In this guide, we take a comprehensive look at internal wall insulation (IWI). We'll begin by explaining what IWI is and what factors should be considered before its installation. We will look at which characteristics of insulation are important for IWI and what fixing methods are typically used. We detail what constitutes a good approach to the installation of internal wall insulation and why a pre- and post-installation inspection is important. IWI can help to bring many benefits to a property, and we explore those and the grants that may be available to help cover the cost of its installation.
What is internal wall insulation?
Internal wall insulation or IWI as it is known, refers to an insulating material that is installed on the internal face of the external walls of a property. The insulation is therefore fixed to those internal walls of a room where heat and cold can pass to the outside of the building and vice versa. The internal separating walls between two heated rooms do not need to be insulated.
IWI is commonly installed on solid wall properties. The date which your home was built as well as the appearance of external brickwork can give you a good indication of whether you have solid walls or not.
If a home was built before 1920 the walls are probably solid and after the 1920s the walls are usually made of two layers with a cavity between them. If you can see the bricks on the outside of a house then the brick patterns, that appear in a government factsheet ‘Keeping the heat in – A simple guide to wall insulation’[1], give a good indication as to the type of wall[2].
IWI can also be used to insulate garage conversions to help improve the thermal performance of a space that was previously unheated.
The insulation of buildings with solid walls can be done externally using external wall insulation (EWI) but factors like planning restrictions in conservation areas or technical issues with the shape or number of external fittings may make internal wall insulation the best suited to the property.
Internal wall insulation will reduce the internal space of a room – the extent of the reduction will depend upon the type of insulation used and the method by which it is installed. Its installation will cause disruption as internal fixings such as cupboards, skirting boards and light switches will need to be removed and replaced after the insulation has been installed.
What factors should be considered before installing internal wall insulation?
Applying internal wall insulation can alter the building's hygrothermal behaviour. The term ‘hygrothermal’ describes how the building reacts to the combined effects of moisture and heat. IWI systems should be chosen with this in mind to act in harmony with the building fabric. For this reason, it is important that an assessment is carried out on the building by a suitably qualified professional before any IWI system is selected or installed.
In the government document issued by the Department for Business Energy and Industrial Strategy entitled ‘Retrofit Internal Wall Insulation – Guide to Best Practice’[3] it recommends that this assessment is carried out in line with the latest version of PAS2030 - Installation of Energy Efficiency Measures in Existing Dwellings[4] and includes a flow chart for the assessment, design, and installation process[5].
The recommended assessment looks at a wide range of factors. These include the condition of the external wall, damp proof courses and guttering to check for areas where water could enter the solid wall.
An assessment of the internal face of external walls should take place to check for any evidence of damp, surface moisture or mould growth. These can indicate that ventilation may be poor, thermal bridging areas exist, or water ingress is taking place.
The existing internal finish of the walls is checked to see if it is vapour closed as this heightens the risk of moisture being trapped between the insulation and the wall surface and being unable to dry out.
This useful summary of the major IWI risks appears in the Guide to best practice[6]:
- Inappropriate external wall finishes for the wind driven rain exposure
- Guttering and external pipework in poor condition
- Surface condensation, fungus, mould growth, salt deposits or damp on internal or external walls
- Damp or rot in joist ends in external walls
- Structural defects
- No damp proof course, or non-functioning damp proof course
- Inadequate ventilation on the property
What characteristics of insulation are important for IWI?
When selecting the most appropriate internal wall insulation there are three main characteristics that should be considered.
- Is the IWI diffusion open or diffusion closed?
- What is the insulation’s thermal performance?
- How does the insulation fit into a project’s sustainability goals?
Diffusion open and diffusion closed internal wall insulation
As the government guide to IWI points out, insulating can affect the hygrothermal behaviour of the wall. Internal wall insulation can either be diffusion open – allowing moisture through the insulation or diffusion closed - where the insulation acts as a barrier to the flow of moisture.
