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.
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.
You would want to use a Lightweight Basecoat with AAC blocks 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 block's 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.
Lightweight Basecoat and Lime
Our Lightweight Basecoat contains a key ingredient, lime. Lime has been used in 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.
Whilst not explicitly part of our Heritage Lime Range, it does sit as an attractive alternative to Lime Basecoat. However, Lime Basecoat has the added benefit of being able to be used as an adhesive for external wall insulation. With a high-strength and high-adhesion formula, you can utilise Lime Basecoat to apply mineral wool insulation boards, which are notoriously heavy. Incorporating mineral wool insulation into heritage restoration systems is crucial to maintaining the overall breathability of the system.
Applying 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 been 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 for a smooth finish or our mineral or silicone renders for a textured finish. These are all highly breathable and through-coloured, so they will offer both an attractive and high-performing finish.
Crucially, Lightweight Basecoat now forms an integral part of our pebbledash coverage system. One Coat Dash Cover is due to be obsolete as a product; therefore, we have been testing ways to improve our pebbledash coverage system. Through extensive testing, we have determined that Lightweight Basecoat is an exceptional alternative. As the basecoat can be applied up to 25mm in one pass, it can sufficiently smooth out and strengthen the pebbledash surface. You can then apply another pass of the basecoat, and proceed to seal the system in with our high-performance silicone renders.
Overview and technical specifications
- Highly breathable.
- Crack resistant.
- Can be applied up to 25mm thick in one pass.
- Perfect for newly built 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.
- Composition: Portland cement, hydrated lime, mineral fillers (including perlite), admixtures
- Density once set: Approx 1.2 g/m3 class LW1
- Density of fresh mortar: Approx 1.5 g/m3
- Air content: Approx 29%
- Compressive Strength after 28 days: CSII
- Thermal conductivity coefficient (λ) (tabular value):• 0.47 W/m*K
- Water vapour diffusion factor: 7.8
- Reaction to fire: A1
- Grain size distribution: up to 1.25mm
- Adhesion to substrate and symbol of cracking model:≥ 0.4 MPa; FP: B
- Capillary water absorption: category W0