The Quick Read
- Internal wall insulation costs between £40/m² and £50/m² — around half the cost of external wall insulation
- There are three main methods of installation — the most common is building a new stud wall. Internal wall insulation can, however, be disruptive and require the removal and refixing of items such as switches, radiators and kitchen units
- Creating an airtight layer is vital, so awkward areas, such as reveals and floor voids, require particular care
Shop insulation and damp proofing for your project
While cavity wall insulation is first and foremost in the minds of most when it comes to insulating walls, it’s obviously not a solution for old solid-walled homes, which do not feature cavities. Options instead include applying insulation externally to the façade of the building, or internally — both could save up to £455 on annual heating bills, according to the Energy Saving Trust. The former will inevitably change the look of the building — not a bad thing in some cases.
Internal wall insulation seems, by comparison, an easier, cheaper and effective option. But there can be issues, not least having to remove all items such as radiators from the interior face of external walls. Internal insulation will change the nature of the building and it needs to be thought through properly.
Achieving U Values and Airtightness
An uninsulated cavity wall will have a U value of around 1.5W/m² and there will also be a relatively high level of cold bridging due to the cavity ties. A solid 225mm brick wall will be around 1.4W/m², and a solid stone wall 1.7W/m² to 1.4W/m² (depending on the thickness). Current Building Regulations require a maximum U value of 0.3W/m² and realistically 0.2W/m². However, achieving that U value for solid walls will mean losing at least 100mm of internal floor space.
It has also been well established that improving airtightness has a greater impact on heat loss than insulation. The solid elements of the wall will naturally be fairly airtight — it is the gaps, cracks and penetrations that can be problematic. These tend to occur as much in awkward places – floor/ceiling voids, below the ground floor, the first floor ceiling – as on the accessible areas of the wall. The insulation applied to the wall can form the airtight barrier, but the benefit will be reduced by up to 50 per cent if the gaps, cracks and penetrations are not also dealt with.
The Impact on the Dew Point of a Wall, and Condensation
The dew point is the point where air meets a temperature that causes the moisture to condense out as water. Insulating the external surface of the wall allows the temperature of the wall to increase, which will push the dew point towards the external surface. Conversely, internal insulation will tend to keep the wall at external ambient temperature and thereby draw the dew point towards the internal surface. If the dew point is too close to the internal surface of the existing wall, moisture can be absorbed by the insulation and appear as damp patches on the plasterboard. Thus, to help prevent damp penetration, a vapour control layer will need to be installed.
The internal surface of an insulated wall will tend to be warmer, reducing the likelihood of condensation forming, but there will be areas – such as where an external wall meets an internal wall – that remain cold. There is a distinct risk of condensation forming in those areas, typically in high-level corners. This is essentially a cold bridge. To overcome this, we need to be sure that it is not possible to draw a (imagined) straight line from inside to outside without passing through a full thickness of insulation.
The Wall’s Condition Can Impact, Too
The condition of the wall’s surface and whether the wall is damp will need to be addressed before installation. The surface condition will determine what preparation work is needed — principally if the plaster needs hacking off. It will also determine if insulation can be fixed to the wall with adhesive, whether mechanical fixing will be needed and if battening is necessary to give a flat surface.
Insulation can make a damp wall worse by reducing the temperature of the wall and by reducing (or eliminating) air movement across its internal surface. There are only two ways of dealing with a damp wall: creating a stud wall with a cavity between the insulation and the existing wall, or finding the cause of the damp and eradicating it. When it comes to the latter, if you believe that rising damp is a myth, then options other than installing a damp-proof course need to be explored. Damp could be caused by rain penetration through the wall itself — in which case internal insulation would be a mistake. It might be a leaky gutter, downpipe or overflow, which is easy to fix. It could be that the external ground levels have been built up above those of the interior wall levels.
Choosing Between the Different Types of Insulation
The options for internal wall insulation tend to include rigid foam boards (Kingspan or Celotex), mineral wools (Rockwool or Knauf) or any of the natural materials, such as sheep’s wool, woodfibre or cork (try Ty Mawr). The rigid foams are better insulators than the alternatives, therefore taking up less floor area, and can also incorporate a vapour barrier. But they are more expensive and might not tick breathability boxes on your checklist.
It is the floor voids, reveals and returns that need particular attention to ensure continuity in the insulation and eliminate cold bridges. Generally, thinner materials are needed and there is a selection available — from paints such as Therma-Coat Acrylic Insulating Primer to aerogels like Spacetherm. What is most suitable will vary with the application, and a bit of research will be needed to find the best solution for your home.