Diffusion open EWI insulation types:
- Mineral Wool
- Wood Fibre boards
Diffusion closed insulation types:
- XPS
- PIR
- Phenolic
Both vapour open and vapour closed insulation can be successfully used for IWI provided that appropriate moisture management measures are undertaken. These should be recommended by the retrofit assessor and be aligned with the standard BS 5250 ‘Management of moisture in buildings.’
Older heritage buildings can have solid walls that were constructed to be vapour open using materials that allow the walls to breathe. In these cases, it may be more likely that vapour open IWI will be selected. In this way the natural hygrothermal behaviour is less likely to be disturbed with less risk to the building fabric.
Level of thermal performance of IWI
The thermal performance of each insulation is known as its lambda value. The lower the lambda value, the better the insulation will be at keeping the building warm during the winter.
Here are examples of lambda values for some different insulation types used in IWI[7]:
Insulation type used in IWI | Form | Vapour open or closed? | Density range (kg/m3) | Typical lambda values (W/(m*K)) |
Mineral wool | Rolls or batts | Open | 30 - 100 | 0.032 to 0.044 |
Wood fibre | Rigid board or batts | Open | 50 - 160 | 0.036 to 0.048 |
Wood fibre | Air injected | Open | 35 – 42 | 0.038 |
XPS (Extruded Polystyrene) | Rigid board | Closed | 25 - 45 | 0.029 to 0.038 |
PIR | Rigid board | Closed | 30 - 32 | 0.020 to 0.027 |
Phenolic | Rigid board | Closed | Approx. 35 | 0.018 to 0.023 |
Vapour closed insulations with lower lambda values can give thinner IWI solutions than the vapour open alternatives to achieve the limiting U-values in table 4.3 in existing dwellings as laid out in Approved Document L - Conservation of fuel and power of the building regulations[8].
However, a view may be taken that other factors may be more important to the dwelling than insulation thickness alone. As the ASBP briefing paper[9] ‘Insulation and retrofit – Finding the sweet spot’ explores, there is a balance to be found between the level of thermal performance and the need to ensure that other important issues such as interstitial and surface condensation are addressed. These factors may include how the insulation reacts to moisture or, as we will cover next, how the IWI fits with a project’s sustainability aspirations.
How IWI fits into project with a focus on sustainability
Different internal wall insulation projects will have different goals, and some will look at sustainability features of construction products they are using as part of the criterion for selection. These can include the source of the material for the product and how much embodied carbon it adds to a dwelling. Although internal wall insulation can help to contribute to the energy efficiency of a dwelling, energy is used in its manufacture.
This means that it comes with its own embodied carbon footprint. This can be measured by Environmental Product Declarations (EPDs). Therefore, the EPDs of different types of products used within IWI can be used as a way to specify one solution over another on projects that place an emphasis on such environmental factors.
Wood fibre insulation from STEICO is made from more than 80% natural wood sourced from PEFC certified forestry and the associated product EPD's reflect the bound carbon dioxide equivalent content and also detail the amount of biogenic carbon that is stored within the products. For example, STEICOflex 036 binds 73kg CO2eq[10] and can thus contribute to climate protection.
What methods are used to install IWI?
Internal wall insulation can be installed in a number of different ways as outlined in the table below
Method of IWI installation | Overview |
Direct fix | Fixing of rigid insulation boards or insulated plasterboards directly to the internal face of the wall. Needs relatively even walls. Vapour control layer (VCL) is required and it is usually the thinnest IWI solution. |
Batten fix | The wall is battened out and insulation is installed between the battens before standard plasterboard and a VCL is applied. |
System fix | There are a range of IWI systems that use a combination of installation methods and bespoke products to form a tested system. The system supplier’s installation instructions should always be followed. |
Stud wall | A new stud wall is built away from the existing wall to provide a cavity. The stud wall is then insulated with rigid boards or flexible batts. This option is likely to have the most impact on room size. |
What is a good approach when installing IWI?
Whatever the method of fixing required, taking the following approach when installing IWI can help the work to follow best practice guidance.
