External walls: Externally applied insulation

Description
External Wall Insulation (EWI) systems applied to the external surface of the wall covered by a weatherproof layer (usually render). Render coats are available in a wide range of finishes to suit a variety of building styles. Commercial systems are usually only available through registered installers.

Unit cost of installation
Full cost up to £8000. Marginal cost if refurbishing external wall - approx. £1500

Bulk cost of installation
5-10% discount for 20 houses close to each other

Indicative energy cost and carbon emissions

For insulation systems achieving a U-value of 0.35 W/m²K

Maintenance
If mastics and sealants are used to form a weather-tight seal around openings they will need to be checked and replaced periodically.

Lifetime
Will last many years with minimal maintenance, although less robust systems can be susceptible to vandalism.

Technical suitability
Suitable for use on any wall including most non-traditional forms of construction, and can easily achieve U-values which would comply with the new-build requirements of Building Regulations. Systems are available that have been certified by BBA and BRE Certification.

Very suitable in conjunction with repair of wall in poor condition. Detailing around windows and at eaves must be designed and implemented with care to prevent water ingress - it may be difficult to fully insulate reveals if windows are not being replaced. Rainwater goods and other items fixed to external walls should be removed before installing the insulation and re-fixed afterwards. Detailing around window and door frames can be difficult particularly with sash windows which have only a very small amount of the frame exposed.

Care should be taken not to block air bricks, bridge damp proof courses, cover the weather drip on windows sills (it is normally necessary to extend sills) or block eaves ventilation. EWI should not be installed on walls with rising damp without first curing the dampness problem. Industry guarantee available.

Most commercial systems use some form of render coat as the weather surface, and are only available via approved installers. However, hung tiles or timber cladding are alternative finishes that can be installed by a competent builder, and can prove more cost-effective especially if small areas are being treated.

External insulation is very effective at removing thermal bridges (cold spots), thus reducing condensation and associated mould growth inside the dwelling. Because the insulation is applied externally the thermal mass of the walls will absorb heat from the dwelling leading to a more even temperature over a complete heating cycle, although the response time of the heating system will be lower than for lightweight structures.

Non-technical constraints
External insulation on terraced housing or only one of a pair of semi-detached can be visually obtrusive if the whole row of houses is not to be treated, although various architectural styles (Georgian, Victorian, Edwardian) can be replicated. In some cases planning permission and or consent from English Heritage may be needed. Need to check whether buildings insurance (specifically fire) is affected.

Potential concerns with fuel poor
No specific issues.

Ancillary benefits
Potential to improve wall condition, appearance, damp penetration, and condensation. Reducing mould growth can have a beneficial impact on occupants health.

Occupants do not need to move out during the installation process and because the insulation is applied externally the internal floor area of dwelling is not reduced.

Trigger points for work
Refurbishment of external walls. (N.B. Building Regulations Approval is required in England & Wales whenever any thermal element is renovated)

Performance Limitations
Can be prone to impact damage at ground level. Consideration should be given to thickening the render coat locally at ground level to mitigate against this.

Technical Guidance Available
» CE118/GPG 293 External insulation systems for walls of dwellings
» BR262 Thermal insulation: avoiding risks BRE, 2002
» CE184 - Practical refurbishment of solid-walled houses
» BBA and BRE Certification certificates
» Insulated Render and Cladding Association 01428 654011 www.inca-ltd.org.uk