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External Wall Insulation, commonly known as EWI or K-Rend, is a comprehensive multilayer insulation system affixed to an existing wall, comprised of various layers such as a insulation, base coat, reinforced mesh, and finishing coat, aimed at enhancing home energy efficiency and preventing water infiltration, thus maintaining the masonry pleasantly dry. External wall insulation is one of three potential ways to insulate the walls of your home, the other options being cavity wall insulation and internal insulation. The effect of external insulation is to warm the wall by separating from cold temperature.

It has been proven by German specialists that in order to improve the insulating properties of the building, insulation must be fitted on the exterior of the walls. By doing so the dew point is simply moved away from the wall. 

Therefore this installation process involves fixing a layer of insulation material to the external walls of a property, and covering it with reinforcement and a special render in a range of decorative finishes.

As with stone walls, the external insulation can be glued and then mechanically fixed to the wall. This method is often required for insulating walls on solid wall properties where there's no option for cavity wall insulation. The external insulation cost is usually higher than internal wall insulation. On top of that, there will be a cost involved in erecting scaffolding and removing all of the pipes that are fixed to the masonry wall.  Even with the higher cost, the advantages external insulation offers over internal mean that it is difficult to disregard. However, EWI and render systems take a fair amount of work to install, and essential to the process is having good tools for the job. It will also change the external appearance of the house. In most cases that will mean gaining planning permission before undertaking the work, so you do need to check with local authority from the outset.

Under Building Regulations, if 25% or more of a wall is to be insulated externally, it is typically necessary to bring the entire wall up to current standards. The thermal performance of the insulated wall must have a U-value of no more than 0.30W/m²K. When it comes to using external wall insulation to insulate a solid wall, you'll be looking at the following U-value figures:

External insulation will add physically thickness to the wall, most evident at the reveals and eaves. You will have to extend your existing window sill using either uPVC or Aluminium window sill extender. In addition, it is usually impractical to return the insulation into the window and door reveals.

There are thin insulation options, like 10mm grey polystyrene that can be used on a reveal to help overcome this challenge.

Installing EWI doesn’t disrupt things inside your property in the way that internal insulation would, so you can easily live in the house during the work. You can expect some noise while walls are being prepared and the insulation is fixed. But disruption should be minimal. EWI can also help to improve weatherproofing and sound resistance. For instance, Rockwool DD sound slab has been specifically developed for use in external wall insulation systems where a higher density slab is required. The structure of the fibres in Rockwool Sound Slab make them ideal for use as a sound absorber. Rockwool Sound Slab works in two distinct ways to reduce noise, either by impeding the transmission of sound through an element of the structure, or by absorption of sound at the surface.

Adding external wall insulation is also a good opportunity to update the appearance of your home. You could make it look pretty much identical to how it was previously or make some improvements to boost your house appearance, possibly even increasing its value. External wall insulation is an invisible part of your home that you won’t be able to see once in place. That is why it is often called fit-and-forget solution.


The first step before beginning the EWI installation process is to do a render test. This will identify whether your existing render is strong enough to support the particular insulation. Second step would be removing an existing pipework from the building so the installation process can begin. On this stage you should also fit a protective layer over your windows, which will prevent them from being damaged during the installation process. External wall insulation does not normally reach the ground but instead begins a few inches above.

Once you’ve decided on the height, this is where the aluminium start track or base track, will be installed. Starter tracks should be installed against a flat surface, with any gaps filled with expanding foam, plastic trims, or silicone sealant, however it would be preferred to dub out the wall locally to ensure full contact with the wall and rear up stand of the trim. Trims are usually set 200mm above ground level to reduce the effects of ‘splash back’ of rainwater, dirt and other contaminants, however it is advised that DPCs should not be bridged.

Once the starter track has been installed, the application of the insulation boards onto the wall can begin. Insulation boards should be laid in a staggered pattern, and should be staggered at the edges of buildings so that they form a toothed finish. Generally boards should be cut in an L shape around the corners of openings, and small cuts of boards should not be allowed. It would be best practice to limit cuts to a minimum of 200mm. Gaps in boards should be limited, and should be filled with expanding foam. Boards should be level so that the basecoat and decorative finish are installed on a flat true surface. Leveling of boards can be undertaken with adhesive renders or dubbing out renders, and should be assessed at contract stage and allowed for within the contract costs. Adhesive renders should be applied to the rear of the board, and should follow either a dot and dab method, with renders to the board edges, or a fully applied adhesive using a serrated trowel.

