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 blockwork. 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!