Insulated external walls are an effective way to prevent energy loss from all types of properties. Ged Ferris, Marketing Manager at Cembrit discusses the incorporation of render-backer board, as a substrate in such constructions.

There are an ever increasing number of ‘green’ solutions becoming available to help cut wasted energy and in the process help the environment. The government driven schemes; the Renewable Heat Incentive and the much-talked about Green Deal, are concerned with improvements that homeowners can make with cutting edge energy saving technologies as well more conventional projects such as improving insulation. In fact, as many contractors recognise, significant heat loss from a building is through outside walls. There is of course cavity wall insulation available, but where that is not possible or desirable – a commercial building for example – external wall insulation can provide the same benefits.

Reduced costs

A typical installation of external wall insulation can cut heating costs by as much as 40%, with the additional benefits to the owner of a reduced risk of condensation and resultant decay of the building fabric. The education market has already taken up this trend and as cuts in education spending start to bite, this type of construction is likely to prove a popular choice for establishments looking for a modern and energy efficient upgrade.

External walls incorporating insulation are constructed as double skin, with the insulation shielding the internal layer and an air gap with a decorative outer skin providing the weather protection and aesthetic effect. Taste dictates that not everyone desires the same finish and there is a multitude of suitable materials, colours and textures available. Although a rendered finish is usually associated with block or masonry, the use of a good render backer gives additional scope for a coarse, textured or smooth finish that can be either painted, pigmented or left natural. The monolithic finish will of course eliminate joint shadowing, which can be an issue to some.

To achieve a desirable monolithic finish that will not only satisfy the design brief but also perform well in the long run, it is important to use a good substrate. A stable render backer will help achieve a better uniform exterior finish. Secure fixing and seamless joints are important preliminary steps that eliminate the potential for cracking.


Installing a render backer is a simple process but for maximum advantage it is a real bonus if the board is lightweight. This helps with manoeuvrability on-site and during the actual fixing process. A case in point is Cembrit’s PB board which incorporates polystyrene beads in its construction. This reduces the density of the board and also makes cutting easier. There is no requirement for mechanical cutting equipment, all cutting can be done with conventional hand tools. This also helps with safety eliminating the risk of nuisance dust.

The installer typically has to be aware of fixing too close to corners / edges as this can damage the boards. A recent development to solve this problem is a board with reinforced edges. In order that fixings can be completely covered with render, the edges are also chamfered meaning the fixing head is below the surface of the façade. Designed to fit neatly together to maximize stability at the butt joints, further reducing the risk of the render cracking along the joints, each of the boards characteristics reduces the risk of the render cracking along the joints or spalling above the fixing, which looks unsightly.

An alternative to rendered finish is brick slip or stone cladding. Such finishes require a strong and stable substrate onto which they are adhered. The best quality render boards can also fulfil this function. Truly versatile boards now coming onto the market will make a huge contribution to the energy efficiency and appearance of buildings for many years to come.

With an increase in demand for lightweight external cladding systems, contractors need to be up to date on the installation of render-backer boards. If specified, and installed correctly, a monolithic finish that is thermally efficient and a pleasing to the eye can be achieved with the minimum of fuss.