OSB board is an engineered wood-based panel consisting of strands of wood which are bonded together with a synthetic resin – these are then pressed together in layers. Unlike traditional wood which is full of knotholes, core voids or points of weakness, OSB board is uniform in strength across the whole sheet. This makes it an ideal building material, especially with respect to load bearing.
It is not only the strength of OSB board that means builders choose it for wall sheathing – it is also the cost. OSB is relatively inexpensive compared to other building materials. In order to increase the energy performance of OSB sheathing, our external wall insulation systems are the perfect solution. Since the insulation board is applied on the outside of the OSB board, the thickness of insulation is not really restricted. We recommend looking at 90mm, as this will mean the build conforms with Part L of building regulations.
When installing our insulation on top of OSB board, we normally recommend attaching a breathable membrane directly onto the board first.
Next we recommend creating a 35mm cavity on top of the OSB board. This can be achieved very easily by installing vertical battens (counter batten) on the OSB board. These timber battens should be 80mm wide and attached to the OSB board 600mm apart.
Once the battens are securely attached, you need to attach a cement/carrier board to the top of the battens. This creates the cavity and allows water (if it were to enter the system) to travel down and away from the building. Once the cement board is attached to the battens, you are then ready to start installing our different systems.
Priming the carrier boards
The first thing that you need to do is to prime the carrier board. We recommend priming the carrier board with EWI-310 Universal Primer, because the presence of silicate in the primer allows the base coat layer to key into it, ensuring a really strong adhesion between the cement board and EPS adhesive.
EWI-310 Universal Primer comes in 20kg buckets and each bucket can cover between 60 and 80m2 depending on absorption of the cement board. The primer is applied to the cement board by a brush or roller and is red in colour allowing you to see where on the wall it has been applied. We recommend allowing this to dry completely before applying the next layer.
Attaching the insulation to the carrier board
Once the EWI-310 primer is dry, you can begin attaching the EWI-410 EPS insulation boards. This is done by applying a layer of EWI-220 to the perimeter of the insulation boards, and then three dots in the middle, and attaching the boards in a staggered formation as shown below:
Once the adhesive has dried firmly, you can then apply the mechanical fixings. When attaching the boards to cement board, we recommend our EWI-745 Plastic fixings for OSB. These allow you to pick a length of screw based on the combined thickness of the insulation and OSB board, to ensure it provides a strong mechanical fix.
If you are looking to hit a U-value of 0.3, then we recommend installing 100mm of EPS in order to achieve this.
The base coat layer
Applying the base coat to the cement board is relatively easy – we recommend using either EWI-220 or EWI-225. The EWI-225 is slightly stronger than the EWI-220 so it depends on how solid you need this façade to be.
Both the EWI-220 Basecoat Adhesive and the EWI-225 Premium adhesive come as a dry mix and therefore need to be mixed with water prior to application. Once the adhesive has been mixed with water and the grey putty has formed, it needs to be applied to the cement board with a notched trowel. We recommend using a 10mm notched trowel, this is the best size of trowel to apply a 6mm layer of adhesive to the cement board.
Once the adhesive has been applied it is time to embed the fibreglass mesh within it; it is this mesh that gives our render systems such flexibility. Our reinforcement mesh comes in 50m2 rolls (1mm wide), and this needs to be applied in vertical strips within the adhesive. The strips of mesh need to overlap one another by 10cm at edges and this is needs to be sunk within the layer of adhesive – this can be done by using the flat edge of the notched trowel and drawing this up from the bottom of the wall – this will pull the adhesive through the holes in the mesh.
If you are looking to further strengthen the wall, then you might want to consider double meshing the system – this involves embedding a second layer of reinforcement mesh at 90 degrees to the first mesh layer. In order to do this, you will need to add a marginally thicker layer of base coat adhesive (minimum 8mm) to give the required depth to add the second layer of mesh.
Remember the final render coat is very thin, so it is very important this base coat layer is smooth otherwise you won’t get a great finish.
Applying the final render topcoat
Once the base coat layer has completely dried (this make take 24 hours or more) it is time to prime the wall prior to applying the final render topcoat. The primer used depends on the render you are going to use, but all our render primers are applied in the same way – it needs to be applied with a brush or roller all over the base coat surface. EWI-330 is used for mineral/acrylic renders while EWI-333 is used for silicone/silicate renders.
The 7kg buckets of primer will cover approximately 20m2, while the 21kg buckets will cover 70m2 of wall.
Finally – to the render!
All our renders are thin coat, so the thickness of the render depends of the granulation of the render being used. For example the thickness of our 1.5mm granulate render needs to be 1.5mm once applied to the wall. It is very easy to get the thickness incorrect, so please practise before applying the render to the wall to ensure you can achieve this thickness. It is important to achieve that depth to ensure the render is flexible and doesn’t crack. Also if a thicker layer is applied, it is more liable to wash off because the weather here in the UK is pretty unpredictable and a thicker layer of render will take considerably longer to dry.