Leeds Beckett research raises questions about the risks associated with government retrofit policy

The UK Government have been funding and supporting retrofits though policy for several decades in an attempt to meet its carbon reduction and fuel poverty targets. Most effort has focussed on the “low hanging fruit”, which has seen 85% of retrofits over the last five years concentrating on installing cavity wall insulation, loft insulation, or new boilers in homes.

More recently there has been a shift to encouraging insulation in homes that do not have cavity walls. There are around 8 million ‘solid wall’ homes in the UK, and these are often among the least energy efficient and are more likely to house occupants living in fuel poverty. Retrofit solutions for insulating solid walls do exist though they are among the most costly and disruptive measures.

External wall insulation (EWI) is one option where insulation is wrapped around the outside of solid walled homes. However, in many parts of the UK EWI is not permitted because of planning restrictions. Additionally, EWI has faced substantial criticisms linked to fire safety risks.

An alternative approach is to install insulation inside homes, which is known as internal wall insulation (IWI). Previous research has shown there can be many problems and risks associated with IWI for the homeowner. However, research undertaken by the Leeds Sustainability Institute (LSI) has highlighted that installing IWI can also pose risks for neighbouring homes.

Dr Matthew Brooke-Peat undertook thermal modelling of two case study solid wall homes, to investigate what happens when one neighbour has IWI fitted, but the other does not. This required the development of a technique to assess thermally asymmetric building junctions.  The results showed that this would lead to the uninsulated neighbour having lower surface temperatures at the party wall / external wall junction, which increased risk of mould growth that can adversely affect health. This is an important finding because the Government is actively encouraging and subsidising the installation of IWI and current guidance does not address this risk to the neighbours.

The reason that risk of mould growth increases, relates to the fact that before any IWI was installed, both homes lost heat equally through their party wall / external wall junction; however, when one side is insulated, the party wall receives less heat and is inevitably cooled down. This means that the insulated home has a warmer wall surface than before, while the uninsulated home has a cooler wall surface. Mould growth and surface condensation are linked surface temperatures, so the risks for the neighbour increases.

Mould growth and condensation may not be inevitable for the uninsulated neighbour because it can be managed through effective heating and ventilation to some extent. However, this research has confirmed that this heat loss mechanism could adversely affect uninsulated neighbours through no fault of their own. This raises the possibility that that installing IWI may have legal ramifications if neighbours suffer health problems, face higher fuel bills, or if their building fabric suffers damage as a result of their neighbour installing IWI. Currently, there is no guidance or modelling conventions for assessing the risk presented by this phenomenon and it is hoped that this research will stimulate more investigations to improve guidance so that future IWI installations are lower risk to both homeowners and their neighbours.