The Zero Energy Building

Energy use in a zero energy building (ZEB) is minimised and all the energy consumed must originate from a renewable source. However, this does not imply energy rationing or utility independence. Such homes are also referred to as ‘zero carbon’ or ‘carbon neutral’, though the latter term more accurately refers to an individual.  A ‘green building’, or ‘eco building’ will include measures to reduce environmental impact, but not to the extent of achieving zero carbon performance.

A large portion of the energy used in the UK is within buildings, and the UK government has identified that CO₂ emission reductions in this sector can be readily made. By 2025 the Government will introduce a Future Homes Standard for new build homes to be future-proofed with low carbon heating and world-leading levels of energy efficiency. By making our homes and other buildings more energy efficient and embracing smart and low carbon technologies, we can improve the energy efficiency of peoples’ homes and boost economic growth while meeting our targets for carbon reduction. (Fig. 1)

The Government has dual ambitions of achieving net zero emissions by 2050 and continuing progress towards achieving 300,000 homes a year by the mid-2020s. These objectives are not mutually exclusive, and with good planning and smart design we can build the homes we need while protecting and enhancing the natural environment and adjusting to climate change.

The introduction of the Future Homes Standard will ensure that an average home will produce at least 75% lower CO₂ emissions than one built to current energy efficiency requirements. In the short term this represents a considerable improvement in the energy efficiency standards for new homes. Homes built under the Future Homes Standard will be ‘zero carbon ready’, which means that in the longer term, no further retrofit work for energy efficiency will be necessary to enable them to become zero-carbon homes as the electricity grid continues to decarbonise.

Homes built under the Future Homes Standard will be future-proofed with low carbon heating and world-leading levels of energy efficiency. By delivering carbon reductions through the fabric and building services in a home rather than relying on wider carbon offsetting, the Future Homes Standard will ensure new homes have a smaller carbon footprint than any previous Government policy.

Though there is much legislation, there is also the barrier that many of the measures required to make a building greener also tend make the building more expensive.

Energy Saving Measures in the Home

There are many different measures that can be taken to limit energy use and eliminate profligacy, but it is first important to assess how energy is currently being used by conducting an energy audit. The space heating (and cooling) requirement is the most important consideration because of the amount of energy used for heating. It is desirable to have a thermostat in each room set at a level which is comfortable but not unnecessarily hot. A temperature of 25 ^oC is hot, but 18 ^oC is tolerable, and there is a big difference in the energy needed to keep the temperature at 18 ^oC compared to 25 ^oC. As a general rule, about 1 % of the total energy used in a building is saved with each degree the temperature is lowered. By experimenting with different settings it should be possible to agree a temperature everyone in the house can accept.

But a heating system is only needed because heat in the home is eventually lost to the outside. To virtually eliminate heating cost, the rule is pretty simple; as far as possible create a sealed box covered by a thick blanket of insulation. This means closing vents and gaps (though this might not be feasible where cavity circulation is necessary to clear moisture ingress). But air circulation is needed for comfort (and safety in the case of fires and boilers to clear dangerous products of combustion), hence energy loss becomes inevitable without sophisticated heat reclamation. Nevertheless sensible measures should be taken wherever possible. Ensure open fires are made efficient (Fig. 3), or replace open fires with central heating (Fig. 4). If there is a choice, install underfloor heating because the circulating water is at a lower temperature than with radiator systems making the boiler run more efficiently.

Replace older boilers with a combination or 'combi' boiler which is both a high efficiency water heater and a central heating boiler in a single compact unit. Combi boilers heat water directly from the mains when you turn on a tap, so you won't need a hot water storage cylinder or a cold water storage tank in the roof space.

Heat rises, so 25% of heat can be lost through the roof area. By insulating the roof, much more heat is retained within the property, which can significantly reduce your heating bill. There are natural and manufactured materials available, some of which can be installed by the home owner; others require a contractor with specialist equipment for installation.

Different materials have different ‘U-values’ relating to its insulating properties, the lower the U-value the slower the rate of heat loss (i.e. materials with a low U-value are the best insulators). Thermal transmittance, also known as U-value, is the rate of transfer of heat through a structure (which can be a single material or a composite), divided by the difference in temperature across that structure. The units of measurement are W/m²K. The better-insulated a structure is, the lower the U-value will be. Workmanship and installation standards can strongly affect the thermal transmittance. If insulation is fitted poorly, with gaps and cold bridges, then the thermal transmittance can be considerably higher than desired. Thermal transmittance takes heat loss due to conduction, convection and radiation into account.

As you are unable to see cavity wall insulation there is no simple way of checking that it has been done properly. You should use a contractor who will issue a cavity wall guarantee certificate produced by the Cavity Insulation Guarantee Agency (CIGA), which covers the work standards and materials used [https://ciga.co.uk/] within 25 years from the installation date.

