This guide was created to help homeowners understand the process and Atlas’ products. It’s not a how-to instruction and is by no means comprehensive. If you want to know more please ask your builder for clarification. A more detailed explanation can be found in the School of Insulation and the Contractor’s Handbook. If you’re an installer, please e-mail us at info@antbs.co.uk. We can help you join a free Atlas EWI course and get an approved installer card.
External Wall Insulation (EWI) using polystyrene is an effective way to improve the thermal performance of a building, reducing heat loss and increasing energy efficiency. Atlas offers high-quality insulation systems designed for durability and excellent thermal insulation. This guide will take you through the layers of EWI. We hope it’ll help you understand the system and it’s components a little bit better.
Layers of the EWI:
Windows and doors should be protected to avoid damage. We offer several tapes suited for external use:
Renderers Tape by Fiberglass Fabrics
Exterior Soft PVC Tape by Deltec
Rough Surface Exterior Masking Tape by Blue Dolphin
Multi-Surface Exterior Masking Tape by Blue Dolphin
Glass can be protected with Window Film, patio with Black Membrane.
Any uneven walls should be patched up with ZW330 mortar – it dries quicker than adhesives and is designed to work well in thicknesses 3-30mm.
Before board installation, a Base Track (sometimes called Starting Edge) is attached to the wall. The Starting Edge provides a stable foundation for insulation boards and prevents moisture and rodent penetration.
A metal Verge Trim is often used as part of an EWI system where the roof is not able to cover the thickness of the insulation. It’s worth considering extending the roof (most grant schemes require this) as it gives a more aesthetic finish and guarantees no water will penetrate into the wall.
Primers are essential and should not be skipped under any circumstances!
Every wall should be prepared before insulation. The wall should be free of mould, algae, dust. Any flaking or unstable coats (such as old render, old paint) should be removed.
All absorptive surfaces should be primed with Uni Grunt. Atlas Uni Grunt is a deep-penetrating primer designed to improve adhesion and regulate substrate absorbency before installing External Wall Insulation (EWI). Here are its key benefits:
Non-absorptive surfaces such as old render, old, well bonded paint should be primed with Cerplast. Cerplast is a bonding primer based on acrylic resin and quartz aggregate. It creates a rough surface, which helps with adhesion and ensures the proper setting of adhesives.
The recommended Polystyrene Thickness is 100mm. It meets the UK Building Regulations requirement of 0.30 W/m²K. We also sell 90mm, 50mm, 30mm and 20mm. Two thinnest are used around windows and doors where the use of 50mm or 100mm is not possible. We sell Graphite EPS only as it’s superior to white.
Graphite EPS contains graphite particles that reflect and absorb radiant heat, improving insulation. It has a lower λ-value (~0.031 W/mK) compared to white EPS (~0.038 W/mK), meaning thinner boards can achieve the same U-value.
Boards should be placed in a staggered pattern, like brickwork, to prevent vertical joint alignment, which can create weak points. Ensure that no four corners meet at one point to avoid cracks in the system. Trim and Fit Around Openings. Use a hot-wire cutter or fine-toothed saw to cut boards around windows, doors, and pipes. Place boards so that joints never align with the edges of openings—always overlap them for better stability. The board should have a L-shape and hug a window/door corner. Minimize gaps between boards. If any small gaps remain, fill them with narrow strips of EPS or low-expansion PU foam (never adhesive) such as Ceresit CT88 or Whiteteq.
Graphite Polystyrene is only used above the DPC. You can (and should, where possible) insulate below DPC with HYDRO EPS. Our boards are blue to easily differentiate them from other types of polystyrene. HYDRO EPS has high water resistance, durability, and compressive strength and very low water absorption.
When using EPS (expanded polystyrene) insulation in external wall insulation systems, mineral wool is often incorporated as a fire break to enhance the system’s fire safety. Mineral wool is non-combustible (Euroclass A1) and capable of withstanding high temperatures without contributing to fire spread. It is typically installed in horizontal and vertical bands, particularly when two properties meet e.g. a semi-detached or terraced house. These strategically placed fire breaks interrupt the continuity of combustible insulation, helping to slow down or contain the spread of fire, thereby improving the overall fire performance of the façade system. This combined approach allows for the thermal efficiency of EPS while meeting fire safety requirements, especially in buildings with increased fire risk or where regulations demand additional protection. Mineral Wool requires K-50 adhesive.
We offer two adhesives, both are dual-purpose, suitable for fixing the boards and meshing. The recommended coat thickness is 2-5 mm. The adhesive should be applied to the insulation in a border-and-dot pattern.