Carry out a pre-installation check
Before IWI is installed, a comprehensive pre-installation survey in line with PAS2030 should be carried out so that the installer is satisfied the building is in a suitable condition for IWI to be installed.
Take measures to minimise thermal bridging
A thermal bridge is an area in a building's insulation where heat flows more easily due to a material with higher thermal conductivity, leading to increased and localised heat loss. This leads to an area of reduced temperature where condensation can form, and mould can grow.
As IWI joins areas such as connecting internal walls, party walls and intermediate floors where thermal bridges are present, IWI installation should include methods designed to help minimise the effect of thermal bridging in these areas.
When timber or metal stud-work interrupts internal wall insulation, it creates repeated thermal bridges. This reduces the overall thermal performance of the system. To minimise this effect, a continuous layer of insulation should be added over the studwork. Stud spacings should be kept as wide as possible to minimise the total amount of thermal bridging taking place but always be within the recommended max spacings required by the lining material.
Avoid the creation of areas for thermal bypass and interstitial condensation
Gaps within and through an internal wall insulation layer can allow heat to escape and this is known as thermal bypass. It can increase the heat loss through an insulated wall.
The risk posed by thermal bypass can be mitigated by ensuring that a robust, continuous air barrier is installed on the warm side of the insulation. Insulation should be carefully and tightly installed to avoid gaps between insulation boards or batts or battens and joints taped where appropriate.
Interstitial condensation occurs within enclosed cavities in walls where moisture-laden air condenses on cooler surfaces inside the structure.
This often occurs where there is an air gap, therefore measures taken to avoid thermal bypass can help to reduce instances of interstitial condensation. Where IWI is directly fixed to the existing wall installers must minimise the creation of air gaps by applying a continuous layer of adhesive to the wall or by applying the correct number and spacings of fixings for non-adhered insulation solutions.
Properly seal any penetrations of the IWI by services
Any unavoidable penetrations through the air barrier or vapour control layer of the IWI must be properly sealed to maintain airtightness and prevent moisture ingress. Common penetrations include incoming utilities, flues, vents, and external service connections to items such as outside lights, taps, sockets and security cameras.
Carry out post installation inspection
Post installation checks are important to ensure that the IWI design has been followed, and installation has taken place to the required standard and in line with installation guidelines. Post installation checks should be supported by survey notes and photographic evidence. It is a good approach to document key stages of the IWI installation process by taking photographs as the work progresses. This can be used as evidence of the quality of work that may be hidden from view once the installation is finished.
What are the benefits of internal wall insulation?
Installing internal wall insulation brings a number of benefits.
IWI can help to lower energy bills
Installing interior wall insulation can help improve the energy efficiency of a property, thus saving heating energy, which can also have an impact on heating costs. The cost savings vary depending on dwelling type as can be seen on data which can be found on the Energy Savings Trust website[11].
IWI can help to improve energy efficiency
IWI can help to improve the energy efficiency of a property and assist with reducing the dwelling’s carbon emissions when assessed using RdSAP[12] for existing buildings. This can improve the Energy Performance Certificate (EPC) of the property, which in turn can help to increase its value. A study carried out by Rightmove[13] of 300,000 properties shows that an improved EPC rating brings about an increase in value of between 3% and 15%. The amount depends on the scale of the improvement in EPC value.
IWI can improve sound insulation
IWI can help to improve the soundproofing of walls to reduce noise pollution within the property. The level will depend on the density and type of insulation used.
IWI can help to keep dwellings cooler in the summer
Insulation helps to slow down the passage of heat no matter which direction it takes so it can assist with keeping the heat in during the winter and keeping the heat outside the house on hot summer days. Therefore, adding IWI can contribute to mitigating the effects of summer overheating. The extent of the mitigation will depend on the type of insulation. In the winter the heat is trying to escape to the cooler temperature outside. Using wood fibre internal wall insulation adds thermal mass to the walls, helping them to keep the building cooler in the summer.
