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Comparing Internal and External Wall Insulation

Insulation is a critical aspect of any building, ensuring energy efficiency and comfortable indoor temperatures. Internal and external wall insulation are two common methods for improving a building’s thermal performance. This blog post will offer a comprehensive comparison of these two insulation types. It will also discuss their benefits and drawbacks.

We would always recommend external wall insulation as the premier option. However, it is important to consider every type of insulation available to you to make an informed choice. This includes cavity wall insulation. Roof and floor insulation can also supplement the major wall insulation types to complete an uninterrupted thermal envelope.

What is Internal Wall Insulation?

Internal wall insulation (IWI) involves adding insulating materials to the interior side of a building’s walls. This method is common in buildings where external wall insulation is not possible. This is either due to planning restrictions, architectural reasons, or the building’s structure. IWI can significantly reduce heat loss and improve energy efficiency by creating a thermal barrier between the interior and exterior environments. However, it may not be as effective as external wall insulation in preventing thermal bridging or providing a continuous insulation layer.

The cost of IWI varies depending on the type of insulation and the size of the area to be insulated. Insulating materials can include mineral wool, expanded polystyrene, or phenolic foam boards. On average, IWI can cost between £40 – £60 per square meter, including labour and materials. The overall cost may be influenced by the complexity of the project, the need for additional work, and labour.

Damp risks

Although effective, IWI can cause damp issues if not properly installed and maintained. One critical aspect to consider when installing IWI is the dew point. Also, consider the incorporation of a vapour control layer to prevent moisture-related problems.

The dew point is the temperature at which air becomes saturated with moisture, causing condensation to form. When insulating a building’s interior walls, the dew point can shift within the wall assembly. If the insulation is not installed correctly, the dew point may be located within the wall itself. This leads to condensation and, subsequently, damp and mould issues.

A vapour control layer is a critical component of an effective IWI system. The VCL helps to manage moisture within the wall assembly. A VCL is a membrane on the warm side of the insulation (facing the interior of the building). It prevents warm, moist air from penetrating the insulation and reaching the cold surfaces of the wall. As a result, it cannot condense and cause damp problems.

  • Address existing damp problems – Before installing IWI, any pre-existing damp or moisture issues should be identified and resolved. This may involve repairing leaks, improving ventilation, or addressing rising damp.
  • VCL position – Ensure that the VCL is installed on the warm side of the insulation and sealed at all joints and penetrations to prevent moist air from reaching the cold surfaces of the wall.
  • Insulating materials – Choose insulating materials that are moisture-resistant and have good hygrothermal performance.
  • Adequate ventilation – Proper ventilation is essential in rooms with high moisture levels, such as bathrooms and kitchens, to minimise the amount of moist air that can come into contact with the insulated walls.
  • Inspect and maintain – Periodic inspections and maintenance can help identify and address any potential damp issues.

Installing IWI

The process of installing IWI is different to installing EWI in that it has various methods. The four common techniques are insulation directly applied to the wall, battering the walls, constructing a stud wall, and applying insulated plaster.

Insulating directly to the wall follows the usual process of external wall insulation. The walls need to be clean, dust free, and relatively flat. Following this, apply adhesive to the insulation boards, following the manufacturer’s recommendations for coverage and pattern. Then press the insulation boards against the wall, ensuring a tight fit with no gaps between the boards. Once the adhesive has cured, apply a moisture-resistant plasterboard or a finishing system (such as a render) to complete the installation.

Constructing a stud wall for IWI involves building a new wall slightly away from the existing wall, creating a cavity for insulation. Stud walls are typically made from timber or metal studs, with insulation fitted between the studs and a finishing layer applied to the frame.

Insulating plaster is a specialised plaster material that serves a dual purpose. It provides both thermal insulation and a finished surface. This type of plaster is formulated with insulating components, such as expanded perlite, aerogel, or other lightweight, thermally-efficient materials, which give the plaster its insulating properties. Insulating plaster can be applied to interior walls. This provides a relatively simple and seamless solution for improving a building’s energy efficiency.


Battens are linear strips made of timber or metal, used to create a frame or support system for insulation and finishing materials. When used for IWI, battens are fixed vertically to the existing wall, creating a gap that accommodates the insulation material. Timber battens are commonly used for IWI, as they are easy to work with and readily available. They should be treated to resist moisture and rot. It is essential to use timber with low moisture content to avoid warping or twisting, which could compromise the installation’s integrity.

Metal battens, such as steel or aluminium, are an alternative to timber battens. They offer several advantages, including resistance to moisture, rot, and pests, and improved dimensional stability. However, they may require specialised tools and skills for installation.

Metal battens

Warm battens are a term used in the context of IWI to describe a method where the battens themselves are insulated, further improving the overall thermal performance of the wall. The idea is to minimise thermal bridging, which occurs when heat passes through the less-insulated parts of the wall, such as the battens, resulting in energy loss and reduced insulation effectiveness.

In a warm batten system, the battens (typically made of timber or metal) are wrapped or covered with an insulating material before being fixed to the wall. This added layer of insulation helps reduce the potential for thermal bridging through the battens themselves.


Ultimately, the choice is yours. However, we firmly believe that the pros of EWI far outweigh those of IWI. Moreover, internal wall insulation has some crucial cons that can pose a serious risk to your home. External wall insulation fits in seamlessly with floor and roof insulation to form a thermal envelope. The risk of thermal bridging is lower, especially when the problem areas are reinforced with beading and verge trims. Secondary membranes around verge trims also prevent water ingress behind external wall insulation.


  • Cheaper than external wall insulation, therefore has a shorter payback period
  • Suitable for conservation areas and listed properties
  • Options to impact breathability


  • Significantly less effective than EWI
  • More likely to cause damp problems
  • Lose internal floor area
  • A highly disruptive process as insulation is installed room by room when the house is occupied

If you are unsure which form of insulation will suit your property best, leave us a comment below!


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