How to install our external wall insulation system

by James Alcock

How to install our external wall insulation system

We are coming across more and more people installing insulation on their homes as a DIY project. In the following blog we look at the process of insulating your home’s external walls.

Before we start, it is worth mentioning that it takes a really competent DIYer to get a nice finish with these products – particularly when it comes to the render finish – so if you have any doubts we always suggest speaking to a professional!

So to start, you need to decide on the amount of insulation you want to add to the outside of your property. We offer 4 different thicknesses:

  • 20mm
  • 50mm
  • 90mm
  • 100mm

90mm of our EPS takes the U-value of the wall down to less than 0.3, which is where current building regulations stipulate you need to get to in terms of the necessary energy savings; however in some cases this simply isn’t possible due to space constraints.

It goes without saying that the thicker the insulation you use, the bigger the energy savings you will see and also the higher the level of thermal comfort – i.e. once you heat the property, it will stay at a consistent temperature for longer.

Installing starter track

Before putting the insulation boards on the wall, you need to locate the DPC layer – this is a barrier that prevents capillary action of water, and hence stops rising damp. The reason this needs to be located is that our external wall insulation needs to be installed above it, so it doesn’t bridge the DPC and encourage water to be pulled up the wall. It is normally pretty easy to identify as the brick pattern will be different, or the wall will stick out a little further above this level – in fact in some newer properties, you can sometimes see the DPC material itself. Once you have identified it, you need to attach the starter track directly above the DPC level. There are two types of starter track: plastic and aluminium. The plastic starter track is the premium product since it is more robust than the aluminium as well as helping to limit thermal bridging. The starter track is attached to the wall using screws which need to be 300mm apart.

The starter track provides a plinth that allows you to build the insulation up from. It is important that the width of the starter track mirrors the thickness of the insulation, otherwise the insulation won’t fit snuggly on the starter track.

Installing insulation boards

With the EWI Pro system, the insulation boards (which are 1200mm x 600mm in size) are first attached to the wall using the EWI-220 adhesive. The adhesive is mixed in a bucket with 5.5 litres of water to provide a nice consistent cement-type mixture. One bag should be enough to attach 8 of the EPS boards to the wall.

First the adhesive is applied to the perimeter of the board, and then 3 dots are placed along the middle of the board:

The boards are then attached to the wall. Start at the bottom, fitting the first piece of insulation into the starter track. The second layer of insulation should be staggered to the first – you can see a nice photo of staggered boards in the photo below:

You can put a little extra adhesive on the boards if you have an uneven wall, to give you a little more to play with and ensure once dry the insulation boards provide a flat surface.

Installing mechanical fixings

The EWI Pro system is a belt-and-braces system; that is to say the insulation boards are held in place with both adhesive and mechanical fixings.

We currently offer two different types of mechanical fixing – plastic (10mm core) and metal (8mm core). These then come in various different lengths depending on the thickness of insulation you end up using – the key is that the mechanical fixing is at least 50mm longer than the thickness of the insulation. This will ensure the boards are held nice and firmly on the wall. Initially you need to drill holes in the wall, using the fixing pattern below. The fixings are then hammered into place. The fixing pattern is shown below:

The basecoat layer

The render that goes on our external wall insulation system is exceptionally thin. It is not like the sand and cement renders from yesteryear. This type of render is relatively new here in the UK, although it has been used on the continent for the last few decades. One of the main advantages of the thin coat renders systems is their flexibility – this makes them especially suited to older properties that tend to expand and contract more over the seasons that newer builds (although it works well on those too!).

Before applying the render to the insulation board, first you need to apply the basecoat layer. This consists of the 6mm layer of EWI Pro Basecoat/adhesive (the same stuff you used to stick the insulation boards to the wall) with an embedded fibreglass mesh.

The fibreglass mesh is one of the key reasons for the flexibility of the render system. It takes a bit of getting used to in order to get this layer right (especially with the embedding bit), but basically first you need to add a 6mm layer of adhesive to the wall using a notched trowel. Once the adhesive is all in place, you then need to cut a strip of the fibreglass mesh – these come in 50m rolls and the rolls are 1 metre wide and embed this into the wet adhesive.

