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Which Basecoat Do I Need?

Last modified: 01.01.2024

When embarking on an External Wall Insulation (EWI) project, decide which basecoat is the best one to ensure long-lasting, effective results. At EWI Store, we stock a diverse range of basecoats, each catering to specific needs and conditions.

Basecoats are an essential component in various construction and renovation projects, particularly in rendering and External Wall Insulation (EWI) systems. Basecoats act as an adhesive layer between the insulation material (like EPS or mineral wool) and the topcoat render in EWI systems. They ensure that the insulation boards are firmly attached to the wall. They also help in creating a smooth, level surface over the insulation, which is crucial for the final finish. Basecoats provide structural strength and protect the insulation layer from physical damage and weather-related wear. Certain basecoats are designed to prevent moisture ingress, thereby protecting the wall and insulation from water damage.

  • Mixing paddle and drill
  • Bucket or mixing tub
  • Trowel and hawk
  • Fibreglass mesh
  1. Preparation: Read the manufacturer's instructions carefully. Each product may have specific mixing ratios and procedures.
  2. Mixing Ratio: Typically, basecoat powders are mixed with water. Ensure you use the correct ratio of water to powder.
  3. Process: Pour the required amount of water into the mixing tub first, then gradually add the basecoat powder. Use a drill and mixing paddle for a consistent mix. Avoid overmixing as it can affect the performance of the basecoat.
  4. Consistency: Aim for a smooth, lump-free consistency. It should be malleable but not too runny or too stiff.
  1. Preparation: Ensure the surface is clean, dry, and free from loose material. Prime the surface if recommended by the manufacturer.
  2. First Coat: Apply the basecoat using a trowel. Spread it evenly over the surface, ensuring complete coverage. The thickness will depend on the product and the requirements of the system.
  3. Fibreglass Mesh: For added strength, embed a layer of fibreglass mesh into the wet basecoat. Gently press it in with a trowel and ensure it's fully encapsulated by the basecoat, with no air pockets or wrinkles.
  4. Second Coat: Once the first layer has been set, apply a second coat if necessary.
  5. Curing: Allow the basecoat to cure as per the manufacturer’s instructions before applying any topcoat or finish.

EWI-225 is at the top of the basecoat range at the EWI Store and has various applications within EWI systems. The primary benefit is the fantastic versatility that the product offers. Due to the chemical additives, it is exceptionally durable and flexible, whilst not sacrificing any load-bearing capabilities. The composition contains white Portland cement and a high concentration of polymer-modified binders. As such, it is the recommended basecoat for EWI systems that utilise EPS, Kingspan K5, and Mineral Wool. Both K5 and Mineral Wool are far heavier in comparison to EPS. Therefore, they require a stronger adhesive. Crucially, EWI-225 is dual-purpose, therefore it reduces wastage on site. Only one bagged product is required which affects cost and time efficiency.

Much like our other basecoats, EWI-220 is hydrophobic and vapour-permeable. The presence of moisture in a substrate is a significant hindrance in retrofits. However, using a system that is designed with vapour-permeability in mind can aid the evaporation of that moisture. Once again, the product is dual-purpose which means that it doubles as an insulation adhesive for EPS and a bedding basecoat for fibreglass mesh. EWI-220 is meticulously manufactured to have greater adhesion to the less tactile surface of EPS boards. Whilst scratching the top layer of EPS is still recommended before the application of the basecoat layer, EWI-220 does not impact the adhesive properties. EWI-220 is also incredibly versatile as it can be used with the following substrates: ordinary concrete, walls of ceramic and silicate elements, concrete, light-aggregate and cellular concrete with an untreated surface, plastered, painted or textured.

EWI-222P is a polymer-rich cementitious basecoat that can be used on many different substrates, providing a breathable and waterproof protective coat. The product is designed for render-only projects and EWI systems, including EPS where it doubles as an adhesive and basecoat. The versatility of the basecoat, including low-temperature application and trowel or spray application makes it highly desirable. The benefit of EWI-222P is that the application temperature stretches down to 3°C. Crucially, all of these benefits also come at a very attractive price point.

EWI-226 Aquabase stands out as an exceptionally water-resistant basecoat adhesive, making it an ideal choice for areas prone to high moisture levels. Its durable and vapour-permeable nature allows it to effectively secure polystyrene insulation boards, such as XPS, onto external substrates. This product is particularly recommended for rendering or insulating applications below the Damp Proof Course (DPC), where moisture resistance is crucial. Beyond its use in external insulation systems, the versatility of Aquabase extends to indoor applications as well. It serves as an excellent solution in bathrooms, particularly around showers and baths, as well as in wet rooms, ensuring a moisture-resistant and reinforced surface. The importance of insulating below the DPC cannot be overstated. Insulation in this area plays a critical role in preventing rising dampness, a condition where water moves upward through a wall or floor. Without proper insulation, moisture can penetrate the building structure, leading to a host of issues like mould growth, heat loss, and damage to the building fabric. Using a product like EWI-226 Aquabase as an adhesive for XPS insulation below the DPC ensures a robust barrier against moisture ingress.

EWI-269 is breathable and heritage-friendly. It is specially designed to be applied in one pass at a depth of 25mm, which also makes it ideal for spray application. A single layer of 25mm can be sprayed quickly and efficiently. Moreover, fibreglass mesh can be embedded directly into this layer, accelerating the project timeline. Separate coats require dedicated drying times whereas one coat reduces this time. Lastly, EWI-269 is also A1 fire-rated which is incredibly important in retrofits.

The EWI-221 is cement-based, manufactured from a mixture of white Portland cement, mineral fillers, and microfibres. Much like our other adhesives, the adhesive can be used as both a bedding adhesive for EPS insulation systems and as a basecoat adhesive on EPS systems once a fibreglass mesh has been embedded within it. The key difference is that EWI-221 can also be used at temperatures down to zero degrees, making it perfect for cold-weather installations. It is also highly durable, elastic and quick drying, offering long-lasting performance that will stand the test of time. Which is your favourite EWI Store Basecoat? Let us know in the comments.