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. And, following numerous grants rolled out for cavity insulation, 2014 saw the Government offer grants towards both internal and external wall insulation in the form of the Green Deal.
But no form of wall insulation, including external wall insulation, is without its challenges — not least of which is dealing with moisture.
(MORE: What is the Green Homes Grant?)
The Issue of Moisture Penetration
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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.
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 – wood fibre, cork or the like – 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.
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.
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.
Splitting the necessary thickness of insulation between cavity-fill and external is a sensible idea. The cavity will typically be 50mm wide; add 20mm external insulation, in the same way as for solid walls, and the wall will achieve a good U value.
Reveals, Cills and Eaves
External insulation will add thickness to the wall, most evident at the reveals and eaves. Eaves width can be a deal-breaker in that if the eaves are not wide enough to accept the insulation, the cost of extending the eaves can outweigh the benefit of the insulation. Whether cills fall into the same category will depend on the width and type of cill, and the cost involved in moving or extending it.
In addition, it is usually impractical to return the insulation into window and door reveals, as there is seldom sufficient door or window frame width to comfortably accommodate the external insulation. Not insulating the reveal will leave a significant cold bridge, negating a good proportion of the value of the insulation. There are thin insulation options, like Spacetherm aerogel from the Proctor Group, that, at 10mm thick, can be used on a reveal to help overcome this challenge.
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:
- 50mm injected foam cavity-fill plus 20mm PUR external gives 0.28W/m²
- 100mm EPS gives 0.31W/m²
- 70mm rigid foam gives 0.30W/m²
- 110mm mineral wool, wood fibre, hemp batts gives 0.30W/m²
External Wall Insulation Costs
The cost will be higher than internal wall insulation. A three-bedroom semi is likely to cost £5,000-9,000, with a larger detached home in the region of £8,000-15,000. There are proprietary systems that require specialist installation, but there are also materials from builders’ merchants that are a lot cheaper.
Whichever is used, there will be a cost involved in erecting scaffolding and removing/replacing all of the pipes and cables that are fixed to the wall that cannot be avoided.
Is Planning Required for 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 consent 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.
Under Building Regulations, if 25 per cent or more of a wall is to be insulated externally, it is typically necessary to bring the entire wall up to current standards — which makes sense in that, if you’re going to go to the efforts and expense of insulating externally, then you may as well do it well. The thermal performance of the insulated wall must have a U value of no more than 0.30.
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. Adding it as part of a larger project will reduce the cost, plus it does not need to be done to the whole house; an attractive front elevation may be more suitable to internal insulation, while less attractive side and rear elevations can be insulated externally. But even with the higher cost, the advantages external insulation offers over internal mean that it is difficult to disregard.
Tim is an expert in sustainable building methods and energy efficiency in residential homes.
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