External Wall Insulation - Technical Detail

External Wall Insulation
External Wall Insulation

External Wall Insulation (EWI) design

External Wall Insulation

External Wall Insulation (EWI) systems involve the installation of an insulating layer to the external fabric of an existing, or new building, the system is usually finished with a render coat, brick slips or clad with timber boards or tiles.

EWI systems should not be used to isolate or hide moisture penetration or damp problems in the existing structure. In accordance with Building Regulations, new solid walls should prevent moisture ingress arising from exposure to rain and snow without moisture penetrating to the inside and damaging the building.

Unless an existing building is already rendered or clad the installation of EWI can dramatically alter and improve the appearance of the external facade. Particular attention will need to be made to window sills, jambs, rainwater goods and the roof eaves as structural changes may be required to accommodate the thickness of the external wall insulation system.

When deciding which EWI system is most appropriate for the circumstances there are several issues that need to be considered. However, the most important issues are the level of thermal performance to be achieved, the insulation type and which finish is the most suitable.

A wide variety of insulation types can be used in EWI systems, however, the best solutions are usually comprised of rock mineral wool or expanded polystyrene (EPS) and each has its own merits. For instance, both rock mineral wool and EPS products have excellent thermal insulation performance, whilst rock mineral wool products are non combustible which negates the need to install fire barriers and EPS has the advantage of being lightweight.

Therefore, the differentiating characteristics of insulation products can dictate where, when and why one product is specified or preferred over another.

Thermal comfort

An EWI system makes use of the thermal mass of the wall which in turn helps to provide less peaks and troughs in the internal environment. This is particularly beneficial in buildings such as hospitals and retirement homes which are often continuously heated and can allow the heating system to be turned down overnight as the energy absorbed by the wall during the day is released back into the internal environment overnight.

Air leakage

Buildings can suffer from excessive air leakage which, if not treated, can lead to high energy costs, occupant discomfort from draughts and external noise, as well as a reduction in indoor air quality. The installation of an EWI system can deliver an airtight structure and reduce draughts and heat loss as any cracks and gaps in the external fabric of the building will be covered and sealed by the system.


Interstitial condensation occurs when warm, moist air from inside a building penetrates into the fabric of a structure and meets a cold surface, where it cools, reducing its ability to carry water vapour and increasing the risk of condensation forming within the construction.

Installing an EWI system prevents interstitial condensation from forming because the dew point temperature is moved closer to the external face of the insulated structure. Installing a vapour permeable insulation product can further reduce the risk of condensation forming as it will offer negligible resistance to the passage of water vapour and in effect allows the wall to `breathe` if used in conjunction with a vapour permeable render coat.

Surface condensation is prevented from forming on internal wall surfaces as the structural wall is on the warm side of the insulation which will always be above the dew point temperature.

Thermal bridging

Thermal bridging can be greatly reduced in existing buildings and almost completely removed in new buildings by installing an EWI system, as the insulation layer forms as complete a layer as is possible around the building. Therefore, overlaying the existing thermal bridges (such as those at external wall to party wall junctions) with insulation removes any significant thermal bridge, reduces heat loss and prevents condensation from forming.

Careful design detailing is required at junctions such as those at window and door reveals, roof eaves and verges and where the EWI system starts close to ground level. The treatment of existing features such as balconies, handrails and satellite dishes also needs careful consideration.

Dampness in walls

The most common causes of dampness in existing solid masonry external walls are, penetrating damp (often caused by deterioration of the existing mortar, blocked gutters or faulty rainwater goods), rising damp and condensation, particularly surface condensation.

For instance, missing or damaged render should be reinstated, faulty or missing flashings should be repaired or replaced and areas suffering from mortar deterioration should be re-pointed with a suitable mortar mix.

Dampness can have a negative effect on the physical properties of the materials used to construct a wall and best practice is to cure any damp problems before installing an EWI system onto an existing wall. This is not a problem for new build walls as they will not be subjected to the potential problems caused by long term exposure to the elements although the EWI system finish will need to be maintained to ensure the long term protection of the facade.

