CPD articles

CPD: External wall insulation

2 September 2017 | By Dale Telling

Fassatherm Classic EPS panels are applied to the ground floor of a Richmond Villages retirement complex in Derbyshire

Dale Telling, render system manager with Fassa UK, explains the advantages of external wall insulation systems in reducing thermal transfer through a building’s walls.

Up to 30% of energy lost in a building is a result of conduction through the external walls. Increasing the thermal insulation of existing and new buildings can be achieved by the addition of external thermal insulation.

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External thermal insulation composite systems (ETICS) are a means of applying layers to the outside of a building with the primary aim of improving thermal performance. ETICS use a thin base coat over insulating material. This stops heat escaping via thermal bridges as well as preventing hot air from entering in summer, rendering the temperature of the inside surfaces as close as possible to the room temperature. Overall benefits include:

Choosing insulation materials

The most common materials used for the insulation itself are expanded polystyrene insulation (EPS), mineral wool and phenolic foam. More sustainable alternatives such as fibre board and cork panels are also available.  

Phenolic foam has traditionally been a popular choice due to its low thermal conductivity, but it has a number of drawbacks. High demand has led manufacturers to shorten production times with possible negative consequences on performance, while in board form it is acidic and reacts with other materials unless separated physically.

It also has such a significant environmental impact that it has not been manufactured in the United States since 1992. Experiments carried out at Taiwan’s Central University in 2002 have also shown that it absorbs water easily, leading to an increase in thermal conductivity of 20% after only two hours’ exposure in a wet climate.

Wet boards can also lead to system failure if they dry out and shrink at a later date, so the boards need to be covered with render quickly once on site and not left out in the open.

The most common choice on grounds of cost and practicality is therefore EPS or GEPS (graphite-enhanced EPS – where graphite has been added to improve the U-value rating). It is easy, clean and safe to work with, as it is lightweight and does not harm human health. It does not degrade over time and has a relatively low environmental impact during production.

Advantages of insulation 

Eliminating mould: Thermal bridges create cold spots in a room which cause condensation, allowing mould to grow. Proper insulation prevents these and improves internal air quality by eliminating condensation and mould.

Protecting masonry: Masonry is protected from the elements and its lifespan is prolonged.

Improving appearance: When refurbishing existing housing stock, the appearance of unsightly masonry can be significantly improved by the addition of EWI.

Flexible application to substrates: ETICS can be used on masonry, timber and steel constructions and can be either mechanically fixed or bonded, whichever is most suitable. 

Insulation systems and fire safety: If applied correctly, external wall insulation panels should have no influence on the start or development of a fire in a building.

Fire-retarded EPS will have a classification of D or E, compared with F for non fire-retarded material. When exposed to heat, fire-retarded EPS will shrink away from the heat source, and as soon as the source of heat is taken away the flame will go out.

However, this is largely irrelevant, as the reaction to fire behaviour should always be examined at the level of the building element: the surface layer of the construction is what will determine behaviour in a fire. When tested in the standardised system build-up which simulates actual site conditions, EPS should comply with a B-s1, d0 classification. 

The fire load of EPS is relatively low when compared to other materials. Although it has quite a high calorific value, it is mainly made of air so the overall fire load is reduced. 

Mineral wool cladding is not combustible and should slow the spread of fire. However, when examined at the system level, it has the same classification (B-s1, d0) as EPS for build-up tests. 

In all cases where the insulation panels used are combustible, fire breaks of non-combustible insulation such as resin-impregnated rock wool lamella should be installed in accordance with BR 135 (the BRE report Fire performance of external thermal insulation for walls of multi-storey buildings) from the first floor up.

Build-up of graphite insulating panel

1. Substrate 2. Adhesive 3. Graphite panel 4. Mechanical fixing 5. Base coats 6. Reinforcing mesh 7. Primer 8. Coloured finish coat plasters 9. Protective finish

System components and installation

The system is built up from the substrate with a layer of adhesive fixing the insulation boards, followed by two reinforcing base coats and a decorative finish which also provides additional protection against rain, mould and algae. The adhesives and base coats differ according to the type of board chosen but the application method remains the same.

Adhesive can be applied either by machine or by hand. Panels are wholly covered or beaded, depending on the evenness of the substrate. In any case at least 50% of the panel’s surface should be covered, with care taken not to get adhesive on the edges of panels as this could cause thermal bridging.

The panels should be laid brick bond, butted up against each other, avoiding gaps, and alternated at corners to ensure stress absorption. Adhesive should be applied far enough from the edge of the panel to avoid getting it on the end of the second panel where they meet at a corner. 

