In the previous issue I wrote about the impact buildings have on greenhouse gas emissions and the potential impact of climate change on the design of buildings. We are all going to be hearing the phrase “net zero” a lot more over the next few years as the UK attempts to balance its environmental books by ensuring the CO₂ we release does not exceed the CO₂ removed from the atmosphere.

To some, net zero means replacing a petrol or diesel car with an electric model and paying someone else to plant a tree. In reality achieving the government’s goal of going net zero by 2050 is going to require some significant changes to the way we approach many aspects of our lives, including how we construct and operate our buildings.

As noted in the previous issue, buildings contribute to the UK’s greenhouse gas emissions through their embodied carbon (that is the CO₂ associated with the building fabric and its construction) and their operational carbon (the CO₂ associated with the operation and use of the building). This article looks a little deeper at a few specific issues relating to a building’s operational carbon and considers the ways in which good building design can play a significant role in meeting the nation’s target of net zero by 2050.

Design strategy for net zero buildings

At the highest level, the overall strategy for achieving a net zero building is to minimise the energy required to operate the building and then to generate that energy from renewable sources (i.e., not fossil fuels). In implementing this strategy, there is a clear hierarchy:

1. Reduce demand for heating, lighting, mechanical ventilation, etc.
2. Deliver the heating, lighting, etc. as efficiently as possible.
3. Use the building to generate renewable energy.

While it may be tempting to think covering a roof with photovoltaic (PV) panels is a quick fix that ticks the sustainability boxes, there is little point generating renewable energy and then wasting it. Indeed, as pressure to eliminate fossil fuels increases, the demand on renewable energy sources will also increase and this scarce resource will need to be used wisely. Energy efficiency, therefore, needs to be at the heart of any building design strategy.

Depending on the use of the building, energy demand can be divided into the following categories:

• Heating
• Lighting
• Cooling/ventilation
• Processes relating to the use of the building (not usually the responsibility of the building designer)

Heating

From a building designer’s perspective, minimising heating demand is a matter of minimising heat loss through the building envelope. Of course, turning the thermostat down by one degree and closing the windows while the heating is on would also help, but that is out of our control. Thermal energy (heat) will always try to move from a relatively hotter location to a colder one and can do so by a combination of conduction, convection and radiation. In a heated building, the particular heat flow paths of concern are conduction through the walls, floor and roof, conduction through cold bridges and convection through gaps in the building envelope. To achieve an energy efficient building, all three of these issues need to be considered by the building designer. Installing 300 mm of insulation but forgetting about the joints around the doors, windows and service penetrations is of little use.

Of the three heat paths, the conduction through the roof, walls and floor is the easiest to tackle and a range of insulation products and insulated panel systems are widely available with plenty of technical literature and support to help building designers specify the product that best meets their needs. The thermal performance of a building element (i.e., an insulated wall or roof) is usually referred to as its “U value” and is quoted in W/m²K, i.e., it is a measure of how much energy escapes per second (in Watts) for every square metre of roof or wall per degree of temperature difference (a temperature difference of 1 Kelvin (K) is the same as a difference of 1°c).

Thermal bridging is a trickier issue to deal with because the strongest materials best suited to load carrying are also the most conductive. Thermal bridging through fasteners is generally considered to be small and is accounted for in the U value for the roof or wall, but larger metal objects, such as steel support beams for balconies or rafters penetrating the building envelope (architects!) can be a major source of heat loss if they are not detailed correctly. Furthermore, the loss of heat will cool the beam or rafter on the inside of the building giving rise to a condensation risk. In some cases, for example balconies, special low conductivity connections have been developed and should be specified where possible. Building designers should seek specialist help if they encounter this situation.

The third source of heat loss, leaky joints, is best dealt with via good detailing, or – more likely – by someone with a tube of sealant. Most building envelope solutions used in industrial and similar commercial buildings are reasonably air-tight by design, especially if the installers follow the manufacturer’s guidance in terms of filler blocks and sealant. The issues tend to be at doors, windows and service penetrations, where gaps are often ignored or left for someone else to fill. This is one case where attention to detail can make a significant impact.

Lighting

The simplest way to limit the demand for artificial lighting is by ensuring there is sufficient natural daylight entering the building. For agricultural and industrial buildings, this is usually achieved by installing rooflights, although north lights are an alternative. The quantity and location of the rooflights is critical to the energy performance of the building. Too many rooflights could result in overheating in the summer (turning the building into a greenhouse) or excessive heat loss in the winter. Poorly positioned rooflights that illuminate the tops of racking storage rather than walkways are of little use, so the building designer needs to co-ordinate the design of the roof with that of the internal layout of the building. The National Association of Rooflight Manufacturers (NARM) has specialist guidance available. The internal layout and decoration of a building can enhance the effect of the natural daylight by reflecting the light and avoiding unwelcome shadows.

Of course, some artificial lighting will always be needed, and the building designer needs to ensure this is as energy efficient as possible. As with rooflights, the quantity and position of artificial lighting is critical to its performance and can be enhanced by a well-designed internal layout and decoration. Specifiers can choose from a wide range of low energy lighting solutions (i.e., LEDs) and should also consider sensor-operated controls to avoid lighting unoccupied rooms.

Cooling/ventilation

As global temperatures rise and rare heat waves become more common, building designers have to give serious consideration to the risk of overheating. To make matters worse, buildings are often filled with heat generating devices such as computers and, in the agricultural sector, livestock can be a significant source of heat. Related to the need to cool the building interior, is the need to supply fresh air for the health and welfare of the building occupants (human and livestock).

Both of these matters can and should be addressed as part of the building design process by ensuring a plentiful, controlled and well-directed flow of natural ventilation, noting that a well-ventilated building is not the same as a draughty one! The alternative is noisy, expensive and energy intensive air conditioning that does little for the welfare of the building occupants and only adds to the building’s carbon emissions.

Conclusions

The operation of our buildings account for a significant proportion of the UK’s greenhouse gas emissions. But reducing the energy demand by minimising heat loss, and through the careful design of the building, building envelope and building services, is within the control of the building designer. Small changes to the specification and attention to detail can often have a major impact on a building’s carbon emissions and improve the welfare of the people and animals who occupy it.

Dr Martin Heywood
RIDBA Technical Consultant