Solution Optimiser and Pathfinder

Knauf Insulation Solution Product U-value Range (High) U-value Range (Low)
External Wall Insulation System Rocksilk®  EWI Slab 0.38 0.13

EXtnernal Wall insulation System


  • Breathable system allows moisture vapour to pass through the construction removing the risk of condensation forming.
  • Can improve the appearance of an existing building whilst providing upgraded thermal performance.
  • Can accommodate wall imperfections to provide a high quality finish.
  • Installed externally with minimal disruption to occupants of existing buildings.
  • Non-combustible Euroclass A1 reaction to fire classification 


Rocksilk® EWI Slab are high impact strength Rock Mineral Wool slabs with a water repellent additive specially developed for external wall insulation systems. Their manufacture has a very low impact on the environment.

Typical construction

Existing or new solid masonry wall insulated with Rocksilk® EWI Slab finished with either a mineral or silicone render.

Typical Installation

The installation guidance in this section is provided for information purposes only. Knauf Insulation advise that system holders instructions are followed to ensure the correct installation of system components.

  • Slabs are easily cut with a large bladed knife or saw. Remove all loose and flaking materials from the wall, making good all hollow areas to provide a smooth, stable substrate.

  • Mechanically fix the starter track at damp proof course level.

  • Rocksilk® EWI Slabs are bonded to the substrate using a suitable adhesive. Adhesive can be applied liberally to even out localised undulations in the substrate.

  • Rocksilk® EWI Slabs should be installed in a stretcher bond pattern. Before the adhesive has set, mechanically fix the insulation slabs/boards with suitable fixings in accordance with the system holders’ instructions. Typically, 4 fixings per m2 are required at low level increasing to 6 fixings per m2 for high level applications however it is recommended to contact Rocksilk® EWI system holders for project specific specifications.

  • Apply adhesive to a minimum thickness of 5mm over surface of the insulation slabs/boards and level out using a straight edge.

  • Trowel a layer of Reinforcing Mesh into the fresh mortar and gently push the mesh just below the surface of the adhesive.

  • Ensure that the Reinforcing Mesh is overlapped by a minimum of 100mm in all instances and trimmed back so none is visible.

  • Primer should be applied prior to the installation of the topcoat in order to provide better adhesion, control background suction and prevent a pitting effect on the topcoat.

  • The background should be dry before application of the Primer.

  • Prior to application of the topcoat the background should be level, dry and free from dirt and dust. Apply a Silicone or Mineral topcoat by hand or spray and ensure that the freshly applied topcoat is protected from the weather to avoid ‘wash offs’ from occurring.

Note: Do not apply when air temperature is at or below 5ºC on a falling thermometer or below 3ºC on a rising thermometer, or when the temperature of the air, or wall surface is above 30ºC and the surface is not protected.


Thermal performance

Rocksilk® EWI Slab has a thermal conductivity of 0.036W/mK.

Compression resistance

Rocksilk® EWI Slab has very high compressive and impact strength providing additional protection when additional resistance to impact damage is required.

Fire performance

Rocksilk® EWI Slabs are non-combustible and classified as Euroclass A1 to BS EN 13501-1. Cavity barriers are not required when installing the Rocksilk® EWI Slabs.

Typical U-values (W/m2K)

USING ROCKSILK® EWI Slab (Refurbishment)

Insulation Thickness (mm) 225mm solid brick wall (0.77 W/mK) 215mm solid brick wall (0.45 W/mK)
200* 0.17 0.16
180 0.18 0.18
160 0.20 0.20
140 0.23 0.22
120 0.26 0.25
100 0.31 0.29

*Thicknesses above 120mm are bespoke and subject to availability and minimum order quantities. Contact us for more details.


Insulation Thickness (mm) 215mm solid brick wall (0.34 W/mK) 215mm solid brick wall (0.19 W/mK) 215mm solid brick wall (0.16W/mK)
200* 0.16 0.15 0.14
180 0.17 0.16 0.15
160 0.19 0.17 0.17
140 0.21 0.19 0.19
120 0.24 0.22 0.21
100 0.28 0.25 0.23

*Thicknesses above 120mm are bespoke and subject to availability and minimum order quantities. Contact us for more details.

Contact our Technical Support Team