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The benefits and issues of insulating the outside of your external walls

A wall does many things, only one of which is to hold the roof up. Since the 1970s the wall’s role in keeping the heat in has become increasingly important. Although External Solid Wall Insulation has been available since the 1950s it has only really become very popular within the last 10 years since Local Authorities and Housing Associations began installing EWI on their properties.

But no form of wall insulation, including external wall insulation, is without its challenges — not least of which is dealing with moisture.

External solid wall insulation being installed

Moisture Penetration

The potential exists 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, the housebuilder generally had a choice to build either an impermeable wall that stops 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 in the house to remove the moist air before it gets to the walls.

Dew Point

The dew point is the point where air meets a temperature that causes the moisture to condense out of the air as water. The temperature will change through a wall as it moves from the outside ambient temperature to the internal temperature.

Ideally the dew point will occur either on the external surface of the wall, where moisture can evaporate away, or in the ventilated cavity (if the wall has a cavity), where the same thing happens. In most cases, it is slightly inside the external surface.

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.

There is, however, a danger that the dew point will occur between the insulation and the wall, or actually in the insulation. Most external insulation systems deal with this challenge through the inclusion of a vapour barrier between the wall and the insulation, but it is worth checking with your manufacturer/supplier.

Solid Stone Walls

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 makes good sense.

Solid Brick Walls

The quality and permeability of bricks varies widely. Spalling – where the surface of a brick flakes off – is a fairly common sight and an indication of freeze-thaw, whereby moisture penetrates the brick, freezes, and in turn leads to this flaking.

As with stone, adding external insulation will have little impact on the performance of the wall (except its thermal performance, of course). In this case a brick wall is not a breathable wall and therefore any of the rigid foam insulations will suit. As with stone walls, the insulation can be mechanically fixed to the wall and clad with render, timber, etc.

Cavity Walls

In cavity wall construction, the cavity will (almost certainly) be ventilated — it is how it does its job as a cavity. But that does mean that heat from the house penetrating the internal skin to the cavity will be exhausted into the atmosphere by that ventilation. That then makes any external insulation almost useless as most of the heat has been lost before it gets to the insulation.

If cavity-fill insulation fails (and there are plenty of stories of it failing) it is because the insulation allows rainwater to penetrate across the cavity. In this case, external insulation with a weatherproof render will prevent the rainwater entering the wall and subsequently makes cavity-fill insulation a useful thermal barrier.

Mineral wool External solid wall insulation being installed

U Values

A 225mm solid brick wall will have a U value of around 1.20W/m². A 450mm stone wall will be virtually the same, and a brick cavity wall about 1.50W/m². The requirement under Building Regulations is to reduce that to no more than 0.30W/m². That means:

Cavity Wall - 50mm injected foam cavity-fill plus 20mm PUR external gives 0.28W/m².

Solid Wall - 100mm EPS gives 0.31W/m²;

- 70mm rigid foam gives 0.30W/m²;

- 110mm mineral wool, wood fibre, hemp batts gives 0.30W/m².

Is External Wall Insulation an Option Worth Pursuing?

So, in summary, there are a number of issues to be addressed prior to installing external wall insulation, but the benefits are numerous, including:

  • Reducing heat loss and energy bills;

  • Reducing draughts and increasing the sense of comfort;

  • It does not disrupt the house while being installed;

  • It does not reduce internal floor area;

  • It allows walls to contribute to thermal mass (the ‘tea cosy’ effect);

  • It improves weatherproofing and sound resistance;

  • It increases the life of the wall;

  • It reduces condensation on internal walls.

External wall insulation is expensive but effective. But even with the higher cost, the advantages external insulation offers over internal mean that it is difficult to disregard. For more information on External Wall Insulation visit our FAQ page.

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