Internal Wall Insulation

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Internal Wall Insulation

Internal wall insulation is one of the most adopted means of cutting back annual heating bills. 

While this insulation comes with a load of benefits, there’re certain things to know before launching into the project. 

Most walls built before the 20th century are solid walls. In these walls, cavity wall insulation is not an option, and choosing between external and internal wall insulation becomes the only available options.

While Energy Saving Trust says both options can help save up to £455 on annual heating bills, most homes go for internal wall insulation. 

It’s cheaper and easier than external wall insulation. However, internal insulation comes with its challenges, including the hassle of removing all items like radiators from the interior surface of the external walls. 

Internal wall insulation also comes with the challenge of reduced internal floor space.

It’s also important to know that internal wall insulation will alter the building’s nature, and therefore requires that its planned properly before embarking on the journey.

This article sums up all you need to know about internal wall insulation. 

What is Internal Wall Insulation?

Like other insulation systems, internal wall insulation is aimed at improving the thermal performance of the building. 

This is done by applying insulation to the interior face of the external walls. 

This can be achieved with four methods, and the most common is building a stud wall from scratch and adding insulation to it. However, this is not the most effective method of internal insulation.

It’s important to plan this type of insulation from scratch to know if it’s the right insulation for the property. 

During the planning phase, it’s important to consider removing items like radiators, switches, and kitchen units. 

It’s best to consider removing these units and know if other types of insulation would be a better option. 

Internal wall insulation also requires creating an airtight layer, necessitating particular care for reveals and other awkward areas. 

How Much Does Internal Wall Insulation Cost?

The cost of internal wall insulation depends on the type of method used. 

While an average cost of internal wall insulation floats between £40 and £50/m², the cost can double to £100/m² if building a new stud wall is required.

Other factors that determine how much internal wall insulation costs include the condition of the wall and the type of materials used. 

It’s worth mentioning that 60% of the total cost of installing internal wall insulation goes to labour cost. 

While it might seem helpful to reduce the labour cost by compromising insulation thickness, it’s worth knowing that the insulation thickness has no effect on the cost of labour.

Building Regulations for Internal Wall Insulation

The Building Regulations require that certain requirements are meant with the installation. 

One common requirement under this law is the specification of the U value of the materials used.

The law frowns at high U value, as this means low thermal performance. 

U value is the rate at which an insulating material loses heat (in watts) through each square meter of the surface.

While the average un-insulated cavity wall has a U value of around 1.5W/m², a solid 225mm brick wall has this value at around 1.9W/m², and depending on the thickness, a solid stone wall may have its U value between 1.7W/m² and 1.4W/m². 

Under the current Building Regulations, a wall is required to have not more than a U value of 0.3W/m². 

This requires installing at least 100mm of rigid insulation, such as Celotex or Kingspan, for solid walls. 

It’s said that improving airtightness says more on heat loss than it does on insulation. 

While internal wall insulation naturally increases the system’s airtight, it leaves cracks, gaps, and penetrations behind, which seems to be the main concern.

This usually occurs in certain areas like the ceiling voids, below the ground and the accessible areas of the wall. 

Though an airtight barrier is achieved from the insulation applied to these areas, the need to deal with the gaps and the likes arises. 

Failure to deal with the cracks, gaps, and penetrations can reduce the benefits of the insulation by up to 50%.

Avoiding Damp with Internal Wall Insulation

Internal wall insulation tends to increase the risk of patches forming on the plasterboard. 

This is because the installation keeps the dew point towards the internal surface when keeping the wall at external ambient temperature, causing the insulation to absorb the moist air. 

The dew point of a wall is the point where air meets with the required temperature that condenses it into water. 

One way to prevent damp penetration with internal wall insulation is to install a vapour control layer. 

This causes the insulated wall’s internal surface to be warmer, limiting the risk of condensation on the wall. 

However, this comes with exceptions. Certain areas, such as where the external and internal walls meet, remain cold, making them vulnerable to condensation. 

How to Install Internal Wall Insulation

There’re four main methods of internal wall insulation, and three of these methods largely follow the same process. Here’re the steps involved in installing internal wall insulation:

  • Check the condition of the wall
  • The exact place where the dew point occurs varies with the thickness of the insulation and the wall condition. This requires letting a professional installer check where the dew point occurs when the preferred insulation thickness is installed.
  • Depending on your need and requirements, choose between the available methods. 
  • Come up with the best ideas of dealing with floor voids, reveals, and other factors that can cause cold bridges.
  • Detach the light switches, plug sockets, radiators, coving, kitchen cabinets, and other things fixed to the wall.
  • Build a new stud wall if necessary

Method 1: 

One of these methods is installing the insulation directly to the wall. 

Common products used for this method are from Kingspan and Celoted. Here, the insulation is bonded to the plasterboard and acts as an excellent vapour barrier.

This is the easiest and the most effective method for flat walls. If the wall is flat with nothing else to deal with, the board can be fixed to the wall directly. 

The installation is carried out by glueing the board to the wall using purpose-specific adhesive. 

