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Materials Required for Insulating Below the DPC

Last modified: 18.11.2024

When it comes to effective home insulation, one crucial aspect that is often overlooked is insulating below the damp proof course (DPC). In this blog, we’ll explore the DPC, its role in your building’s structural integrity, and why proper insulation below this level is essential. We’ll also discuss the materials needed to achieve optimal insulation performance below the DPC.

The damp proof course (DPC) is a horizontal barrier layer within a building's walls, usually made of slate, plastic, or bitumen, designed to prevent moisture from the ground from rising into the structure. Typically installed at least 150mm above ground level, the DPC is a vital component in modern building construction, protecting the internal walls and floors from moisture ingress.

The primary function of the DPC is to act as a barrier against rising damp, a condition in which moisture from the ground travels upward through the capillaries of brickwork and masonry. If unchecked, rising damp can lead to significant problems, such as mould growth, damage to plaster and internal decorations, and even structural decay. By preventing the upward movement of moisture, the DPC safeguards the building’s structural health and helps maintain a dry, habitable interior.

While the DPC is essential for stopping moisture, insulating below it further enhances the building’s thermal performance and prevents heat loss at the base of the structure. Insulating below the DPC minimises the cold bridging effect, which helps maintain a consistent internal temperature, reduces energy costs, and improves overall comfort.

If you’re installing your XPS insulation system below the DPC of a brickwork substrate, it will need priming with the Water-Based Primer. We recommend applying this to the substrate with a roller or brush. Water-based primers are great for porous and friable surfaces as they coagulate and form an impervious layer. However, if you are rendering onto a very smooth substrate, we recommend applying the EWI-310 Universal Primer, as this contains aggregates that will provide a rough surface for the basecoat to adhere to. Ensuring that your substrate is adequately prepared is essential to ensuring that your render system will last.Coverage rates:5 litres covers 15-30m2 depending on the substrate.Drying times:1 coat takes 4 hours to dry.

Because the DPC area is so prone to damp, we recommend using Aquabase, a highly water-resistant basecoat/adhesive. Aquabase is a very versatile, dual-purpose product that can also be used in the subsequent basecoat layer. The adhesive is applied to the entirety of the back of the board using a notched trowel. Coverage: 1 x 25kg bag covers 4/5m2Drying times: 24-48 hours

XPS insulation boards are ideal for below the DPC because they offer a closed-cell structure which makes them more waterproof and impact-resistant. We sell XPS by the square metre in a range of thicknesses. For insulating a solid brick wall the general recommendation to reach building regulations is to opt for a 110mm thickness. A flat surface must be achieved while mounting the boards to ensure a smooth topcoat. We recommend using a spirit level to regularly check that everything is flat. XPS should not be used anywhere else but below the DPC due to its non-permeable nature. The rest of the EWI system needs to be able to breathe to help alleviate problems with moisture.Coverage:1m2

EWI Pro recommends that you install one fixing per corner, with an additional two in the middle of the insulation board. Plastic fixings should be used with the XPS insulation system as they significantly reduce thermal bridging. The fixings are available in a wide range of lengths to accommodate different insulation thicknesses. The fixings are installed in this pattern so that they go through the adhesive, not just the board. This adds strength to the overall system.Coverage:Plastic pin fixings are the recommended fixing for XPS insulation. Depending on the length, these come in boxes of 200 or 100.

Beading is essential for reinforcing external areas that are more prone to damage, for example, the corners and the areas around windows and doors. The required beads will depend on the property and which particular areas need reinforcing. We have a broad range of render beads that should be selected and applied as necessary, our beads are uPVC and are designed to be embedded within the basecoat layer. For security against condensation and cold spots, you may also want to use expanding foam tape between the starter track and the top of the insulation boards.

For insulating below the DPC with XPS insulation, we recommend using the EWI-226 Aquabase in the basecoat layer again for added protection against damp and water penetration. Coverage rates:1 x 25kg bag covers 4/5m2Drying times:24-48 hours

A render primer should be used to increase the adhesion of the render to the basecoat. It can be tinted to match your render's colour for increased opacity. The type of primer will depend on the type of render you are using. When using Mosaic Render, we recommend using the Mineral & Acrylic Primer, which comes in two different bucket sizes.  Coverage:1 x 7L bucket covers 20m21 x 21L bucket covers 60-70m2Drying times:12-24 hours

Once the primer is dry, the render topcoat can be applied. We recommend using the EWI-050 Mosaic Render, which is very durable and waterproof, making it perfect for below the DPC. Mosaic Render is available in various colours to achieve a unique finish. Coverage:1x25kg bucket covers c. 3-4m2Drying times:24 hours (+)