Joints around window and door openings and any other changes in the substrate, for example from brick to concrete, must also be staggered in a dog-leg pattern.

Where gaps do occur between panels they must never be filled with adhesive; strips of insulating material for larger gaps or polyurethane foam filler for gaps under 4mm should be used to avoid creating thermal bridges. 

Anchoring the panels

Once in place, panels need to be fixed mechanically to counter perpendicular stresses caused by an incorrectly prepared substrate and/or wind stress. Failure to anchor correctly can lead to the “mattress” effect where panels curve away from or towards the building.

The type of anchor chosen depends on the type of substrate and panel. The pattern of screws depends on the type of panel, with EPS, cork and wood using a T formation with anchors at intersections and one centrally, whereas resin-impregnated rock wool panels require a W pattern with three anchors at about 5-10cm from the edge of each panel.

Fixings should be increased by 20% on the corners of buildings and any openings to help resist wind damage.

The anchorage penetration into the wall surface must match the anchorage depth. The length of the anchor is determined by adding the thickness of the insulation, adhesive and plaster and the anchorage depth together. Anchor length = (thickness of insulation + adhesive + old wall covering) + anchorage depth.

It is important to ensure that the actual depth of any existing render or other covering that is still on the wall is taken into account to ensure adequate anchoring. 


Once the panels are fixed, a base coat is applied to them using a metal trowel or by machine. Reinforced mesh is put on top of this and a second layer of base coat applied. Once the base coat is completely hard, approximately two to three weeks after the second coat, a primer followed by a colour coat is applied. This also gives added water repellence and mould and algae resistance. 

In areas of intense sunshine, a light to medium colour with a Y reflective index of over 30 should be chosen to avoid overheating of the wall which could affect the lifespan of the ETICS by damaging the protective coating and the panel. A Y reflective index of at least 25 is recommended elsewhere.

The walls of a house are responsible for the greatest proportion of heat loss

Common causes of failure

Moisture: Walls need to be free of damp before the application of external wall insulation. Otherwise the damp will be trapped in the wall and unable to escape, causing the failure of the system and damage to the building fabric.

Insulation panels must be completely dry. Phenolic board and mineral wool insulation with high levels of moisture will shrink as they dry, making the render crack and fail and opening up gaps in the building envelope. 

Inadequate anchoring: This causes the “mattress” effect where perpendicular stresses make the panels warp and curve in both concave and convex directions. To avoid it, panels must be anchored to the wall with an adequate number of fixings using the correct pattern and screws for the type of panel. 

The wind loads on the walls should be calculated in accordance with BS EN 1991-1-4: 2005 using both pull-off and pull-through tests. This is especially important on high-rise buildings in areas of high wind load. Manufacturers are now tasked by the BBA to specify the fixing pattern for buildings using an approved software system.

As part of the installation process, a layer of mesh is applied to the first layer of basecoat on top of the EPS panels

Measuring thermal properties

The thermal properties of a system are normally shown using a U-value (w/m²K = the measure of structural heat loss per unit of surface area). This is calculated on the rate at which heat transfers through 1sq m of a structure, where the temperature difference between the inner and outer face is 1 degree celsius. 

Thermal conductivity (lambda value λ) is calculated by taking the heat flux and the temperature gradient and dividing one by the other: k = QL/A∆T, where k is the thermal conductivity in W/m K, Q is the amount of heat transfer in J/S or W, A is the area of the body in square metres and ∆T is the difference in temperature in K. 

Thermal resistance (R-value) is the temperature difference between two defined surfaces of a material that induces a heat flow rate through a unit area. It is obtained by dividing the thickness by the thermal conductivity. The lower the thermal resistance, the more heat will be transferred. 

U-values are calculated as the reciprocal of the sum of R-values, that is: 1/total sum of R-values. The lower the value, the higher the insulating properties. 

Dale Telling is render system manager at Fassa UK. www.fassabortolo.com.

Editor’s note

Following recent events at Grenfell Tower, and the ongoing investigation into fire safety with regard to cladding systems, any ongoing and future advice provided by the relevant government advisory panels and trade bodies should be properly examined before progressing with cladding works.

This article is sponsored by Fassa Bortolo

Fassa UK is the British subsidiary of Fassa Bortolo, an Italian firm trading for more than 300 years which offers an integrated system of building materials.

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I would like to point out that vapour permeability should be high up on the list of the desired properties of any retro-fit external wall insulation system used on pre-1925 era buildings due to their build type.

James Browne, 20 March 2018

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