A crucial step in insulating party wall cavities


With the welcome announcement from DECC that insulating existing party wall cavities is now included as a measure in the latest RdSAP calculations for both the Green Deal and ECO funding, Nick Ralph from MIMA explains why measures such as this are so crucial…

MIMA was instrumental in influencing the latest changes to RdSAP, through its work with Leeds Beckett University and the BRE; which proved the case for retrofitting existing party cavity walls using blown fibre mineral wool.

Over recent years MIMA has funded a series of co-heating trials and forensic investigations by the Buildings and Sustainability Group of the School of the Built Environment at Leeds Beckett University, to gain a detailed understanding of the factors influencing and contributing to party wall bypass, as well as quantifying its effect.

Historically, there was an assumption that cavity party walls were an area of thermal equilibrium between two heated spaces and not a source of heat loss. However, initial studies between 2005 and 2007 showed that, for example, in a mid-terrace dwelling the heat lost through the untreated party cavity walls could be greater than that which is lost through all of the other external elements combined.

The study demonstrated that heat energy from both dwellings can escape into the party wall cavity. This causes free moving air in the cavity to warm and rise up through the cavity, bypassing the loft insulation and – in a majority of cases – continuing to the roof line where the air and heat energy escape to the external environment.

Where cold air enters the uninsulated cavity at exposed edges, the uninsulated cavity creates a ‘chimney stack effect’ as the cold air rises and is warmed by heat conducted through the leaves of the party wall from the adjoining homes, before escaping from the cavity to the external environment – either into the loft space or through the roof. Additionally, windy conditions can induce differential pressure that leads not only to heat losses at the junction of the party cavity with both external walls and suspended floors, but also increased heat loss due to the stack effect of the cavity.

Once this highly detailed work had been undertaken and widely accepted, it was possible to monitor a number of dwellings in lower detail, whilst still making quantitative measurements of heat flux, to show that the heat loss phenomenon was common to all party walls with cavities to the roof. The quantum of heat losses was also considered to be consistent. Leeds Beckett University’s work also demonstrated that filling the cavity with insulation would consistently reduce this heat loss. Taking a mid-terrace house, which was built between 1990 and 2001, the study demonstrated an annual saving of 1,978 kWh of energy and 0.38 tonnes of CO2 – equating to a £70 reduction in household energy costs. RdSAP attributes a heat loss equivalent to an effective U-value of 0.50 W/m2K to an unfilled party wall with a cavity to the loft and a U-value of 0.20 W/m2K when it is filled.

When you take into account estimates that there are 3.77million bypass walls in England alone, equating to 5 million households, the potential to reduce fuel usage and CO2 emissions through filling party cavity walls with blown fibre mineral wool is therefore huge. In fact, the BRE has estimated it would save approximately £465m per year and 2.5 million tonnes of CO2.

Putting that into the context of increasing fuel poverty and the government’s ambitious CO2 emission reduction targets and the importance of such a measure being included in RdSAP becomes clear.

According to a recent report from Cambridge Econometrics, millions of people are living in fuel poverty in the UK; and one of the biggest causes is the poor condition of our housing stock, which is one of the least energy efficient in Western Europe.

The report undertook detailed modelling to assess the economic, fiscal and environmental impact of a recommended investment programme aimed at bringing homes up to Band C on an Energy Performance Certificate. Included within the recommendations is a national super-insulation scheme that would result in £8.5bn annual energy bill savings for British households.

In addition to making all low income households highly energy efficient and reducing the level of fuel poverty, it also demonstrates the comprehensive economic benefits of taking radical action to fix Britain’s energy wasting homes. Overall, it is estimated that a radical programme to make all homes highly energy efficient would add £13.9bn annually to the UK economy by 2030, with the government receiving £3.20 through increased GDP for every pound they invest.

With the UK’s existing housing stock posing the greatest barrier to us achieving ambitious CO2 reduction targets and over 5,000 people a year dying from cold housing, recognising those measures that can make a significant contribution to improving the energy efficiency of our housing stock – such as insulating existing party wall cavities – is crucial. And as per the Cambridge Econometrics report, tackling these measures has an economic benefit too.

Nick Ralph

Mineral Wool Insulation Manufacturers

Association (MIMA)

Tel: 020 7935 8532




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