External Wall Insulation

ABOVE: Alumasc’s Swisslab EWI system was used on this house in Hertfordshire. It comprises a polymer modified base render and lightweight silicone top coat, applied over phenolic foam insulation boards, and achieves a Uvalue of 0.3W/m²K

Adding insulation to the exterior of a property is a more effective way of creating a continuous layer of insulation around the entire external wall area, without cold bridges across internal partition walls. Interstitial condensation problems, which can cause potentially damaging damp problems, especially to timbers, are also less of an issue. The downside is that it has a significant impact on the external appearance of a building. In the case of a rendered property, or a property with mixed external materials, this may not be an issue. It is not suitable, however, where the external materials are of an attractive brick, natural stone, timber and lime plaster, and therefore a significant part of the building’s character. Nor is it likely to be an option for protected buildings. It is also essential that the solution is breathable to prevent the build-up of potentially damaging damp within the walls.

Adding external insulation will also reduce the overhang of the roof (soffit) and increase the depth of the reveals at window openings, possibly necessitating alterations. Most systems have technical solutions for all of these issues, such as treatments for window surrounds, vents, avoiding bridging the dampproof course, and extending window cills.

The EST’s Best Practice guideline for external wall insulation is a maximum Uvalue of 0.35W/m2K. Installation of external wall insulation is typically undertaken by specialist contractors and adding external insulation and new cladding should also solve any problems from penetrating damp.

There are several solutions and manu - facturers have details of suggested build-ups and the U-values they achieve. A typical solution is to bond rigid insulation board such as expanded polystyrene (EPS) to the external walls, finished in a sheet material or render mesh cloth, followed by an external render system. Modern thin-coat polymer modified renders are ideal for this specification. They are flexible and therefore crack resistant, breathable and self-coloured.

An alternative solution is to batten out the existing exterior walls and insulate the void in between, followed by a semi-permeable vapour barrier and then a further layer of counter-battens laid in the opposite direction and insulated to reduce cold bridging, followed by an external cladding material, such as timber weatherboarding.

ABOVE: This house in London has been insulated externally with an expanded polystyrene wall insulation system from Sto. These synthetic (and therefore crack-free) systems are either fixed adhesively or mechanically (by rail). Prices for the StoTherm EWI system start at around £75/m²

How it works

For owners of homes where external walls can be re-rendered, a rigid phenolic insulation core (a) can be attached onto a bedding compound (b). A mesh is then installed, allowing a new render finish (c)

Solutions for Insulating Walls:

• Insulating Walls
• Internal Wall Insulation
• Insualting Walls: Traditional Construction