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Internal Wall Insulation Cost Guide

In this article, we’ll be looking at the internal wall insulation cost along with its pros and cons and tips on how to make the most of this type of insulation without creating problems for the home.

Internal wall insulation can be a cost-effective way to improve the thermal performance of certain homes. Typically, solid wall homes made from solid stone or brick that were built before the 20th century are the main candidates for this type of insulation.

For houses built after this time, there’s often a cavity wall as part of the home’s construction, offering the opportunity to use cavity wall insulation instead.

While internal wall insulation tends to be cheaper than external wall insulation, it can eat into valuable floor space, making it worth careful consideration.

Additionally, it changes the fabric of the home and can cause issues such as damp and condensation if not employed properly.

internal wall insulation cost

What is Internal Wall Insulation?

Internal wall insulation is the process of insulating the interior face of external walls to enhance the thermal performance of a property. The most common method of installation is building a new stud wall, to which insulation can be added.

However, this method is not always the most effective. It can also be disruptive and require the removal and re-fixing of items such as switches, radiators, and kitchen units.

Therefore, it is important to consider if internal wall insulation is the best solution for the specific home. It is crucial to create an airtight layer, so particular care must be taken in tricky areas such as reveals and floor voids.

Internal wall insulation is an effective way to reduce heat loss and energy bills, but it is important to consider the potential disruption and costs before deciding to install it.

How Much Does Internal Wall Insulation Cost?

Internal wall insulation is a cost-effective way to improve energy efficiency and reduce heating bills. The cost of internal wall insulation will vary depending on the type of insulation specified and the condition of the existing wall.

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. Internal wall insulation is much cheaper than external wall insulation, in some cases less than half the price.

Up to 60% of the cost of internal wall insulation will be labour, so skimping on the thickness of insulation is not recommended as it will not significantly reduce labour costs.

Qualifying households can benefit from grants available for internal wall insulation for solid walls under the government’s ECO scheme. It is important to note that the cost of internal wall insulation can vary depending on the insulation type, wall thickness, and labour costs.

Does Internal Wall Insulation Cause Damp?

Internal wall insulation can cause damp if it is not installed correctly or if the existing wall structure is not suitable for insulation. The insulation changes the fabric of the home, and if not employed properly, can cause issues such as damp and condensation.

The dew point is the point where air meets a temperature that causes the moisture to condense out as water. Internal wall 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.

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.

Is it Worth Insulating Internal Walls?

Insulating internal walls can be a cost-effective way to improve the energy efficiency of a home. While it is cheaper than external wall insulation, it is less effective and more disruptive. However, it might be the only option if external insulation is not possible, such as in a conservation area.

There are options available for those who are concerned about breathability. It is important to plan carefully to avoid any potential damp problems. It should be noted that insulating internal walls will result in a loss of internal floor area. The process is also more disruptive, and occupants may need to move out of the rooms being worked on.

Overall, insulating internal walls can be a worthwhile investment, especially if it is the only option available. It has a better payback than external wall insulation, making it a cost-effective solution. However, careful planning is required to avoid any potential problems.

Pros:

  • Cheaper than external wall insulation (up to 50% less)
  • Might be the only option if external insulation is not possible
  • Options available for breathability concerns

Cons:

  • Less effective than external wall insulation
  • More likely to cause damp problems
  • Results in a loss of internal floor area
  • More disruptive process

What Building Regulations Apply for Internal Wall Insulation?

When it comes to internal wall insulation, certain Building Regulations must be followed to achieve compliance. The regulations stipulate requirements for airtightness and heat loss that must be met to ensure the home is compliant.

The U value of a material is the rate at which heat (in watts) is lost through each square meter of the surface. The lower the U value, the better the thermal performance of the material. For example, an uninsulated cavity wall will have a U value of around 1.5W/m2, while a solid 225mm brick wall will be around 1.9W/m2, and a solid stone wall will be around 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 this U value for solid walls will mean installing at least 100mm of rigid insulation, such as Celotex, Kingspan, or similar. Kingspan’s solid board internal wall insulation is an insulated plasterboard with a thermal conductivity of 0.018 W/mK.

It is important to note that improving airtightness has a greater impact on heat loss than insulation. The solid elements of the wall will naturally be fairly airtight, but gaps, cracks, and penetrations can be problematic.

