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How to install external wall insulation-step by step


External wall insulation, also known as EWI is a type of insulation that is attached to the outside of your walls. 

Installing EWI will add around 100mm to your house, depending on the thickness of insulation used.

In order to improve the insulating properties of the building, insulation must be fitted on the exterior of the walls – installing insulation on the interior would decrease the usable space and room sizes.

EWI creates a barrier that stops heat escaping through your walls. This barrier is attached to the outside of your walls in several layers. The main insulating component is normally made of expanded polystyrene (EPS), phenolic boards or mineral wool. EPS is most commonly used.

Here at Insulationgo LTD, you will find it all


Phenolic boards

Dual Density mineral wool


It is always recommended that the following is undertaken prior to the application of an EWI system:


Check for rising damp and the causes

Check for existing movement joints and make provision to reflect this in the new installation

Check for efflorescence (lime bloom) or if similar problems have been treated and remedied that the masonry has dried out sufficiently

Test the walls for any defects

Test the existing substrate for pull out loads so that fixing type and quantities can be determined

Carry out a line and level survey to determine if a dubbing out or levelling coat is required

Remove moss, lichen, mould, and treat with an anti-fungicidal wash if required

Identify any unusual detailing issues

Determine if any architectural features need preserving or re replicating in order to be repositioned and placed within / on the new EWI system

Evaluate access to the property, heights of the building and security of tenants

Discuss finishes, colours and textures with the client / interested parties








Installing base track/starter profile

Specified starter track should be installed at existing DPC level, with specified fixings at correct centres. 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 set 150mm 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.

                                                                                                                                                     man installing base track for external wall insulation 










Bonding / Fixing Insulation

Boarding works should only occur in dry conditions, and boards should be protected from adverse weather, direct sunlight and possible impact damage. 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.                                

                                                                                                                                                    polystyrene boards fixed  in l shape











Minimum cuts will vary from system supplier, however 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. Levelling 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. This will be subject to the system designer’s recommendations.

 white polystyrene board  with adhesive                                                                                                          












  petal pin fixings used with white polystyrene                                                                                                         












Meshes should over lap by a minimum of 100mm at all edges and should overlap with integral meshes on the corner beads and starter track clips. Check with 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 carryout 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. Meshes should be continuous, and should have a minimum 100mm (10cm) lap with the adjacent mesh. Any meshes that are integral to beads and trims should also allow for 100mm (10cm) lapping. 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 final finish i.e. suitably scarified in the case of thick coat finishes or sponged for fine textures.


Beads and trims

      Beads should be installed as per system designer recommendations and manufacturers guidance. Beads should be fit for purpose, and installed by a competent person. Trims at verges and eaves and soffits can be a concern, and these should be formed to ensure a good watertight seal is provided. Should there be a detail that standard trims would struggle to overcome, then it is recommended that site specific trims be allowed for and measured on site.

Detailing of window reveals should indicate if a stop bead is required. Sills and flashings should be installed so that they provide sufficient overhang to shed water away from the face of the finished system. Best practice is to allow for a minimum of 40mm overhang to the throat or drip.













Each EWI system will consist of the following components:


Insulation layer

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. 



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.

Panel Fixings



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.




Glass fibre mesh 

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




Ceresit ZU

Ceresit CT87

Ceresit CT84

K-Rend HP12

Render finish

This is the final layer of EWI and has a decorative function, enhancing the look of your home. There are various different 
renders available.

Ceresit CT74

Ceresit CT174

Ceresit CT60

K-Rend TC15