How to Install Internal Wall Insulation
There are three basic methods of installing internal wall insulation, and the process is broadly the same for all three:
- Check the condition of the wall and undertake remedial work;
- Ask the preferred insulation manufacturer to check where the dew point will occur with the preferred thickness of insulation;
- Decide on which of the three methods will be best (as below);
- Decide how to deal with reveals, floor voids and other potential cold bridges;
- Remove everything fixed to the wall — plug sockets, light switches, curtain tracks, radiators, pipes, skirtings, covings, kitchen cabinets, fitted wardrobes, etc;
- Carry out any preparation work to the wall;
- Build the new stud wall (if required) and/or fix insulation;
- Seal joints and skim plasterboard to finish;
- Reinstate light switches, plug sockets, etc.
The first option for installation is whereby the insulation is fixed directly to the wall. Kingspan and Celotex offer products specifically designed for this method, with insulation bonded to plasterboard and with a vapour barrier. If the wall is relatively flat and in good condition, this can be an effective, quick method. These are expensive products, but this is offset to some extent by the speed of installation.
Boards can be glued directly to the wall with an adhesive specific for the purpose. Mechanical fixings (screws) can also be used, if necessary. Gaps between boards, at the ceiling and floor edges, should be filled with mastic and taped over before plaster skimming to ensure continuity of the vapour barrier. Ensuring a continuous, unperforated vapour barrier is the only effective way of dealing with a dew point that occurs in the wall.
The problem with this method is refixing heavy items such as kitchen cabinets, and hanging pictures, mirrors, etc. Special fixings are available for this but, over time, it can become a headache.
The second option is to batten on the wall. There are two ways of doing this: fixing battens to the wall to provide a more even fixing for the insulation, or fixing the battens over the insulation, known as the ‘warm batten’ method. Both would use 25x50mm battens. The first method is the more common and, when the wall is very uneven, can be the best option. However, the insulation will be rigid and is screwed to the battens, inevitably leading to perforations in the vapour barrier.
The warm batten method is less common but it has some distinct advantages. In this method, a semi-rigid wool batt is placed against the wall. Appropriately spaced battens are placed on top and screws driven through the batten, through the insulation and into the wall. Rigid or semi-rigid insulation can then be installed between the battens with plasterboard then installed. The advantages this method offers is that the battens are kept warm by the insulation and so are less likely to rot; the battens are accessible directly below the plasterboard and so pictures can be hung with more ease; and extra battens can be installed to allow heavier items such as kitchen cabinets to be refixed.
Option three – the generally accepted method of installing internal wall insulation – is by constructing a new stud wall, generally 100mm thick inside the existing wall, with a 50mm cavity between the two. This option takes up more floor space than the other options, is more expensive and no more effective, except in the case of very damp walls. For the latter, the cavity between the new stud wall and the existing wall must be ventilated to the atmosphere so any moisture can be carried away.
The Cost of Internal Wall Insulation
The generally accepted cost for internal wall insulation is between £40 and £50/m². The cost will vary with the type of insulation being used and the condition of the existing wall can have a big impact. In any event, up to 60 per cent of the cost will be labour. The implication of that is that skimping on the thickness of insulation as a means of reducing capital cost is a false economy; the labour cost will remain the same whatever thickness of insulation is installed.
So, is it Worth it?
Internal wall insulation presents its own set of problems. It is cheaper than external wall insulation – up to 50 per cent less – but less effective, potentially more problematic and more disruptive, certainly if the house is being occupied while the work is going on. Perhaps the way to look at it is that it is better than doing nothing and it has a better payback than external insulation, which possibly makes it worthwhile overcoming the problems it presents.
This solid-brick-walled Victorian house in Camden, North London, is situated in a Conservation Area, which meant that an external insulation solution was out of the question. As part of a package of insulation measures, Kingspan’s Kooltherm phenolic urethane insulation was specified.
For most walls on the ground floor and above, 102.5mm-thick Kooltherm K18 Insulated Plasterboard was fixed on battens, protected from the wall by a strip of DPC. The U-value of the walls is now 0.19W/m²K. 62.5mm of the same plasterboard was fixed for the basement walls. All plasterboard joints were taped to get the best possible airtightness.
Junctions between the insulation and other materials were sealed with flexible mastic or foam to further improve airtightness. At the edges of floors and walls, the floorboards were cut away and the insulation drawn up to meet the neighbouring layer. Making the insulation continuous in this way prevents a thermal bridge (or heat loss) around the floor junctions.
How it works
For those unable to fix insulation externally, an insulated dry-lining board might be an option, fixed to timber battens.