Zero-carbon housing is housing that does not emit greenhouse gasses (GHGs) into the atmosphere, either directly (Scope 1), or indirectly due to consumption electricity produced using fossil fuels (Scope 2). Most commonly zero-carbon housing is taken to mean zero emissions of carbon dioxide, which is the main climate pollutant from homes, although fugitive methane may also be emitted from natural gas pipes and appliances.
Definition There are nevertheless a number of definitions of zero carbon housing, particularly concerning the scope of emissions in the housing lifecycle (eg construction vs operation or refurb), and whether it is acceptable to count off-site emissions reduction (eg due to renewable energy export) or other external reductions against any residual emissions from the house to make it a Net Zero Home. The Chancery Lane legal climate project gives 6 definitions of zero carbon housing or buildings, of which 2 explicitly allow for the inclusion of off-site emissions reductions, via off-site renewables or other carbon offsets, and one is a net zero definition, allowing for net renewable energy export to be included. Some definitions are at odds with the apparent meaning of zero carbon, with the UK government at one point proposing to define a zero carbon home as one with "70 per cent reduction in carbon emissions against 2006 standards" - ie by definition not literally zero, as it allows up to 30% of conventional emissions.
Construction vs operation: Some scopes cover operation only, some give the option of including construction too. For the purposes of present day policy to reduce emissions, it is most useful to include construction and operation in the scope of new buildings, and refurbishment and operational emissions in the scope for existing buildings (as their construction impacts cannot be changed in retrospect). For a refurbishment to be genuinely zero-carbon, the embedded carbon needs to be "paid back" by the emissions saved by the house within a timescale relevant for action on climate change (normally within a few years), and well within the lifetime of the equipment concerned. Where a new zero carbon house is constructed, the embedded carbon of the whole building must be considered and paid back. As there is substantial embedded carbon in conventional building materials such as brick and concrete, a new zero carbon home is a bigger challenge than a retrofit and is likely to need more novel materials.
Another way in which a home can become zero carbon in operation is simply that it is powered, heated and cooled purely by a zero carbon electricity grid. While these are currently (2024) few (eg Iceland, Nepal), a significant number of countries are targeting zero carbon electricity grids by 2035, including Austria, Belgium, Canada, France, Germany, Luxembourg, the Netherlands, Switzerland and the UK.
Retrofitting existing conventional homes to become zero carbon in use The following main changes are required:
Eliminate direct greenhouse gas emissions Most conventional houses in countries where space heating is required use fossil fuels or wood for space heating, hot water and cooking. In order to become zero carbon, these heating systems need to be replaced with zero emission heating methods. The main options are:
Heat pumps — powered by electricity, deliver high efficiency by drawing on the heat energy in ambient air/ground, heat pumps can deliver an apparent effiency of 400% or more, i.e. heat delivered is 4x the electrical energy in. May use air, water or ground as the heat source, and can deliver heat as warm air (air to air heat pump) or via a wet system using radiators or under-floor heating (air source heat pump). Most buildings in Norway already use heat pumps, and they are being rolled out with some government support in many countries in Western Europe, while heat pump installations now exceed gas furnace installations in the USA. As heat pumps often use lower flow temperatures than traditional fossil fuelled systems, homes may need improvements in insulation and larger emitters (eg radiators) when they install a heat pump. One study estimates the embodied carbon in a UK installed heat pump at 1563 kg. For average UK heat demand of 10,000kWh per year and a sCOP (efficiency) of 4, this would use 2,500kWh electricity at 156g/kWh=390 kg. Whereas gas would have emitted 10,000/0.85 x181g=2129 kg. Therefore 1621 kg is saved per year, and the heat pump carbon payback is 11 months. The payback would be longer in a country with higher grid carbon intensity. Direct (resistive) electric heating — Already widely used, but less efficient than heat pumps (max output 100% of electricity in), and therefore significantly more expensive for space and water heating. In the form of induction heating, is a replacement for fossil fuelled cooking. Hydrogen fired boilers/furnaces — a hydrogen boiler emits only water vapour, so is zero emission locally. However, it will only be zero emission overall if the hydrogen is from zero carbon sources, eg green hydrogen from electrolysis powered by renewable or nuclear energy. Also requires significant distribution infrastructure. Hydrogen boilers have been widely demonstrated but have no adoption at scale. They face competition from heat pumps, which make much more efficient use of available renewable electricity. Passive solar heating — uses large window areas, appropriately orientated to the sun, to absorb solar energy directly for space heating. In a retrofit situation this approach may need significant building remodelling to enlarge or reorient windows, although a conservatory or sunspace may be an easier add-on alternative. Passive solar heating does not work at night and is likely to only provide a part of the home's heating demand. It may also cause overheating in summer if not appropriately controlled, eg with shades, shutters or blinds. Solar thermal heating of domestic hot water — uses roof panels with fluid circulated through it to heat domestic hot water directly. Increasingly consumers choose to use solar PV panels instead, which can also be used to heat water through a diverter or heat pump, but supply electricity for other uses too. Biomass — Under some circumstances use of wood burning in a stove or biomass boiler may be considered zero carbon, if the source of the wood is known and it can be confirmed that carbon equivalent to that produced from burning has been captured or will be within a short timescale. However this information is rarely available in practice and biomass has become highly contentious as a zero carbon solution. Additionally, biomass systems normally produce significant local air pollution due to wood smoke. In some locations fossil fuelled generators would need to be replaced by solar PV/battery systems.
