Internal Wall Insulation Guide

When it comes to insulating walls, we tend to think of cavity-fill insulation, but this is not a solution for solid-walled homes (i.e. solid stone or brick properties typically built before the 20th century).

For solid-walled homes, we can either insulate 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 – and the later may encroach on internal floor space.

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.

Ultimately, internal insulation will change the nature of the building and it needs to be thought through properly.

What is Internal Wall Insulation?

Internal wall insulation involves the application of insulation to the interior face of external walls in order to improve the thermal performance of the property.

There are four main methods of installation — the most common (but not always the most effective) is building a new stud wall, to which insulation can be added.

Internal wall insulation can, however, be disruptive and require the removal and re-fixing of items such as switches, radiators and kitchen units, so you need to be sure it is the best insulation solution for your home.

Creating an airtight layer is vital, so awkward areas, such as reveals and floor voids, require particular care.

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Achieving U Values and Airtightness with Internal Wall Insulation

The U value of a material is the rate at which heat (in watts) is lost through each square meter of the surface. A lower figure means better thermal performance.

  • 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 1.9W/m²
  • 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. Achieving that U value for solid walls will mean installing at least 100mm of rigid insulation (Celotex, Kingspan or similar)

It has 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 — but 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.

Impact on the Dew Point – and Condensation – When Insulating Walls

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.

How to Prevent Damp

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. Overcoming this typically means extending the insulation to cover that cold bridge.

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 walls to be insulated — plug sockets, light switches, curtain rails, radiators, pipes, skirtings, covings, kitchen cabinets, fitted wardrobes, etc
  • Carry out any preparation work to the wall (i.e. knocking off old plaster if damaged)
  • Build the new stud wall (if required) and/or fix insulation
  • Seal joints and skim plasterboard to finish
  • Reinstate light switches, plug sockets, etc
  • Insulating the wall directly

    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.

    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 pros and cons are:

    • These are expensive products, but this is offset to some extent by the speed of installation.
    • 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.

    Fix the insulation with battens

    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 the warm batten 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.

    Insulating with a stud wall

    Option three involves constructing a new stud wall, generally 100mm thick inside the existing wall, with a 40mm cavity between the two.

    This option takes up more floor space than the other options. It is also more expensive and no more effective, except in the case of very damp walls (see below).

    The cavity between the new stud wall and the existing wall must be ventilated to the exterior so any moisture can be carried away, but this then impacts on airtightness.

    Installing a breathable insulation

    There is a fourth internal wall insulation option: applying an insulating plaster directly to the wall.

    This is most useful on stone walls, where breathability is important. This would involve either a hemp or cork-lime mix (the hemp or cork providing the insulation) or layers of lime plaster sandwiching a cork or woodfibre board.

    This option will not get the desired U-value – typically around 0.5W/m2 is the best you will get – but it has a few distinct advantages:

    • It significantly improves airtightness by sealing all the cracks and gaps.
    • It provides a warm internal surface.
    • And most importantly, being breathable, it prevents and damp patches.

    How to Prepare a Wall for Internal Insulation

    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 old plaster needs hacking off. It will also determine:

    • if insulation can be fixed to the wall with adhesive;
    • whether mechanical fixing will be needed;
    • 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.

    How Much Does Internal Wall Insulation Cost?

    The generally accepted cost for internal wall insulation is between £40 and £50/m² — but it can reach over £100/m2 when a new stud wall is built.

    The cost of internal wall insulation will vary according to the type of insulation specified and the condition of the existing wall. In any event, up to 60 per cent of the cost will be labour. The implication being that skimping on the thickness of insulation as a means of reducing capital cost is a false economy; the labour cost will remain largely the same whatever thickness of insulation is installed.

    How to Choose the Right Type of Insulation

    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.

    • Rigid foam boards
      Rigid foam boards of insulation (such as Kingspan or Celotex) are better insulators than the alternatives and so they are thinner and take up less floor area. They can also incorporate a vapour barrier. But they are more expensive and might not tick breathability boxes on your checklist.
    • Mineral wools
      Mineral wools such as Rockwool or Knauf are widely used. These tend to be available as semi-rigid batts or quilts.
    • Natural materials
      Sheep’s wool, woodfibre insulation or cork (try Ty Mawr) are good options for people looking for good levels of breathability and those interested in the eco-credentials of a product. Natural materials do not ‘gas-off’ (release toxins) either.
    • Thin insulation
      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 here and there is a selection available — from paints such as Therma-Coat Acrylic Insulating Primer to aerogels like Spacetherm.

    Is Internal Wall Insulation Right for Your Home?

    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 wall insulation, which possibly makes it worthwhile overcoming the problems it presents.

    Pros:

    • It is cheaper than external wall insulation (up to 50 per cent less).
    • It might be the only option if you can not insulate externally (if you are in a conservation area for example).
    • There are options where breathability is a concern.

    Cons:

    • It is less effective than external wall insulation.
    • It is more likely to cause damp problems (so needs careful planning).
    • You will lose internal floor area.
    • It is a more disruptive process (you may need to move out of the rooms being worked on).

    Internal Insulation: A Case Study

    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.

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