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How Much to Budget for External Wall Insulation

Last modified: 23.08.2024
Piggy bank wrapped in a scarf on top of a radiator, symbolising energy savings. Investing in EWI Store insulation systems significantly reduces heating costs and improves indoor comfort year-round. A smart choice for sustainable savings.

External wall insulation is a more expensive endeavour compared to simpler measures like loft insulation. However, the impact on thermal comfort and energy bills is significantly higher. Therefore, it is crucial to consider external wall insulation as the premier option. That raises the question: How much should you budget for external wall insulation, and how much do the costs vary? We will discuss the answer in today's blog and provide some avenues for grant funding for EWI.

Due to their larger size and surface area, detached homes generally require the most significant investment in external wall insulation (EWI). On average, a detached house with a size of around 147 square metres and four bedrooms will cost between £15,000 and £20,000 for EWI. The potential savings in energy bills are estimated at £1,000 per year, with a payback period of about 20 years.

The cost of EWI for a semi-detached house, which typically has a size of 96 square metres and three bedrooms, ranges from £8,000 to £10,000. Homeowners can expect to save approximately £700 per year on their energy bills. The payback period for this type of property is around 15 years.

Mid-terraced houses tend to have smaller external areas that require insulation, which can lower costs. For homes within the size range of 64 to 100 square metres, EWI costs from £6,000 to £8,000. Energy savings for these homes are typically between £500 and £700 per year, with a payback period ranging from 12 to 15 years.

Bungalows around 77 square metres also fall into a similar cost range as mid-terraced houses. The cost for EWI for a bungalow is estimated to be between £6,000 and £8,000. Annual savings in energy bills can be around £700, with a payback period similar to mid-terraced houses, ranging from 12 to 15 years.

An external wall insulation system has several components, all integral to the system's functionality and performance. A typical EWI system will include the following:

EWI-310 Universal Primer is used to prepare the walls before applying our adhesives. It improves the adhesive's adhesion to the substrate, facilitating application and reducing groundwork absorbency. The presence of quartz aggregate in the primer creates a strong, textured surface onto which the adhesive layer can bind easily.
  • £60.03 per bucket = £0.90 per square metre
When installing insulation systems, a starter Track is required. It sits just above the DPC layer (near the bottom of the wall) and acts as a stable, protective ledge on which the insulation boards can be built.
  • £21.99 for 2 metre track = £11.00 per metre (as the starter track is only required for the bottom of the insulation, you only need to account for the total length of the wall, as opposed to the square metreage of the walls)
EWI-225 Premium Basecoat is breathable, high-strength, and durable. This product is ideal as a basecoat/adhesive for Mineral Wool, Kingspan K5, and Wood Fibre insulation boards or as a render-only solution for various substrates, including render carrier boards. The adhesive should cover at least 40% of the board in a dot-and-dab method or full coverage.
  • £19.36 per bag = £3.87 per square metre
Kingspan K5 is an external wall insulation board designed specifically for external wall insulation systems. It is made from high-performance, rigid thermoset phenolic insulation, offering excellent thermal efficiency with a lambda value of as low as 0.020 W/m²K. The board is faced on both sides with a glass-fibre tissue, enhancing its durability and providing a stable substrate for plaster or render.
  • £16.79 per 60mm thick board = £23.17 per square metre
Metal Pin Hammer Fixings (metal pin, plastic body) can be used on various insulation thicknesses. The mechanical fixing is made from a metal pin with plastic bodies, reducing thermal bridging within the system. The metallic rods are hammered in, giving the system stability.
  • £35.94 per box of 200 = £1.26 per square metre
Corner Beads with Mesh, designed for 90°external corners in thin-coat render systems, are made from rigid PVC and incorporate fibreglass mesh 'wings'.
  • £5.09 per 2.5 metre bead = £2.03 per metre
Orange Fibreglass Mesh, crucial for render and EWI Systems, is embedded in the cement-based basecoat for a strong, impact-resistant yet flexible finish that accommodates seasonal building movements. It is also used for extra strengthening. 300mm x 200mm stress patches are applied at 45 degrees to corner openings for added reinforcement.
  • £49.90 per roll = £0.99 per square metre
EWI Pro Topcoat Primer, compatible with silicone renders, enhances adhesion, limits basecoat absorption for proper curing—vital in summer—and can be tinted to match the render colour, preventing basecoat bleed-through. It's essential for achieving a flawless finish.
  • £69.10 per bucket = £0.86 per square metre
EWI-075 Silicone Render is one of our most popular Silicone Render products. This polymer-modified, ready-to-userender is normally applied externally to produce a decorative finish on buildings. The render is suitable for use on brick, concrete, render carrier boards, traditional plasters, gypsum substrates and external wall insulation systems. EWI Pro Silicone Render is highly breathable. This makes it ideal for application on external wall insulation systems where breathable insulants have been used. The render is also flexible, meaning you can apply it as part of a render-only system (i.e. no insulation). Unlike thicker sand and cement or monocouche renders, our Silicone Render provides a robust, long-lasting decorative finish that is unlikely to crack over time.
Render type Size Bag or bucket Coverage Cost per bucket/bag (ex. VAT) Cost per m² (ex. VAT)
EWI-075 Silicone Render (1.5mm) 25kg Bucket 2.4kg/m² £71.49 £6.86
EWI-076 Premium Bio Silicone Render 25kg Bucket 2.4kg/m² £80.87 £7.77
EWI-077 Nano Drex Silicone Render 25kg Bucket 2.4kg/m² £128.49 £12.33
EWI-040 Silicone Silicate Render 25kg Bucket 2.4kg/m² £61.42 £5.90
EWI-010 Acrylic Render 25kg Bucket 2.4kg/m² £46.92 £4.50
EWI-747 NHL 3.5 Lime Render 23kg Bag 15 kg/m² per cm thickness £16.62 £10.86
EWI-090 Monocouche Render 25kg Bag 25kg/m² £11.99 £11.99
EWI-060 Mineral Render 25kg Bag 2.5kg/m² £32.09 £3.07