The cost of these measures to householders is naturally a critical factor. Because conventional systems benefit from economies of scale and installation skills are widely available, new zero carbon technologies may have a higher capital cost, although this may be offset by lower operating costs or efficiency savings, depending on the relative costs of electricity and fossil fuels. For this reason some governments provide householders with grants or subsidies towards the cost of the shift, for example the Boiler Upgrade Scheme in the UK, which helps to fund heat pump installations.
Ensure that the house generates more electricity than it consumes from the grid
In almost every case the renewable source of choice for dwellings is solar photovoltaic (PV) power. Use of solar PV power is now becoming routine worldwide, as solar power costs have fallen to become the cheapest source of electricity. Solar panels are typically placed on roofs, outhouses, or on the ground near the home, and it is practical for almost all scales of dwelling and most parts of the world. The only exception may be flats / apartments in dense urban areas, which may lack a roof or even any exposure to the sun.
To deliver a zero carbon house, the size /generation capacity of the PV array must match the annual consumption of the house. This is often straightforward, even if the home is using electricity for heating, directly or via a heat pump, or for cooling. In the case of cooling the solar energy availability will match the cooling demand quite well, but this is not the case with winter heating in higher latitudes. In this situation the house will typically import electricity for heating and other purposes in the winter, and export excess solar power in summer. To be net zero the export must exceed the import.
Home batteries are widely used with solar power, to provide electricity at night or dull conditions, and for cost advantage where export rates are low. In this situation it may make financial sense to store rather than reimport electricity.
Other forms of renewable power are possible in domestic situations, including micro hydro and wind turbines, but the larger size of this equipment restricts it to larger farms or estates, or to communal facilities, eg a wind turbine on an apartment block.
Maximise energy efficiency
Energy efficiency is not strictly necessary to achieve zero carbon housing, so long as the house is able to cover its electricity demand with renewable energy generated on site. However, greater energy efficiency reduces the scale of renewable generation required, and the cost of electricity imported, and may increase comfort by reducing temperature variations. At a national / economy level greater domestic energy efficiency reduces the need for large scale grid generation and transmission infrastructure, and electricity imports. The main energy efficiency approaches are:
Building fabric insulation to reduce space heating and cooling needs: Existing buildings can have their energy consumption cut significantly by insulating walls, floors and roofs, and by related measures such as draughtproofing. While some measures, eg loft/attic insulation using rockwool, are cheap and simple, others such as external wall insulation are more disruptive and expensive. Householders have to make careful analysis of the costs and benefits in terms of energy costs saved. In some countries there is state support for some home insulation measures.
Efficient appliances and lighting: these enable a cut in energy consumption without any change in occupant behaviour. For example, modern LED lighting uses 75% less electricity than traditional incandescent bulbs. Almost all appliances including white goods, computers, TVs and refrigerators have been developed to use less electricity, such that even since 1995, when they were a mature product, refrigerators in the EU are estimated to have had their power consumption cut by 60%. But more efficient appliances can more expensive, and consumers find it hard to know or calculate whether the more efficient products are worthwhile. For this reason certification including the Energy Label in the EU, and Energy Star in the United States have been developed to help consumers.
Efficient behaviour: home occupants have a large influence on the energy consumption of the a home. Typical behaviours include:
whether lights and appliances are switched off when not in use frequency of use of washing appliances such as clothes washers, dishwashers and tumble driers, and energy intensity of programs selected (eg washing temperature) whether high energy but optional equipment like hot tubs, tumble driers and electric power showers are installed and used preferences regarding internal temperature settings for space heating and cooling.
Fabric first?