The easiest way of doing this is cutting the strip of mesh and then gently pressing it into the adhesive using the flat edge of the notched trowel. Don’t worry about getting it perfect yet – just make sure if you take your hands away the mesh won’t fall off the wall.

Now, using the flat edge of the notched trowel, drag the edge up from the bottom of the mesh applying a moderate amount of force. This will pull the adhesive through the mesh so that it will embed within it.

The key to getting a nice finish is to ensure this basecoat level is really flat. If you can see any bits of mesh sticking out, or the surface isn’t flat,  then you are going to have a problem, so take time with the basecoat layer to try and achieve the best finish you can.

If the adhesive goes off (dries!) and you are not happy with the finish, then this is the time to add another very thin layer of adhesive to ensure you have a completely flat surface on which to put the render. It helps if the first layer of adhesive hasn’t completely dried as your ‘fixing layer’ will key into the first layer you have added, so try and do this if possible. It may require a layer of primer (EWI-301) if the first layer of adhesive has completely gone off prior to adding your ‘fixing layer’ of adhesive.

Remember overall we don’t want the basecoat layer to be thicker than 6mm as you begin to lose flexibility of the system!

Priming the wall prior to render

This part is very easy. You simply wait until the basecoat layer has dried completely and paint or roll on the primer. We offer two types depending on the render you are looking to use – Acrylic Primer or Silicone Silicate Primer.

Applying the render to the wall

Applying render to the wall is the final part of the process. This part is the bit we would suggest practising prior to making a start on your actual project. The key is to apply the render incredibly thinly; both our renders come in two different granulate thicknesses 1mm and 1.5mm. The idea is to ensure the layer of render added on top of the basecoat layer is only 1mm or 1.5mm thick in total. People sometimes struggle to get their head around this, but it is just another reason why these renders are so flexible and don’t suffer cracking like the older sand and cement renders.

The first step is to apply the render to the wall using a trowel. We normally use a metal trowel at this point, and you should try to remove as much of the render as possible leaving only a very thin layer. A telltale sign you are applying the render too thick is the appearance of waves of render. If this is the case, you need to work hard to remove the excess.

One bucket of 1.5mm render should cover approximately 10m2 of wall, while the 1mm render will go a little bit further, ideally about 11-12m2.

Once you have a smooth, thin layer of render on the wall that you are happy with, this then needs to be left for 15-20 minutes to slightly go off. At this stage, you need to use our plastic float to rub the render in circular motions across the whole of the render surface. This gives the render its characteristic finish.

We would always recommend looking at the weather prior to rendering (and even applying the basecoat layer), since rain hitting the wet render will cause a messy and expensive ‘run-off’. This basically means you need to start the whole rendering process again, so we suggest ensuring there are at least a couple of reasonably dry days once you have finished.

During the summer months, both the adhesive and renders will dry much faster, so you may want to do the rendering in the early morning or late afternoon to ensure you have time to work the render. Likewise, during the summer months, never render a façade in direct sunlight as this will go off too quickly!

Both our Acrylic and Silicone Silicate renders are through colour renders. This means that that the render comes coloured in the bucket. If you are interested in using a coloured render, then please look at the EWI Pro colour tool – there are 57 colours to choose from. Please keep a note of the colour number (under the picture of the house on the tool) as you will need to use this to order the correct colour!


It is worth spending a couple of minutes discussing beading. Beading is used for a few reasons with EWI/render systems. The first is to strengthen different elements of the system, but also it helps create straight lines in the render (90 degree corners etc). Finally, the beading also helps provide neat finishes where the render stops.

4 types of beading:

  • Stop bead
  • Corner bead
  • Movement bead
  • Bellcast bead

These are applied directly on top of the insulation boards prior to adding the basecoat layer. You will need to use a bit of basecoat to hold these in position. All our beading is 2.5m in length.


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