How is External Wall Insulation Installed?

EWI installer

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A Step-by-Step Guide

An external wall insulation system is fitted to a property in layers by an accredited installer. These layers include insulation boarding, render and a decorative waterproof top coat.

Approximate time for installation: 7 – 10 days for an average semi-detached house.

Factors to consider: The weather can affect EWI installation. Cold weather means the system takes a lot longer to dry and any rain could wash off the final primer coating. Installers will check the weather forecast on the days when different coatings are to be applied and may not undertake work if rain is predicted.

If rain does occur during installation, your installer may need to re-apply the render for the system to seal effectively. YES Energy Solutions’ installers are highly trained EWI professionals. We will only sign off an installation once the topcoat has set properly and that ours customers are happy with the work undertaken.

Level of disruption: There is minimal disruption during an EWI installation since the majority of the work takes place on the outside of the house. The only potential disruptions are:

  • Scaffolding in place throughout the course of the install which could hinder access to walkways and doorways.
  • Heating/water being switched off if piping or a gas flue has to be extended, but this shouldn’t be for more than an hour.
  • One day of intermittent drilling whilst the insulation boards are being mechanically fitted.

How long does the topcoat take to dry?

It depends on how warm it is. In 20°C heat, the system will be dry 4 hours after the application of the final coat, in 6°C it could take 24 hours.

The 12 Steps of EWI

In order to give you a realistic sense of exactly what will happen during installation, we spoke to one of YES Energy Solutions experienced EWI installers - Stuart Player and asked him to take us through the process step-by-step:


Step 1 – A survey is conducted to assess the suitability of the property
A surveyor will visit the property and take measurements of the outside walls to determine how much insulation is required. Some properties (e.g. those with cavity walls) may not be suitable for external wall insulation. Find out more about how to identify suitable properties.

The survey will also highlight what remedial work needs to take place. Fixtures and fittings that are currently on the external walls of the house will need removing or extending for installation. This can include: gas flue pipes, alarm systems, outdoor lights, satellite dishes and down pipes. The survey will assess what preparations need to be made before any work can take place. Following the survey, a quotation will be produced for you to consider and sign off.

Step 2 – Scaffolding is erected
Most properties require a scaffold to be erected for external wall insulation to be applied. This gives the installer access to fit insulation to the walls and also take down or extend fixtures and fixings. The scaffold will remain in place during the whole installation and will only be removed once the topcoat has dried and the system has been signed off by the installer.

Step 3 – Any fixtures and fixings that need removing or extending are dealt with
This work has to be done by the relevant tradesperson. So, for example, if the gas flue pipe needs extending a gas engineer will need to carry out the work. Likewise, a satellite dish will have to be removed by the original installer, for example, Sky.

Your installer can arrange all of this work for you so you don’t need to worry. If you would prefer, you can arrange for the work to be carried out by different tradespeople yourself.

Step 4 – Plastic sheeting is laid down
For many installations, plastic sheeting is laid down on floors and flat roofs to protect garden areas and patios from splashes and spills during the work.

Step 5 – Window protection film is fitted
This is fitted to all windows and any glass doors to protect the glass during installation.


Step 6 – The insulation boards are prepared and fitted
The boards are cut to the necessary size, and adhesive is applied. They are then placed on the walls in a pattern that looks a bit like brick work.

Step 7 – The insulation boards are mechanically fixed to the wall to secure them further
Once they have been stuck with adhesive they are then drilled into place. Self-tapping screws are usually specified.

Step 8 – A meshcoat is applied
The meshcoat is a three-layer system that adds strength to the insulation. A polymer-modified base coat is applied, a glass fibre reinforcing mesh is fixed on to this base coat and a second coat of polymer-modified render is applied over the top.

The structure of the polymer-modified coating is comparable to a lot of little threads or shards, which means it is very flexible and extremely strong. The whole meshcoat protects and strengthens the insulation boards.

Step 9 – Once the meshcoat is dry, a primer (top coat) is applied
More commonly this is a grained-textured silicon top coat. This is the final layer of the system and gives a decorative finish. The coating is pre-coloured according to the customer’s preference.

Silicon is considered a smooth finish. Other finishes are available such as dry dash, which is effectively small stones that will produce a more textured finished, and brick slip effect, which is a render that looks like a brick wall.

Step 10 – Corners and edges of the system are protected with reinforced beading
Beading is used to stop EWI from touching other material that can affect the performance of the insulation, and it helps to ensure clean, neat edges around window and door openings.

Step 11 – Fixtures and fittings are reinstated and protective sheeting is removed
Satellite dishes, lights and alarm systems are placed back on to the outer walls, and all protective sheeting and film is now removed.

Step 12 – Scaffolding is taken down
Scaffolding is taken down and the installation is complete, enabling you to enjoy a warmer home with a brand new look.

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