The EWI Pro system specification only covers the installation of the insulation at the level of the DPC and above. However, this doesn’t mean that you can’t insulate below the DPC because some properties have a sub-level or go below the ground level and there may be a need to insulate at this level to prevent significant thermal bridging or damp issues.
Also many properties actually have a warm basement or habitable space, it therefore makes sense to also insulate any exposed walls to prevent heat loss and increase the energy efficiency of the walls. Also the property may have damp issues at the base and while it is being treated with damp-proof injections, it could be insulated and rendered at the same time, providing a much more robust and long-term solution. However the insulation of the DPC part would only be undertaken if the rest of the property was being insulated.
Insulating with extruded polystyrene boards or XPS
We would recommend insulating the sub-ground level or basement external walls with the XPS or the extruded polystyrene boards. The XPS boards are of the closed cell format and highly compact in their composition meaning they are fantastic at preventing water being absorbed.
The boards are also compact and can withstand additional force, which is always useful to have this prevention at the base of the building.
Preparation of the substrate for DPC insulation
The design of the DPC on each property could be different. It is very common to find solid brick throughout – from the base of the property right up to the DPC. Some properties on the other hand may have a concrete base or concrete cladding at the base. While others have just the brick that is painted. Some properties even have render or pebble dash throughout the property including the DPC layer.
If the property is simply of brick construction you may want to prime it with our EWI-301 water based substrate primer. This will penetrate deep into the brick and provide a nice substrate to work with. For any small cracks at the base you can also use the EWI-260 Levelling Mortar in addition to the primer.
If the base it pebble-dashed, we recommend stripping off the render at least 10-15cm from the ground level, especially if you are also preparing the area to allow for anti-damp-proof injections. You may also want to do this to simply stop the bridging of any moisture from the ground level up.
Digging a ditch below the base of the property
As the CAD drawing suggests it is actually recommended to dig a ditch at least 30cm below the ground level foundation of the property. The XPS boards do not absorb moisture and therefore by having the insulation start at sub-ground level will prevent the moisture penetrating the layer between the insulation and the foundation of the property.
If, however you have a concrete perimeter around your property or a gully that helps with drainage, then you may simply want to place the XPS boards on top of the existing concrete. Ensure the area is nice and dry and fill any cracks.
Injecting property base with DPC injections
As you can see from the photos you can use a solution like Dryzone, which is simply inserted into the brick at 10cm intervals – once inside the fabric of the building it will help dry out the affected areas and prevent future rising damp.
This doesn’t have to be done if you are insulating below the DPC and is only recommended to do if the property has had previous rising damp issues.
Sticking XPS board to the substrate
The XPS boards come in different thickness – for example: 20mm, 50mm and 100mm. To then cut to them to size we recommend using a hot wire cutter or a similar cutting device. As mentioned the typical property bases vary in depth and angle. If the property is on an incline, you will have a different starting position at the front relative to the back. Therefore, you will need to be precise when cutting the boards to make sure they are sunk in appropriately into the ground (if possible).
To stick the XPS we recommend using the EWI-226 Aquabase. This product is ideal for below the DPC because it is highly water-resistant and will therefore help to prevent rising damp and water ingress.
Do not use the dot-and-dab method, instead, you should cover the board liberally with the adhesive using a notched trowel. The reason for this is that using adhesive is the only way to fix the insulation board to the structure and therefore 100% coverage is imperative.
When this point has been reached in the install, you don’t need to use mechanical fixings but you can proceed straight to the basecoat and mesh. At this stage you can use the EWI-226 as your basecoat. Ensure you overlap the mesh where they join and use the same technique to sink it into the basecoat as you would do to the normal wall.
While it is understandable that the finish of the sunk mesh may not be as consistent as you would have on a normal wall where the access is ok – here it may be rather awkward, which would mean you may have to do a little touch-up basecoat once the first layer has dried.
Finishing off the DPC insulation with mosaic render
For the mosaic render you can use a number of our primer range, but we suggest going for the EWI-330 Mineral or Acrylic Primer. This will be quite robust given the top-coat that is going to be applied.
Depending on the area that is being insulated you may only need 1 or 2 buckets of the mosaic render. We recommend you use a small trowel as every drop is precious!
The good thing about the Mosaic Render is that the little pebbles that are part of its texture can be mixed with various pigments to produce a range of fantastic finishes. Please get in touch if you would like to explore some of these possibilities.