Hoter U is a universal adhesive, suitable for both EPS and XPS boards. It’s grey, suitable under all colours from the standard pallet (SAH 400). It must be primed with the appropriate primer before the top coat. Main advantages:
K-50 is also a universal adhesive, it’s suitable for EPS and mineral wool. It’s perfect where a fire break is installed. It’s off-white in colour, suitable for all colours of the top coat including the intense range (SAH 401 – 480). No primer is required between the base coat and the top coat (acrylic, hybrid, silicone), which saves time on-site. Main advantages:
A fibreglass mesh layer is essential to provide long-lasting, durable insulation. It resists impact and cracks and reinforces the wall. It can be purchased in full rolls or we can cut the required amount from the roll (in increments of 1m). A mesh should overlap to avoid weak spots. The window and door corners should be additionally reinforced with a stress patch applied diagonally.
Mesh and all beads should be embedded into the adhesive in one layer.
PVC corner beads are available in 2.5m and 3m lengths. In Atlas’ system, the corner profiles have a 90° angle without a lip, with the mesh on the outside of the corner, to be embedded into the adhesive layer for a secure and seamless finish. They reinforce the fragile corners and provide a straight, visually appealing line.
Drip PVC corners help to redirect water away from window reveals. The bead is white.
A very useful bead is the bottom mesh drip bead (sometimes also called clip-on profile with mesh) – it redirects water away from the wall and base track. The mesh helps to reinforce and blend the bead into the adhesive layer.
Window profiles have a seal and an extra protective self-adhesive strip that gets removed at the end. They provide a neater finish than stop beads or rendering up to the window. The mesh can be partially trimmed if the reveal is shallow.
A stop bead is used in EPS EWI systems to create a clean, straight edge at terminations such as window reveals, wall ends, or transitions to different materials. It helps ensure a neat finish, protects the render edge from cracking or chipping, and provides a clear separation line where the system stops, enhancing both aesthetics and durability.
UK regulations do not always mandate mechanical fixings, but for added security and durability, insulation should be installed using both adhesive and mechanical fixings, regardless of wall height or building location. The adhesive should be completely dry before the holes are drilled. The plugs’ length should be chosen in consideration of the insulation thickness.
To reduce thermal bridging, you can use KES cutter to cut the holes for the plugs and fill them with KES covers.
If the wall has metal or timber elements, Insulation Washers with appropriate screws can be used instead of plugs for secure fixing.
Priming the reinforcing layer in an EWI system serves several important functions, ensuring durability, adhesion, and a high-quality finish. Here’s why it’s essential:
KT16 Cerplast should be applied before Acrylic, Hybrid (Acrylic-Silicone), Mineral and mosaic (DEKO M) renders. We can tint Cerplast to a similar colour as the top coat, which could be helpful when applying bright or intense colours. Tinting Cerplast is always recommended for mosaic renders.
ANX Silkon should be applied before Silicone render.
K-50 Stopter base coat doesn’t require a primer for the acrylic, hybrid and silicone renders. Many builders still decide to prime it to enhance durability.
The rendering work can be commenced after the mortar has dried (min. 3 days) and when atmospheric conditions correspond to the requirements indicated in the Technical Data Sheets for thin-coat plasters.
In this system the final coat is a thin coat render. Three main types are Acrylic, Hybrid and Silicone. Acrylic and Hybrid can be tinted in 400 colours, Silicone in 480. The colour chart is available here, please choose the orange rectangle 🟧 https://strefaarchitekta.atlas.com.pl/wzornik_kolorow/index.php?lang=en
Render types:
✅ Best for: Budget-friendly projects, areas with minimal exposure to rain and pollution.
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✅ Best for: A balance of durability, breathability, and affordability.
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✅ Best for: High-end projects, buildings in harsh weather conditions (e.g., coastal, urban, or high-humidity areas).
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✅ Best for: High breathability, eco-friendly applications.
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Atlas Salta Silicone Paint is a high-performance, water-repellent paint designed for external facades. It provides excellent protection, durability, and aesthetic appeal, making it ideal for various applications. It enhances water resistance, preventing moisture penetration while maintaining breathability. Salta improves UV resistance, preventing fading and surface degradation. It can be mixed in 400 shades (SAH 400 pallet 🟧).
✅ Best for: Base areas, plinths, and surfaces exposed to high mechanical impact. Can be used indoors as a decorative, protective wall finish. You can use stone or brick stencils with this render.
Key Features:
Best Used On:
✅ Best for: Base areas, plinths, and surfaces exposed to high mechanical impact. Can be used indoors as a decorative, protective wall finish. You can use stone or brick stencils with this render.
Key Features:
Best Used On:
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