As well as the adhesive, mechanical fixings are also used to secure the insulation boards. Around 8 – 10 fixings are hammered in per square metre, to ensure the insulation boards will not get blown off or fall off during periods of bad weather.

Next step would be to add corner beds to the corner surfaces. Corner beads and stop beads are a great way to ensure the insulation remains secure and tight around windows and doors.

Once the boards and beading have all been installed, two layers of render with strengthening properties are applied, with a fibreglass mesh sitting between the two layers of render. Meshes should overlap by a minimum of 100mm at all edges and should overlap with integral meshes on the corner beads and starter track clips. Check with the system supplier as laps can reduce to 75mm if agreed. Stress patches should be installed to all corners at openings and should be sized by the system designer. Base coat application should only be carried out when the weather is fine and free from rain. It is recommended to carry out a daily check on the weather forecast, for a minimum of 48 hours prior to the proposed application of system designers.

Polypropylene or glass fibre meshes should be installed into the wet basecoat, using the back of a steel trowel and pushed in, so that they sit in the top third. This ensures strength and continuity of the basecoat. A second basecoat is then applied, and there should be no visual sign of the mesh once this is applied. Once the basecoat has been applied, it should be left to cure in its appropriate state to receive the priming paint and then final finish i.e. suitably scarified in the case of thick coat finishes or sponged for fine textures.

The final coat of render can now be applied. This render is the last coating in the installation process, meaning that it is important to choose the exact colour and texture you like, as it will be in view on the outside of the building.



This is the main insulation normally made of either expanding polystyrene or mineral wool. Technologies such as phenolic resin insulation are also available. All of these materials are used to prevent heat escaping through walls. The insulation forms the main thermal layer as well as being the background for applying the basecoat and finishes to. Boards are fixed either directly with adhesive, with mechanical anchors, or a combination of both. 

The spider diagrams below are an extract from a recent multi-criteria comparison of insulation materials carried out for EUMEPS (European Manufacturers of EPS). These are taken from the External Wall Insulation comparison and shows EPS having the lowest overall environmental impact.

external wall insulation diagram




Fixings are specified to suit the existing substrate, the height and shape of the building, its location and elevation, and proximity to other buildings. Fixings should be specified by the system designer, and calculations made to check the pull-out value of the specified fixings. All mechanical anchor suppliers will provide printed literature with characteristic pull-out values and these can be used to specify the fixings, however best practice would be to check the site pull-out value and carry out wind and fixing analysis calculations based on these findings against localised conditions. This is the approved method to ensure the correct fixings and quantity are used.


The primer helps waterproof the EWI system and act as a barrier to moisture passing from the outside in. Some primers also have inherent insulation properties adding to the overall efficiency of the system.


This part is used to hold the basecoat in place whilst it dries. Another layer of primer is then applied over the top of it.


Adhesive provides the perfect base for fixing all types of external insulation boards. The adhesive is waterproof, frost proof and vapour permeable, ensuring that your EWI system remains stable and structurally sound for years to come. Adhesive renders should be applied to the rear of the board, and should follow either a dot and dab method, with renders to the board edges, or a fully applied adhesive using a serrated trowel.


This is the final layer of EWI and has a decorative function, enhancing the look of your home. There are various different renders available.
Silicone Render is a flagship silicone-based  thin coat render. Renowned for being long lasting and aesthetically pleasing, this render is the classic choice for homeowners and businesses alike. With hydrophobic properties, the Silicone Render has self-cleaning capabilities and also increased resistance to biological growth. It’s a customer favourite because once it’s installed it requires minimal upkeep and continues to look fantastic.

Silicone Silicate Render is the most popular hybrid silicone render, offering great performance and great value. Silicone Silicate is fantastically versatile, providing the key benefits of a thin coat render with the added boost of self-cleaning and breathability that the silicone technology provides. Silicone Silicate is perfect for a standard classic finish that will maintain its brilliance in the long term.

Acrylic Render is the best value render. It is impact resistant and is most well known when it comes to coloured render as it retains the colour pigment even after extended exposure to UV. Acrylic Render achieves a vibrant, stand out finish. 


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