Is planning permission required for internal wall insulation?
As there is no change in the external appearance of the property, planning permission is not normally required. However, if the property is a listed building or is in a conservation area the local planning authority should be consulted for advice and guidance before going ahead with any IWI work.
What grants are available for internal wall insulation?
There are two government schemes where grants for internal wall insulation are available – the Great British Insulation Scheme[14] (GBIS) and the fourth iteration of the Energy Company Obligation[15] (ECO4). The schemes are set to run until April 2026.
Both schemes have eligibility criteria and offer different insulation measures including IWI. To be able to fit IWI measures under GBIS or ECO4, installers must be registered with the Trustmark government endorsed quality scheme. Installers must also follow retrofitting standards PAS 2030[16] and PAS 2035[17] and their work can be inspected for its quality. Failure to meet the required standards can result in installers no longer being eligible to carry out work under the schemes.
Where can I find out more information about wood fibre insulation solutions for internal wall insulation?
Wood fibre insulation from STEICO is made from more than 80% natural wood sourced from PEFC certified forestry.
STEICOinternal is a vapour open wood fibre insulation board for internal masonry and timber frame insulation. It can be combined with recommended lime and clay plasters and can contribute to the regulation of indoor air moisture to deliver an IWI solution that can help to support the energy efficiency of buildings.
STEICOflex 036is a flexible thermal insulation batt for insulation between studs of IWI systems. It is vapour open, has good retention abilities and forms well to its surrounding IWI components.
All the above STEICO products have EPDs that reflect the bound carbon dioxide equivalent from the wood component of the insulation. For example, STEICOinternal binds 270kg CO2eq[18] and can thus contribute to climate protection.
To enquire about using our internal wall insulation on your next project, please contact our expert team who will be happy to help.
[1]assets.publishing.service.gov.uk/media/5a81b046ed915d74e33ff8f1/DECC_factsheet_11.11.16_WALLINSULATION_LOCKED.pdf
[2]Graphic taken from page 1 of ‘Keeping the heat in – A simple guide to wall insulation’
[3]assets.publishing.service.gov.uk/media/614b30aad3bf7f718a54c0be/iwi-guidance.pdf
[4]www.bsigroup.com/en-GB/insights-and-media/insights/brochures/pas-2030-installation-of-energy-efficiency-measures-in-existing-dwellings/
[5]Graphic from page 7 of Retrofit Internal Wall Insulation Guide to Best Practice
[6] List from page 24 of Retrofit Internal Wall Insulation Guide to Best Practice
[7] Values stated in the table are typical and may vary from manufacturer to manufacturer – always check the declared product performance with the manufacturer. Values can vary due to different densities, formulations and manufacturing techniques.
[8]assets.publishing.service.gov.uk/media/662a2e3e55e1582b6ca7e592/Approved_Document_L__Conservation_of_fuel_and_power__Volume_1_Dwellings__2021_edition_incorporating_2023_amendments.pdf
[9]asbp.org.uk/asbp-news/finding-the-sweet-spot
[10] Storage refers to CO2 equivalent in life cycle phase A1 according to EN 15804’
[11]energysavingtrust.org.uk/advice/solid-wall-insulation/
[12]files.bregroup.com/SAP/RdSAP10-dt13.02.2024.pdf
[13]www.rightmove.co.uk/news/articles/property-news/green-premium-epc-ratings/
[14]www.ofgem.gov.uk/environmental-and-social-schemes/great-british-insulation-scheme
[15]www.ofgem.gov.uk/environmental-and-social-schemes/energy-company-obligation-eco
[16]www.bsigroup.com/en-GB/insights-and-media/insights/brochures/pas-2030-installation-of-energy-efficiency-measures-in-existing-dwellings/
[17]www.bsigroup.com/en-GB/insights-and-media/insights/brochures/pas-2035-retrofitting-dwellings-for-improved-energy-efficiency/
[18] Storage refers to CO2 equivalent in life cycle phase A1 according to EN 15804’