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How to Render External Insulation

Updated on:
April 30, 2024
External Insulation

Image Source: Canva

For many households, external wall insulation is the natural choice, carrying an extensive range of benefits in thermal efficiency, comfort, durability and enhancing property values. You'll find external wall insulation systems improve thermal performance.

Preparing external wall insulation

Before you consider installing insulation to external walls, ensure the initial layer is dry, clean and free of dust and all substances that might negatively impact the insulation or adhesive. If you have the time or means, clean the wall quickly with a pressure wash, or clean it manually with a wire brush.

Check to ensure the wall can host external insulation. With some surface materials, such as pebbledash, the wall might not be structurally sound for insulation work.

Prime the exterior layer

Once you are happy with the condition of the solid brick wall, prime the substrate, ideally with a water-based primer. This substance seals porous surfaces. The depth of the initial layer and how absorbent it is will influence the amount of primer you need.

Depending on how absorbent the layer is, you might need extra coats of primer. If you are applying additional primer coats, please allow sufficient time before adding a new layer.

Installing the starter track

After you prime external walls, you should install a starter track. This creates a level surface, making the insulation installation more accessible and enhancing the level of protection against the elements.

Commonly, you can choose between a uPVC or aluminum starter track. Both have their benefits, with uPVC minimizing thermal bridging while the mesh layer added with the aluminum starter track helps to tie additional layers together.

Rawl plugs, positioned at 300mm centers, ensure the starter track is firmly in place.

Using adhesive to attach expanded polystyrene insulation boards

Adhesive is suitable for attaching expanded polystyrene insulation (EPS) boards to the wall, and there is no shortage of commercially available basecoats and adhesives for you to choose from.

Please speak with a professional if you require assistance selecting the most appropriate sealant for your property. In most cases, a professional firm will likely carry out the work for you, and they’ll know the most appropriate solution.

The range of adhesives gives you greater control of external wall insulation cost

Please follow manufacturer instructions to mix the adhesive with clean water, and a heavy-duty power plaster mixer is helpful.

The “dot and dab” method is the most common way to apply this mixture and you can apply it with a trowel. You should apply adhesively around the edge of the board in a consistent manner, and then dot and dab spots in the middle.

It is common to use three adhesive spots, and you should look to cover at least 40% of the board with adhesive.

Image Source: Canva

Applying the boards to the wall

Following a staggered formation, attach the boards to the wall. When you reach a corner, interlink the boards from both sides.

When you place insulation around doors and windows, the joints between boards shouldn’t line up with these openings. If you do so, it is more likely cracks will occur, so avoid this.

Ensuring the insulation boards are level

As you can imagine, it is vital to ensure the boards are appropriately aligned vertically and horizontally. A simple way to do this is to use a spirit level to ensure you are satisfied with the placement.

If you have a gap between boards larger than 2mm, you should fill this space with EPS strips. You can take offcuts from insulboards for this, do not use EPS adhesive directly, as this often leads to cracks.

Mechanical fixings to add peace of mind

You can fully secure the insulation by using mechanical fixings. Please allow at least one day after attaching the EPS boards to the wall, and then you can drill holes for each mechanical fixing.

A good way to space this out is to place one fixing for every corner of the insulation board and add a further one or two fixings in the middle of the board.

The fixing can be hammered into position or driven directly through the insulation with a power tool, depending on the product you use.

Ensure the facade is level

Now, take the time to ensure the insulation board surfaces are level and flat; if you need to smooth this surface, a polystyrene rasp or similar product is suitable.

You can reinforce insulation by adding beading

If you are keen to strengthen some areas which might be susceptible to pressure, such as corners, you can add beading. This step provides additional support in directing water away from the render, and most professionals advocate using uPVC beading, as this will not rust.

Cut the beading material to an appropriate length and place them into the basecoat reinforcement layer.

Adding a reinforcement layer improves thermal performance

Whatever reinforcement layer product you choose, follow the instructions to mix it correctly. A similar process to the mixing and layering process of the adhesive section is acceptable.

Apply this layer with a trowel, and you can follow a one-pass or two-pass process, depending on your required reinforcement. Once you have applied the layer(s) you need, skim the surface so it is flat.

Prime the surface before you render when installing external wall insulation

You should allow a minimum of 24 hours for the base coat layer to dry, although it might take up to 48 hours in damp weather conditions. Once dry, apply a topcoat primer, using a roller or paintbrush.

This layer should fully dry before rendering, and it commonly takes between 12 and 24 hours for this to occur.

Apply the insulating render

When cleared to do so, use a trowel to apply a thin layer of weatherproof render to the surface, matching the thickness of the render to the grain size. Once you have applied render, smooth it out with a trowel, and remove excess elements to create a thin layer.

To create a textured finish, apply the render in circular movements.

It is best to complete this work in one sitting, as doing it in more than one sitting will make each section look different.

To maintain consistency, mix your render into one large bucket, and remix it regularly to ensure the finished appearance is satisfactory.

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