This method allows screwing when required. However, to ensure continuity of the vapour barrier, the gaps between the boards should be dealt with by filling them with mastic. 

This should be done before moving on to plaster skimming.

Though recommended products for this method are expensive, the ease and speed of installation cover-up for a portion of the cost.

However, one challenge often faced with this method is the need for re-fixing heavy items, such as kitchen cabinets. 

While there’re special fixings for this, it’s one common challenge why most homes prefer other methods.

Method 2:

The second method of installing internal wall insulation is by battening on the wall. 

This method also comes with two options. While one involves fixing battens to the wall and later fixing the insulation, the other is where the battens are fixed to the insulation. 

Both methods use 25x50mm battens.

The first option is often used for uneven walls in that, fixing the battens first evens the wall, making it a more suitable option than the other. 

However, the option requires screwing the insulation to the battens, causing perforations in the vapour barrier. 

While only a few people prefer the latter option, it has its advantages. 

This method requires a semi-rigid wool batten placed against the wall and spaced battens placed on top and screwed to the wall. 

Semi-rigid or rigid insulation is fixed between the battens, after which the plasterboard is installed. 

The major advantage that stands this method out is that it keeps the battens warm, reducing the likelihood of rotting. 

Another advantage is that the method gives room to access the battens directly below the plasterboard, making it possible to hang pictures – with ease. 

When heavier items are to be fixed later on, more battens can be installed to the system.

Method 3:

The third option is where a new stud wall is constructed. With 100mm thickness, the stud wall is built inside the existing wall, and a 40mm cavity is installed between them. 

This is the least common option as it’s the most expensive and the least effective. However, it’s an ideal option for damp walls.

It’s worth mentioning that there must be provision for ventilation between the new stud wall and the existing wall to the exterior. 

This is necessary to ensure moisture is carried away, which is more impacts on airtightness.

Method 4: 

The fourth option of installing internal wall insulation is where an insulating plaster is directly applied to the wall.  The method is most effective on stone walls.

Though the option doesn’t deliver the required U value as it always has around 0.5W/m2, it comes with a few advantages:

  • It seals all the gaps and cracks, improving airtightness.
  • It offers a warm internal surface.
  • The risk of patches in the method is significantly reduced compared to other methods, as it’s a breathable system. 

Preparing a Wall for Internal Insulation

The condition is a major factor that determines the level of preparation needed. 

If the wall is not in good condition, such as when the old plaster needs being hacked off, this should be addressed first. 

Among many others, the condition of the wall also determines if:

  • It’s right to fix the insulation to the wall with adhesive.
  • Mechanical fixing is needed.
  • Battening is needed to achieve a flat surface.

Insulation often has more negative impacts on damp walls at it reduces air movement across the internal surface of the wall and its temperature. 

While this can be dealt with eradicating the cause of the damp, another way is by building a stud wall having a cavity between the insulation and the existing wall.

If finding and fixing the cause of the damp is the preferred option, installing a damp-proof is not the way to go where the damp is on the rise. 

Installing internal insulation on a wall where the damp is caused by rainwater penetrating through the wall is not ideal. 

Common causes of a damp wall include downpipe or overflow, leaky gutter, all of which are easy to fix. 

Another cause of damp is when the external ground level is higher than the interior wall levels.

Choosing the Right Type of Insulation

Knowing the right type of insulation for your home depends on the application and your wall condition. 

Also, doing a bit of research is handy when deciding which type of insulation to go for. Here’re the common insulation materials:

Mineral wools: these are the most used type of interior wall insulation. These materials come in semi-rigid battens and quilts. 

Rigid foam boards: these insulation materials are a great choice going by their favourable thickness. Since they’re thin, they take less floor area than other options, and most. 

They are also a great choice for their flexibility, making them more suitable to incorporate into a vapour barrier. 

However, they’re more expensive and don’t have a good level of breathability. 

Natural materials Sheep’s wool: this and wood fibre insulation are a great choice when breathability is a priority. This option is also eco-friendly as they don’t gas-off.

Thin insulation: though it’s a good material, it requires proper care during installation to promote continuity to reduce the risk of cold bridges. 

This also requires thinner materials, and a load of them are available, including Therma-Coat Acrylic Insulating Primer and Spacetherm.

Is Internal Wall Insulation Right for Your Home?

Before embarking on installing internal wall insulation, the first question to ask is if the insulation is right for your home. 

While internal wall insulation is cheaper than its exterior counterpart, it comes with its certain disadvantages. 

Generally, interior wall insulation is not recommended for homes occupied already, as it causes disruptiveness. 

However, it’s worth noting that installing interior wall insulation is far better than having nothing installed, and the benefits it offers are a great payback for its initial problems.

Here are the pros and cons of interior wall insulation:


  • It usually comes at half the cost of external wall insulation
  • It’s a great option where external wall insulation is not a great deal
  • It offers options with high breathability levels


  • It’s not as effective as external wall insulation
  • Damp problems are always a major challenge
  • It reduces internal floor area
  • It’s more disruptive