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Tips for Installing Kingspan K5 Insulation

Kingspan K5 is a great product for insulating your property. We’ve put together a few tips for installers to consider when installing Kingspan insulation boards as part of external wall insulation systems.

What are the benefits of installing Kingspan insulation?

The Kingspan Kooltherm K5 External Wall Board is primarily composed of phenolic foam. This core material is what gives the board its superior thermal insulation properties.

  1. Phenolic foam core: The core of Kingspan K5 is made from phenolic foam, a type of foam manufactured through a reaction between phenol and formaldehyde. This foam is known for its excellent thermal insulation capability due to its closed-cell structure, which traps air and minimises heat transfer. Phenolic foam also has a low thermal conductivity, typically better than many other insulation materials.
  2. Facing materials: The phenolic foam core is typically faced with a composite foil on both sides. This foil facing serves multiple purposes: it acts as a barrier against moisture, enhances the board’s mechanical strength, and aids in the installation process by providing a more durable surface.
High thermal efficiency

The high thermal efficiency of Kingspan Kooltherm K5 External Wall Board is primarily due to its phenolic foam core, which has an exceptionally low thermal conductivity. This property means that the board is incredibly effective at inhibiting heat transfer. The U-value of Kingspan is 0.021 W/m²K. In practical terms, this results in less heat escaping from a building during cold months and reduces the amount of heat entering during warmer periods. Consequently, this maintains a more stable and comfortable indoor temperature year-round.

This efficiency results from the phenolic foam’s closed-cell structure, which traps air and minimises the movement of heat. Because air is a poor conductor of heat, the trapped air in these cells significantly slows heat transfer. Additionally, the reflective foil facings on the board contribute to its insulative properties by reflecting radiant heat, further enhancing its overall thermal performance.

Moisture resistance

The moisture resistance of Kingspan Kooltherm K5 External Wall Board is a critical attribute that contributes significantly to its overall performance and durability, especially since it’s designed for external use. This resistance is essential for preventing water ingress, which can lead to structural damage, mould growth, and reduced insulation effectiveness. Kingspan K5 boards have a water vapour resistivity of 300MN.s/g.m.

The board’s core, made from phenolic foam, inherently resists moisture absorption due to its closed-cell structure. This structure ensures that the tiny air pockets within the foam do not allow liquid water to penetrate easily, which is crucial in maintaining the board’s insulative properties even in damp conditions.

Additionally, the board’s surfaces are faced with a foil as an additional barrier against moisture. This foil repels water and protects the phenolic foam core from external humidity and rain. It ensures that moisture does not seep into the board, which helps to maintain its structural integrity and thermal performance over time.


The durability of Kingspan Kooltherm K5 External Wall Board is an essential feature, especially given its application on external walls where it is exposed to a range of environmental stresses. One key aspect of this durability is its compressive strength, rated at 120 kPa. This measure indicates the board’s ability to withstand mechanical pressures without deforming or breaking down, which is particularly important for external insulation subject to various loads over time.

A compressive strength of 120 kPa means that the board can resist significant forces, such as wind loads, potential impacts, and even the stress from installation and maintenance activities. This strength ensures that the board maintains its shape and integrity. Due to its durability, we recommend Kingspan for many urban projects and refer to it as a space-saving system. Thanks to the exceptional thermal performance, Kingspan insulation satisfies Building Regs at lower thicknesses. 

In addition to its mechanical robustness, the phenolic foam core’s closed-cell structure contributes to its overall resistance to environmental factors such as temperature fluctuations and UV exposure. The protective foil facing on both sides protects the core from direct exposure to moisture and sunlight. Moisture and sunlight contribute to degradation.

Securing Kingspan K5 boards to the substrate

Installers should use the Premium Basecoat as an adhesive when installing Kingspan K5 insulation boards. This material must be dotted and dabbed (as usual) on the back of the boards.

The fundamental difference between installing Kingspan K5 and EPS boards is rasping. You cannot rasp Kingspan insulation boards due to the glass tissue facing. This means that any slight imperfections can’t be masked by rasping. Therefore, the boards must be perfectly aligned during application to ensure they are flat. Otherwise, you’ll be able to see the joints underneath the render. You’ll need a long spirit level, patience, and strength to do this.

The boards need to be cut precisely to ensure the right level of accuracy. You can’t use a hot wire cutter with K5 boards. As a result, it’ll have to be done by hand. Once secured to the substrate, cracks should be filled with spray foam and smoothed out. Please note that you should never fill the gaps using the adhesive.

We recommend using the metal pin screw fixings (as opposed to hammer fixings) because the K5 boards are far tougher than EPS. The screw fixings help achieve a flat finish.

  • Premium Basecoat EWI-225
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    Premium Basecoat (EWI-225) – 25kg

    £19.36 Excl. VAT

Applying the Basecoat to Kingspan K5 Boards

You can’t put a levelling coat on insulation boards with external wall insulation. Any basecoat thicker than 3-5mm will crack—the basecoat is not for levelling. It’s for enhancing the tensile strength of the system. This action heightens the importance of getting a level surface with the insulation boards. Any basecoats over 5mm thick will void a system warranty.

Because of the glass tissue outer facing, applying the basecoat to the installation of Kingspan K5 insulation boards is slightly more complicated than with EPS. This shouldn’t be a problem if you’ve installed Mineral Wool before. You need to use a very thin, flat basecoat layer before applying the second notched layer (taking up to a maximum of 5mm) and embedding the mesh.

Once the basecoat reinforcement layer has gone off, the render can be applied the usual way. The result should be a cosier home with a seamless render finish!

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