New Building Design

Buildings are designed in such a way that glazing faces the full southern sun in winter when energy is most needed.  The house should be located away for obstructions. Glass should have a solar heat gain coefficient (SHGC), between 0 and 1. This is a measure at how effectively sunlight is converted to internal heat. In Scotland, glass ideally should be around a SHGC value of 0.75 or higher to compensate for respective environmental conditions.

Alternatively, the Scottish Building Standards Agency say that windows must have a U-value (thermal insulation rating) of not more than 1.8 W/m²K.

Overglazing should be avoided: The building will be too hot in summer and too cold in winter. Materials with a high thermal mass should be used for construction and placed in the path of direct sunlight.

When buying a house it is extremely rare to find one that ticks all the boxes. One way to get the ideal home is to start from scratch, and this can be profitable too. If the new build is environmentally friendly and generates more power than it needs, the excess energy can be sold back to suppliers without incurring income tax or capital gains tax. Grants for microgenerators, central heating and insulation make the prospect even more appealing, especially with value added tax cut of 5 to 17.5 per cent on a range of energy-saving products.

Under the UK Government’s Renewable Heat Incentive (RHI) you are entitled to receive quarterly cash payments over seven years if you install an eligible renewable heating technology such as a heat pump (air source or ground source) or solar thermal (hot water) system. The RHI will reduce your payback period considerably and in some cases will cover the cost of installation.

Government grants of up to £2,500 per UK household are available for the installation of microgeneration technologies through the Department for Business Enterprise and Regulatory Reform's Low Carbon Buildings Programme. Grants can contribute to the cost of installing a certified product by a certified installer and the technologies supported include:

• Solar photovoltaics
• Wind turbines
• Solar thermal hot water
• Ground source heat pumps
• Small hydro
• Fuel cells
• Bio-energy
• Renewable CHP (combined heat and power)
• MicroCHP

In Scotland, the croft house grant scheme can help crofters build new homes. Rates of grant vary depending on where the croft is located but can be up to £22,000, with high priority going to remote areas with declining populations.

Energy Monitoring and Non – Residential Buildings

Energy monitoring in residential buildings often amounts to no more than keeping an eye on the quarterly bills or installing a smart meter . This is fine when it is easy to control and limit energy use. However in a non-residential building, the people with the responsibility for controlling energy use cannot go around switching off lights and turning down the heating. The energy transfer processes in a large building are very complex and environmental parameters such as temperature and air quality should be under the control of an automated Building Energy Management System (BEMS).

The BEMS sensors can also be used to monitor how the building is performing, or a separate network of sensors can be installed. It is important to monitor the temperature and energy performance of a building to check if the building functions according to its design,  with any discrepancy feeding into and improving on the design process. Buildings also degrade over time and information from monitoring systems can be used to trigger repairs and improvements.

A building may be continuously monitored with a permanently installed sensor network, or can be periodically surveyed by attaching sensors to the flow and return pipes of the heating system, recording the data captured at regular intervals with a data logger. The data can be processed later with reference to the measured room temperature to understand energy demand as a function of space and time. From this the BEMS may be reconfigured to reduce energy use and improve efficiency.

Figure 8: Green Building Design; Bosco Verticale (Milan, Italy) [source] (CC BY 3.0)

Non-residential buildings can be more energy efficient than domestic buildings because of their size difference, but are subject to more rigid legislative controls. There are rules governing air quality, temperature, acoustic emissions and illumination that may be in direct conflict with the desire to reduce energy use.

Figure 9: PV Solar panels situated on roof of commercial building. Making use of wasted space [source] (public domain)

In many cases the design will be more complex because of the additional constraints. Indoor Air Quality (IAQ) is an important issue which should be recognised in current designs. Indoor Air Quality refers to the air quality within and around buildings and structures, especially as it relates to the health and comfort of building occupants. Understanding and controlling common pollutants indoors can help reduce your risk of indoor health concerns. Health effects from indoor air pollutants may be experienced soon after exposure, or possibly, years later.

But with new non-residential (commercial) designs, the architect has the opportunity to experiment with  medium-scale energy generation measures that would just be inappropriate in the design of a residential building (Fig. 8): Large ground source heat pumps can be installed; medium-sized wind turbines can be used (10-75 kW) - at these generation capacities turbines become very economic; ventilation heat recovery becomes viable; PV panels may also be included. It becomes necessary to understand the ways new and renewable energy sources can be harnessed in buildings, and modelling must be used effectively to get a credible understanding of how the building will work. These skills are increasingly in demand in architecture and engineering consultancies, building analysis and design consultancies, utilities and regulatory organisations, local and national government, and academia.