While external wall insulation is one of the more extensive and costly forms of insulation, other types of insulation can also significantly improve your home's energy efficiency at a lower upfront cost.

  • Loft Insulation: Loft insulation is one of the most cost-effective insulation solutions. An average-sized home typically ranges between £550 and £960. The potential savings on energy bills are around £300 per year, with a payback period of 2 to 3 years. This makes loft insulation one of the quickest ways to reduce energy bills and improve your home's efficiency.
  • Cavity Wall Insulation: Cavity wall insulation suits homes built with cavity walls (usually those built after the 1920s). Depending on the property size, the cost ranges from £1000 to £4,600. The energy savings can be around £250 to £400 per year, leading to a payback period of 5 to 10 years. Cavity wall insulation is a relatively low-cost and effective way to reduce heat loss in your home.
  • Floor Insulation: Floor insulation can vary significantly in cost depending on whether you have a solid or suspended floor. A suspended timber floor costs between £800 and £1,200. The potential savings are around £60 to £150 per year, with a payback period of 8 to 12 years. While the savings are smaller than loft and cavity wall insulation, it can still contribute to overall energy efficiency, particularly in older homes.

The budget for external wall insulation will naturally exceed the costs listed above. However, there are several avenues that allow you to receive grant funding for the installation of external wall insulation. The most popular scheme is the Great British Insulation Scheme.

The Great British Insulation Scheme is a government initiative in the United Kingdom designed to help households reduce energy consumption and lower energy bills by improving their homes' energy efficiency.

The scheme is part of the UK's broader strategy to tackle climate change, reduce carbon emissions, and make homes warmer and more energy-efficient. It can also help you budget for external wall insulation. The scheme focuses on supporting low-income households, particularly those receiving certain benefits such as Universal Credit, Income Support, or Pension Credit. Households with members who are elderly, disabled, or have specific health conditions that make them more vulnerable to cold may also be eligible. In England, homes in council tax bands A to D are generally eligible for the scheme. This inclusion ensures that a larger proportion of the population can access the funding for insulation improvements. Another critical factor for eligibility is the property's Energy Performance Certificate (EPC) rating. The scheme targets homes with lower energy efficiency, specifically those with an EPC rating of D, E, F, or G.

The best way to check your GBIS eligibility is with our Insulation Grant Funding partners. They have developed a calculator that allows you to determine the approximate amount of funding you may be eligible for.