Climate change: evidence review of mitigation options in the Built Environment sector

7 Cross-cutting themes

Climate change mitigation options in the built environment which are aimed at reducing the use of fossil fuels, either through avoided energy consumption ( chapter 4), the use of less energy-intensive materials ( chapter 5) or the use of low carbon energy sources ( chapter 6) offer air quality and future energy security benefits and these are addressed in turn below.

7.1 Air quality

As fossil fuels are the main source of air pollution, most of these mitigation options will, therefore, have the co-benefit of improving air quality, with the exact level of benefits depending on the fuel displaced and on the magnitude of the energy savings. In the case of biomass combustion, emissions displaced from fossil fuel combustion will be diminished (or increased, if replacing gas) by those produced by the biomass, unless appropriate mitigation is put in place.

Building fabric improvements reduce the total energy consumption for heating. Insulation can also prevent over-heating, leading to electricity savings in cases where fans or air conditioning are used for cooling. Air pollutant emission savings therefore depend on the heating fuel and technology. This could be a gas or oil boiler or electric heating. For reductions in electricity used for heating or cooling, the emission savings depend on the mix of sources used to generate electricity across the whole network. Both the location of emissions and the pollution abatement technologies will be different for boilers located in residential or commercial buildings compared to gas or coal-fired power stations used to generate electricity.

Behaviour change has similar impacts except that there is potential to also address energy used for lighting and appliances, thus saving additional emissions from electricity generation.

The use of more sustainable building materials such as timber, cellulosic material or sheep's wool insulation can reduce the emissions of air pollutants from extraction, processing and manufacture of the building materials that are displaced, such as steel, aluminium, concrete, bricks or synthetic insulation foam. Life-cycle analysis would be needed to properly compare the total emissions embodied in these different materials.

Green infrastructure can reduce emissions in a different way, by absorbing or adsorbing pollutants on to vegetation. The amount of pollution removed is generally thought to be proportional to the leaf area, and so is greater for larger trees with a dense canopy. There are numerous modelling studies in the literature, and the iTree-Eco model has been used to estimate the value of air pollution removal at various sites in the UK, but empirical data to demonstrate the size of the effect in real life is scarce. Most studies of urban trees estimate that pollution levels are reduced by only a few percent, but the economic value of this removal can still be significant because of the high levels of mortality and morbidity related to urban air pollution.

Solar PV panels will displace the national electricity generation mix, and solar hot water panels will displace the fuel that would have been used for hot water - probably gas, oil or electricity. Both heat pumps and more efficient boilers will displace the gas, oil or electricity that would have been used for space heating, but in the case of heat pumps the emission savings will be offset by emissions from the electricity used to run the pump. More efficient lighting and cooling technologies will reduce the use of electricity.

A key potential adverse side-effect is emissions of particulate matter ( PM) from biomass combustion. Emissions vary considerably depending on the nature of the fuel and the combustion device, so estimates are highly uncertain, but although biomass boilers produce less PM than residential coal combustion, they produce more than oil-fired boilers. PM emissions from gas and biogas are negligible.

Quantitative evidence

There are numerous articles in the literature that show substantial air quality co-benefits from climate change mitigation actions, but few of these present separate estimates for the building sector, and none were found that quantify potential co-benefits specifically for Scotland. However, a study for the whole of the UK was carried out in 2013 for the UK Committee on Climate Change, which estimated the air quality benefits that would arise from achieving the CCC's Medium Abatement Scenario by 2030. This scenario involved a shift to biomass boilers, biomass district heating, biogas, solar hot water and heat pumps in residential and non-residential buildings and industry, plus a range of building energy efficiency measures.

Using emission factors for different heating technologies and standard DEFRA estimates of damage costs per tonne of each pollutant, the study estimated the costs of health impacts from emissions of NO _X, SO ₂ and PM ₁₀ for biomass and biogas heating, and the costs avoided from savings in coal, gas and oil (changes in electricity use were assessed as part of the whole power sector, not just for buildings, so are not shown here). The findings are summarised in Table 7.1. This shows that the cost of emissions from biomass and biogas boilers, which totals £169 million in 2030 for the residential and non-residential sectors, partly offsets the large savings from avoided coal, oil and gas combustion (£316 million in 2030 for the shift to biomass, solar and heat pumps, and £92 million for building energy efficiency measures). However, there is still a large net benefit of £239 million.

The study emphasises that the estimates of emissions from biomass boilers are highly uncertain: the main estimate assumes that the Renewable Heat Incentive standard can be met, whereas the sensitivity test assumes a value measured for Swan Eco-stoves, which is a factor of ten larger. If this value was used, the biomass emissions would more than offset the savings from avoided coal, oil and gas for this scenario. This emphasises the importance of optimising biomass combustion devices to reduce emissions as far as possible, and avoiding the use of biomass in urban areas where population exposure is high.

Table 7.1: UK Estimates of the value of air quality co-benefits from climate change mitigation action in the buildings and industry sector (building energy efficiency and shift to solar, biomass and biogas) for the UK, based on damage costs for emissions of NO _X, SO ₂ and PM ₁₀.

Numbers in brackets are a sensitivity test with a much larger PM ₁₀ emission factor for biomass stoves (see text). Source: ApSimon and Oxley (2013)

7.2 Energy security

The climate mitigation methods that reduce building energy consumption will also have benefits for future energy security, by reducing reliance on fossil fuels. Despite production of North Sea oil and gas, a considerable proportion of the UK's fossil fuel energy is imported from other countries. For the energy supply technologies, solar energy is of course provided locally and therefore will also provide energy security benefits. Biogas is also generally provided locally, typically from anaerobic digestion of farm or household organic waste. For solid biomass, it is important to ensure that supply is from a sustainable source, such as sustainable forestry waste, rather than from forests of high biodiversity value. The size of the forestry sector in Scotland should ensure that sustainable biomass can be provided locally, so this option should also improve energy security. However, for both biomass and biogas it is necessary to ensure that a continuous supply is available locally to avoid short term disruptions, ideally with a choice of suppliers.