What to consider when installing external wall insulation

27 September 2021 Kingspan Insulation UK

External wall insulation (EWI) can provide an effective means of cutting heat loss from solid walled properties, reducing energy bills whilst also enhancing their external appearance.

What to check before installing an EWI system

As an EWI installation can significantly alter the performance characteristics of a property, we strongly recommend that a risk assessment and whole dwelling assessment are carried out by a suitably qualified expert in line with PAS 2035: 2019. This should help to confirm that the property is suitable for the installation of an EWI system as part of its medium-term improvement plan.

Learn more about PAS 2035.

These inspections should cover the following aspects specifically relating to EWI:
•    assessing the condition of the walls and roof including brickwork, pointing, and guttering for issues including damp or damage;
•    checking the damp proof course is at least 150 mm above the path and that the ground runs away from the wall;
•    considering the materials used in the original construction and whether the building uses a vapour open or closed strategy;
•    assessing the local exposure conditions. In addition to looking at the exposure zone, this should also consider building specific factors such as aspects that are open to prevailing conditions and local objects such as trees or other buildings;
•    assessing current ventilation provision and how this may be impacted by the EWI system. This should include considering whether airbricks need to be retained (typically as a source of oxygen for open fires or other combustion devices) or can be replaced with controlled ventilation systems;
•    identifying external obstructions such as satellite dishes, flues, guttering and gas or water penetrations which should be removed before installation of the EWI system; and
•    considering the condition of all complementary building elements including windows and doors.

Any issues identified during this assessment must be dealt with before EWI installation can begin. 

What to consider when specifying EWI

When selecting which EWI system or insulation material to use on a home, it is important to consider their thermal conductivity. This is the measure of how well it limits heat loss through conduction. The lower the value the more effective it is at preventing heat loss. This means a slimmer thickness of insulation can be used to achieve a compliant U-value, limiting the expansion of the building footprint. The specific U-value targets for each home will depend on where the property is located and whether the work is being completed as a refurbishment or new build.

Find out more about the minimum U-value requirements within the Building Regulations.

By specifying a more thermally efficient insulation such as Kingspan Kooltherm K5 External Wall Board (which has a leading thermal conductivity value as low as 0.021 WmK), it is possible to reduce any loss of space in side-access areas. In addition, in some cases it may also be possible to accommodate the system within the existing eaves and gable overhang, avoiding the need to extend these elements. 

The thinner wall construction also helps to limit changes to the window reveal depth. Research has shown that this can notably increase daylight levels within the space, creating lighter, healthier environments.

Care should also be taken to ensure the system is compliant with the fire regulations for the particular building (these may vary depending on building use, height, construction and location).

What to consider when detailing an EWI system

As with any insulation installation, it is essential to pay attention to potential thermal bridges, such as window reveals and the junctions with the roof and floor. Designers should seek to achieve continuity of insulation at these junctions or, failing this, an overlap of the insulation layer with low thermal conductivity materials. If neither is possible then additional condensation risk analysis should be carried out.

It is also important to ensure the system is designed to handle excessive driving rain. This includes ensuring appropriate overhangs and rainwater goods designed for the level of exposure. Render drips and parapets should be detailed so that water is shed away from the wall and directed into gutters and downpipes. All such measures should be robust, durable and easily maintained.

Some of the key design considerations to pay attention to include:

Window reveals

•    Where existing windows are being retained, detailing should consider the potential that these may be replaced in the future. To address the linear thermal bridge around the reveal in these applications, it should be designed to accommodate a 20 mm thickness of Kingspan Kooltherm K5 (min R-value 0.6 m2K/W). 
•    If new units are being installed, these should be positioned so the outer face is flush with the masonry wall and overlaps the EWI.
•    projecting sills should be used to improve the weather-tightness of the detail
•    In both cases, heat loss through the reveal can be further reduced by fitting a 32.5 (min) thickness of Kingspan Kooltherm K118 Insulated Plasterboard internally.

Floor junction

•    Wherever possible, the ground at the base of the walls should be excavated and an insulation materials such as Kingspan GreenGuard GG300 fitted to a depth of at least 300 mm below finished floor level. In new build constructions in the UK, the wall insulation must extend at least 150 mm below the top surface of the floor insulation for solid floors or 200 mm below for suspended timber floors. Improvements to external drainage could also be considered when undertaking such work.

Roof junction

•    If the EWI system extends beyond the existing eaves and gables, these should be extended, and the loft insulation should overhang the EWI system to maintain insulation continuity between the wall and roof. Guttering and downpipes should also be repositioned where necessary to maintain insulation continuity and increase robustness at interfaces from driving rain.

Wall penetrations/attachments

•    As mentioned, external obstructions such as flues and satellite dishes should be removed and re-fixed to the surface of the EWI system. In some instances, separation may be required between the EWI system and flues/air ducts per manufacturer instructions.

As insulated render systems are proprietary, installation guidance should always be sought from the render system manufacturer. In all cases, however, the walls should be dry (moisture content 6-8% for masonry walls) and in good condition before the system is installed. If walls are uneven then a bedding compound should also be applied. Minimum total render thicknesses should average 8 mm and not drop below 6 mm at any point.

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