These gaps tend to occur in awkward places, such as floor/ceiling voids, below the ground floor, and the first-floor ceiling, as well 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% if the gaps, cracks, and penetrations are not also dealt with.

To comply with Building Regulations for internal wall insulation, the U value of the material used must be no more than 0.3W/m2, ideally 0.2W/m2. Achieving this requires the installation of at least 100mm of rigid insulation. It is also important to address gaps, cracks, and penetrations to improve airtightness and reduce heat loss.

What Kind of Insulation Should I use for Interior Walls?

The type of insulation suitable for interior walls depends on the application. Rigid foam boards are the best insulators and take up less floor area, but they are more expensive and might not be breathable. Mineral wools are widely used and are available as semi-rigid batts or quilts.

Natural materials like sheep’s wool, woodfibre insulation, or cork are good options for people looking for eco-friendly products and good levels of breathability. These materials do not release toxins and are a good choice for those interested in the eco-credentials of a product.

Thin insulation is particularly suitable for floor voids, reveals, and returns to ensure continuity in the insulation and eliminate cold bridges.

Thinner materials are needed here, and there is a selection available that includes paints such as Therma-Coat Acrylic Insulating Primer and aerogels like Spacetherm.

It is important to do a bit of research to find the best solution for your home. Keep in mind that the most suitable insulation will vary with the application.

How to Prepare a Wall for Internal Insulation

Before installing internal insulation, it is important to address the condition of the wall’s surface and whether the wall is damp.

The surface condition will determine what preparation work is needed, such as hacking off old plaster. It will also determine if insulation can be fixed to the wall with adhesive, whether mechanical fixing will be needed, or if battening is necessary to give a flat surface.

It is important to note that 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. If the wall is damp, there are only two ways of dealing with it.

One way is to create a stud wall with a cavity between the insulation and the existing wall. The other way is to find the cause of the damp and eradicate it.

If the cause of the damp is rain penetration through the wall itself, then internal insulation would be a mistake. In this case, it might be a leaky gutter, downpipe, or overflow that is causing the problem, which can be easily fixed.

Alternatively, it could be that the external ground levels have been built up above those of the interior wall levels.

Rigid foam boards offer the best insulation compared to the alternatives. However, it is important to ensure that the wall is not damp and that the surface is properly prepared before installing insulation.

How to Install Internal Wall Insulation

There are several methods of installing internal wall insulation, but the process is broadly the same for all of them. Here are the steps to follow:

  1. Check the condition of the wall and undertake remedial work.
  2. Ask the preferred insulation manufacturer to check where the dew point will occur with the preferred thickness of insulation.
  3. Decide on which method will be best.
  4. Decide how to deal with reveals, floor voids, and other potential cold bridges.
  5. Remove everything fixed to the walls to be insulated, such as plug sockets, light switches, curtain rails, radiators, pipes, skirtings, covings, kitchen cabinets, fitted wardrobes, etc.
  6. Carry out any preparation work to the wall, such as knocking off old plaster if damaged.
  7. Build the new stud wall (if required) and/or fix insulation.
  8. Seal joints and skim plasterboard to finish.
  9. Reinstate light switches, plug sockets, etc.

Fix Insulation Directly to the Wall

One option for installation is to fix insulation directly to the wall. Kingspan and Celotex offer products specifically designed for this method, with insulation bonded to plasterboard and with a vapor 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 vapor barrier.

Ensuring a continuous, unperforated vapor barrier is the only effective way of dealing with a dew point that occurs in the wall.

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.

Battening the Wall

The second option is to batten on the wall. There are two ways of doing this:

  1. Fixing battens to the wall to provide a more even fixing for the insulation.
  2. 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 vapor barrier.

The warm batten method is less common but 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 are 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.

Construct a New 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. 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.

Apply Insulating Plaster

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 wood fiberboard.

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 any damp patches.

How Much Floor Space Will Be Used With Internal Wall Insulation?

When installing internal wall insulation, expect to lose at least 100mm on each wall. The thickness of insulation varies from brand to brand, but it can range from 60mm to 100mm.

Generally, the more expensive the insulation, the thinner its profile. It is important to factor in this loss of floor space when planning your insulation project.

Interested in learning more about Internal Wall Insulation?

If you want to learn more about internal wall insulation or would like one done for your property, get in touch with us for a free quotation.

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