blocks. A cement mortar would also negate the AAC blocks ability to breathe.

This is why you would use our Lightweight Basecoat. Both AAC and the Lightweight Basecoat are lightweight, soft and breathable. Therefore, when used together the chances of cracking are minimal and functionality is optimal.

The Lightweight Basecoat and Lime

Our new Lightweight Basecoat contains a key ingredient, lime.

Lime has been used in building and construction for thousands of years, it is a truly tried and tested building material in the UK and is proven to stand the test of time – you might have noticed certain areas in the UK (e.g. Bath) where vernacular buildings are made with lime.

Lime as a component in building materials (such as in our Lightweight Basecoat) adds the benefit of breathability. We talk a lot about breathability here at EWI Store, and essentially it means the buildings’ ability to transmit moisture. If a building is not able to expel moisture and water vapour, then damp can occur internally and externally. This can cause damage to the building structure and can also pose a health risk due to poor indoor air quality.

For these reasons, it’s incredibly important for a building to be able to breathe, which is why using our Lightweight Basecoat on breathable AAC blocks provides the perfect combination for allowing this function. 

Furthermore, insulation and breathability go hand in hand. Humidity and moisture can severely disrupt a buildings’ thermal performance. Because the lime within our Lightweight Basecoat is porous, it absorbs and releases humidity (it breathes), therefore helping to regulate humidity and maintain the thermal comfort of a building.

Applying the Lightweight Basecoat

The Lightweight Basecoat is applied in a very similar way to our other basecoats. It comes in a 25kg bag and is simply mixed by hand or using a mechanical mixer with 5 litres of clean, potable water.  It is then applied to the substrate by spray machine or by hand using a plastering trowel. Fibreglass mesh can also be embedded within the basecoat. The only real difference in application is that because it’s so lightweight, this basecoat can be applied up to a thickness of 25mm.

Because it’s a simple grey colour, once it has set it will need finishing with a decorative render. It’s essential that a breathable render is applied on top of the Lightweight Basecoat, so we recommend using either our Lime Render (coming soon!) for a smooth finish, or for a textured finish use our Mineral or Silicone renders. These are all highly breathable and are through-coloured, so will offer both an attractive and high performing finish.

Lightweight Basecoat – An Overview

  • Highly breathable.
  • Crack resistant.
  • Can be applied up to 20mm thick in one pass.
  • Perfect for new build properties.
  • Works well with AAC blocks – Poroton, Porotherm, Ytong.
  • Can be primed and painted with a breathable render finish.
  • Has a thermal conductivity of 0.47 W/(mK).
  • Internal and external use.

Lightweight Basecoat available for order now!

Order by phone or online! We hope this was useful and provided clarification about our Lightweight Basecoat but if you have any more technical questions, give us a call we are always happy to help!

Stay tuned to our blog! We upload new content every Tuesday and Thursday all about EWI materials, technical advice and more.

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