The UK Government has estimated that there are 8.5 million solid walled homes in Britain. These walls are the poorest performing, leaving homeowners especially vulnerable to fuel poverty. Only 8% of these properties have wall insulation. Correctly fitting solid wall insulation could help to reduce heat loss and energy bills for these properties. Before carrying out this work, however, it is essential to carry out some checks to ensure the solution is suitable for the property.
Should I fit Internal Wall Insulation (IWI) or External Wall Insulation (EWI)?
When insulating solid walls, the options are to either fit insulation on the external or internal side of the wall.
Find out more about insulating solid walls.
In most cases, External Wall Insulation (EWI) will provide a faster and more straightforward option for insulating solid walls. As the insulation layer wraps around the envelope, detailing is simpler and the external wall construction is kept warm – reducing potential condensation and damp issues.
However, it may not always be possible to fit EWI. For example, some solid walled properties are subject to additional planning restrictions which can limit external alterations (such as being listed or located in a conservation zone). In this case, the only option is to fit insulation on the internal face of the wall.
IWI offers flexibility and reduces constraints around installation, but it can present technical risks that should be managed. If recommendations are not properly considered, there could be a high risk of interstitial condensation and mould growth.
IWI can be installed on most construction types provided the risks have been well understood and mitigated. This includes carefully considering whether the IWI system is moisture open or moisture closed.
Learn more about where moisture open and moisture closed IWI systems should be used.
Who can carry out an IWI installation?
IWI installations are amongst the most technically challenging energy retrofit projects. For this reason, we strongly recommend the property is first assessed under the process laid out within PAS 2035: 2023 (Retrofitting dwellings for improved energy efficiency. Specification and guidance). This will require a Retrofit Coordinator to be appointed, the property to be carefully assessed and a 20-30 year medium term improvement plan for the entire property to be drawn up. This thorough approach should help to ensure that the work is appropriate for the property.
PAS 2035: 2023 supersedes PAS 2035: 2019 + A1: 2022, which remains current and will be withdrawn on 30th March 2025.
Please note that whilst PAS 2035: 2019 + A1: 2022 can still be used until 2025, PAS 2035: 2023 can optionally be used now ahead of the 30th March 2025 withdrawal date.
As part of the PAS 2035: 2023 process, all installations need to be carried out by installers qualified to PAS 2030: 2023 (Installation of energy efficiency measures in existing dwellings. Specification). Trade associations such as the National Insulation Association (NIA) provide a directory of registered installers. Similarly, the Solid Wall Insulation Guarantee Agency (SWIGA) offers a quality framework for members which includes an independent assessment scheme – providing customers with assurance that qualified members will provide a good standard of work.
What to look for when checking the condition of the property
As part of the PAS 2035: 2023 framework, a Risk Assessment and Whole Dwelling Assessment should be carried out. In relation to IWI, there are a number of aspects should be considered:
- Any planning restrictions on the property.
- A full moisture risk assessment following the guidance in BS 5250: 2021 (Management of moisture in buildings. Code of practice) - learn more about what to consider when carrying out a moisture risk assessment for solid walled properties.
- Local weather conditions including the exposure zone (level of driving rain exposure for the area) and building specific factors such as aspects which are open to prevailing conditions and local sheltering.
- How the work will impact ventilation within the property, as installation of insulation measures can improve airtightness dramatically. Any existing provisions (such as air bricks) will need to be considered and the overall ventilation strategy may need to be improved.
- A survey of the building characteristics (such as whether walls are vapour open or vapour closed) and the materials used in all constructions. For example, fines concrete constructions are not suitable for IWI.
- The condition of external walls, including any cracks in bricks or render, missing pointing and broken guttering or downpipes.
- Suitability of sill overhangs, roof eaves and verge overhangs.
- Planning detailing for penetrations such as pipes, cables or ducts.
- Current insulation levels in neighbouring properties and if other elements (floors, roof or ventilation) could receive upgrades in the future.
- Whether a damp proof course is present and if so, its type, if it is sufficiently raised above paths and that paths run away from the external wall.
- Whether there are any signs of damp (such as mould or peeling paint) or insect attack in any part of the wall, floor or roof construction including the timbers.
Any issues that are identified, must be fully dealt with and the walls should be dry and in good condition before any IWI installation begins.
How to prepare a property for IWI
Once any underlying issues are dealt with, the installers will need to carry out some additional preparation work including:
- Removing all external and party wall (if being insulated) fittings, including skirting boards, picture rails, wallpaper, gloss/silk emulsion paint, kitchen cabinets, toilets etc.
- Determining whether sockets, radiators or pipes need to be relocated.
- Confirming if window reveals can be insulated if the existing plaster is removed (it is not recommended to leave any wall areas uninsulated).
- Sealing any gaps including around windows and doors and at the perimeter of the room (including junctions between the wall and floor and ceiling).
- Depending on the approach taken and the condensation risk analysis, system specific issues will need to be addressed e.g. with a moisture closed system, there should be a high level of vapour resistance on the warm side of the construction; alternatively with a moisture open system, the insulation thickness may need to be restricted to avoid moisture becoming trapped within the construction beyond its buffering capacity.
- Ensuring there is adequate protection against driving rain. This may include applying cladding, render or a barrier cream to the external face of walls. Or it could mean ensuring that render drips and parapets throw rainwater away from walls and making sure that all penetrations are effectively closed.
- Checking walls are dry (a typical masonry wall should have around 6-8% moisture content by volume).
