Using External Wall Insulation to improve thermal performance

External Wall Insulation,

Simon Lloyd, technical project manager at Kiwa Building Products, discusses everything you need to know about External Wall Insulation (EWI)

External Wall Insulation is used to augment the thermal insulation performance of existing external walls by improving their thermal resistance value.

External Wall Insulation is the preferred method of insulating a solid wall property, refurbishing non-traditional housing or that which requires external repair work, or to improve a building that has severe heating problems. It should only be applied to walls that do not suffer from pre-existing rising damp.

EWI ‘system’

An EWI ‘system’ comprises an insulation layer fixed to the outside of the wall and covered with a protective and decorative render, brick effect or cladding finish.

The insulation layer can be expanded polystyrene (EPS), phenolic foam or polyurethane foam boards; or mineral wool (MW) batts. A render finish is a cement-based, mineral or synthetic base coat reinforced with mesh; and over-coated with a primer. A topcoat render is then applied and finished to a smooth, textured, painted or pebble-dashed finish. Dry cladding includes timber boards and aluminium panels; whereas brick effect finishes can be stone or clay tiles or brick slips.

Benefits of External Wall Insulation

  • Improved protection of external walls against rain penetration to the inside
  • Reduced heat loss
  • Reduced CO2 emissions
  • Lower heating/energy bills
  • Reduced draughts through walls and around window frames
  • Improved sound resistance and an enhanced appearance.

Warmer walls reduce the risk of condensation on internal wall surfaces and help with damp problems; and increase a property’s energy efficiency (EPC) rating.

Planning and building regulations for EWI

EWI will change the external appearance of a property. In most cases planning permission or consent from local council planning authority is needed before undertaking any work. External Wall Insulation must be installed in accordance with the national Building Regulations. The reaction to fire classification of EWI (which is largely determined by the materials selected for use) may restrict its use in terms of location and height.

Prior to installation, each property to be treated must be surveyed to establish suitability, as incorrect treatment can be detrimental to the building fabric in the longer term. Where appropriate, a condensation risk analysis must be undertaken by a competent specialist. If the ‘dew point’ will occur between the wall and the insulation, or within the insulation itself, a vapour barrier must be included between the wall and the insulation.

The project-specific choice of EWI is determined by the existing wall type. For example, solid stone walls may be breathable and allow moisture to penetrate a little and be evaporated away. Using breathable MW insulation with a breathable non-acrylic external render or other finish will allow the wall to maintain its breathability.

Solid brick walls may be impermeable to moisture penetration and non-breathable. Any rigid foam insulation will suit such a wall.

Window trickle vents and extract fans in high humidity areas need to be installed to meet room space ventilation requirements.

As with all property fabric-related work, a high standard of installation is required. Poor detailing e.g. around eaves, allows water to pass behind the insulation where it can become trapped and cause hidden damage to the property in the longer term.



Simon Lloyd

Technical project manager

Kiwa Building Products


  1. What remedy would be recommended when a previously trouble-free solid wall (victorian) has developed some low level damp issues after the installation of expanded polystyrene external wall insulation?


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