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Is an energy-efficient retrofit with IWI (internal wall insulation)  the right option for me?

For many homeowners, using internal wall insulation is the only option if they want to improve the warmth and comfort of their homes. Constraints from local planning departments and aesthetic and practical considerations can often mean that adding external wall insulation is not an option. However, using IWI is potentially more fiddly and complex than using external wall insulation. It also carries risks of potential moisture build-up within the insulation (‘interstitial condensation’) and mould growth. Moisture build-up and mould growth can lead to serious health problems for occupants as well as major damage to the building fabric.

To avoid problems with IWI, it is very important to ensure that retrofits adopt the four key principles that are essential for successful and effective energy-efficient retrofits.  The principles are central to the Passivhaus low energy building standard and methodology but can equally be applied to retrofit projects.  All four principles need to be considered during a low energy refurbishment to avoid any ‘unintended consequences’, such as cold spots, condensation or mould growth.


For walls, floors, roofs, lofts and windows, energy efficiency obviously depends on good levels of insulation. The materials used for Internal Wall Insulation (IWI) need to be carefully chosen, depending on the wall build-up, to cope with what are often complex moisture issues.  The use of ‘breathable’ or ‘capillary active’ materials may be necessary to mitigate moisture risk, which can also be modelled using WUFI software for additional peace of mind.


Airtightness (or reduction of draughts) is an often overlooked aspect of energy efficiency which can make a huge impact on the warmth and comfort of a home.

Continuity of insulation (or minimising ‘thermal bridging’)

To work best, insulation needs to work in a continuous ‘blanket’ around the house, minimising any gaps in the insulation (known as ‘thermal bridges’). Examples of ‘thermal bridges include through stones through cavity walls or gaps in the insulation where the wall meets the roof, which breaks the continuity of insulation and loses heat out of the building. The importance of reducing ‘thermal bridges’ increases as the level of insulation increases.


As airtightness improves in a building, it is also vital that suitable approaches to ventilation are considered carefully, so that there is no impact on occupant health or building structure.

Whole house plans

The complex interaction of  the 4 key principles means that the order in which home improvements are undertaken is very important. It helps if you think of your home as a complex system which needs to be considered as a whole. Piecemeal improvements to a home, without fully considering the 4 principles, can result in ‘unintended consequences’ To avoid this, it is very useful to think about a ‘whole house plan’ before you begin any refurbishment work.

Improving the energy efficiency of our existing homes is a lot more difficult than designing it into a newbuild from scratch,  and even more so when using Internal Wall Insulation (IWI). However, by using  the four key principles (insulation, airtightness, continuity of insulation, and ventilation) we can create warm and comfortable homes.

 Green Building Store

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Heath House Mill,
Heath House Lane,
HD7 4JW,

01484 461705

Proud to exhibit at the Homebuilding & Renovating shows

Green Building Store is committed to low energy building to help reduce carbon emissions and cut heating bills. The most effective way of doing this is to focus on the building fabric, using high levels of continuous insulation, together with stringent airtightness and advanced ventilation strategies. This approach leads us to be strong proponents of the Passivhaus approach to low energy building, which can cut a building’s heating need by 90% compared with the UK average.