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External Wall Insulation

The role of external wall insulation is to increase the thermal efficiency of houses built with solid walls. However, installing it is not always straightforward. Our expert guide explains everything to consider before installation.

External wall insulation refers to a layer of insulation that's fixed to the outside of an existing wall. This layer will be finished either with a coat of render or with an alternative cladding material to protect it from the elements.

This is a common method of insulating walls used by those with solid walls, where cavity wall insulation is impossible. That said there are some other situations where external wall insulation make sense.

What is External Wall Insulation?

External wall insulation involves adding a layer of insulation to the outside of a building’s walls, often covered with render or cladding to protect against weather.

his is a common method of insulating walls used by those with solid walls, where cavity wall insulation is impossible. That said there are some other situations where external wall insulation make sense.

Upgrading and adding insulation should be something anyone who wants to improve the thermal efficiency of their home should consider, but it still pays to be aware that no form of wall insulation is without some form of challenge — often centred around how your home deals with moisture.

Advantages of External Wall Insulation

Will external wall insulation cause damp in my home?

There are several causes of damp within the home. There is potential for moisture to enter a wall from both sides: rain on the external skin and moisture from people and the activities they undertake (cooking, drying clothes, washing, breathing, etc.) on the internal skin.

Prior to the widespread introduction of the cavity wall, there was generally a choice to build either an impermeable wall to stop moisture penetration to both surfaces, or to build a breathing wall that allows moisture to penetrate (to a degree) and be evaporated away. A solid brick wall will tend to be the former and a stone wall will tend to be the latter.

The cavity wall by comparison is an impermeable wall in that the cavity is intended to form a ‘barrier’ to prevent moisture penetration; any rainwater entering the wall is evaporated away by air movement in the cavity.

Many modern insulation materials are usually non-permeable and will be rendered or clad to prevent rainwater penetration. However, they do not stop moisture reaching the internal surface of the wall from people and what they do — and that can mean that more ventilation is needed to remove the moist air before it gets to the walls.

Getting the detailing right is key to avoiding damp problems," advises Ian. "For example window sills need to project sufficiently so they’re clear of the thicker insulated wall surfaces, otherwise the sills will need to be extended or the windows replaced, adding to cost.

Another risk is where the walls being lined are poorly maintained and are suffering from existing damp issues, in which case cladding them externally will only exacerbate the problem. Accurate detailing at junctions is critical, particularly around window and door reveals to reduce thermal bridging.

"Detailing is also important at DPC level to avoid bridging, and at roof level where the rafters may need to be extended to accommodate the thicker walls So it’s essential that the work is carried out to a high standard. Approved EWI installers are registered with INCA (Insulated Render and Cladding Association)."

Adding insulation to a wall will change the place where the dew point occurs. The effect of external insulation is to warm the wall and this in turn moves the dew point outwards, towards the colder external air, thereby reducing the risk of condensation appearing on the internal surface.

Is planning permission required to add external wall insulation?

Whatever else external wall insulation does, it will change the external appearance of the house. In most cases that will mean gaining planning permission before undertaking the work, so do check with your local authority from the outset. For homes in Conservation Areas and for listed buildings it is quite possible that consent will not be forthcoming..

Can you add external wall insulation to solid stone walls?

Can external wall insulation cause dampness?

Solid stone walls tend not to be ‘solid’ at all. They usually have a rubble-filled ‘cavity’ between two stone skins. The natural dew point will be between that ‘cavity’ and the external surface, where any moisture can either fall out of the wall to the ground or evaporate away. External insulation has no great impact on this. The dew point will move a little further outwards and any internal moisture penetrating the wall can still be dealt with within the wall.

However, stone walls tend to be breathing walls and maintaining this breathability ensures that the wall continues to operate as designed. In this case, using a breathable insulation – natural insulation like wood fibre, cork or the like works well – with a lime external render makes good sense.

One ideal solution would be a 90mm layer of wood fibre board – Diffutherm, Pavatex or similar – mechanically fixed to the wall. These materials are ready to accept render. A 20mm thickness of hemp-lime render would be applied in two 10mm coats. The hemp-lime can be coloured to suit or painted with a lime wash.

An alternative would be a 70mm rigid foam insulation (Kingspan, Celotex or similar) again mechanically fixed to the wall and then clad with a sand and cement render, timber cladding or whatever other weatherproof layer is preferred. This option will be considerably cheaper but means the wall is no longer breathable — which may or may not matter. If the wall is in good condition and there is no sign of damp penetration, and the interior is well ventilated, then turning it into a non-breathable wall will have little impact.

What U-value does it provide?

Insulation requirements usually bring the U-value down to 0.30 W/m² for optimal thermal efficiency.