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Steps to Net Zero: Step 1 - Passive Design Optimisation

Net Zero Carbon By Simon Wyatt, Partner, Sustainability – 30 March 2020

Wilsthorpe Community School exterior red brick building


Simon Wyatt in front of office facade in a dark suit and blue shirt

Simon Wyatt

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As outlined in our recent blog, Step by step: How to achieve a net zero building, everything starts with passive design optimisation - without this a building cannot hope to be net zero carbon.

Passive design is the fundamental principle of utilising the external climate to maintain a comfortable environment with minimal or no active lighting, heating, cooling or ventilation systems. The best known example of this is the international Passivhaus Standard which uses the building envelope, to balance heat gains and losses to minimise the need for traditional heating.

This starts with the façade design, as the façade is the intersection between the building and the external climate where it is able to passively harness solar heating and daylighting; fresh air for breathing and removing heat gains; in addition to restricting heat losses and optimising internal thermal comfort.

The primary consideration should be to deliver the best possible daylight for occupants. This does not mean as much as possible, as too much glazing leads to glare, excess solar gains, poor uniformity and typically results in the overuse of internal blinds. A more appropriate metric is the useful daylight index which sets maximum and minimum values for daylight – typically leading to glazing to wall ratios of up to 50%.

A key requirement for becoming a net zero carbon building is meeting the challenging energy intensity targets. In order to achieve these, buildings will need to be fully or partially naturally ventilated for a substantial proportion of the year, supplemented by mechanical systems to provide heat recovery in winter and comfort cooling in summer where required.

By designing the façade to optimise daylighting then it allows for natural ventilation for longer periods of the year when compared with over glazed solutions. Natural ventilation openings should be designed into the façade to allow for effective ventilation for much of the year. In locations with air quality and/or acoustic concerns, openings should still be included but may not be utilised from the outset. But as the air quality in our towns and cities improves with the use of electric vehicles then these openings will be utilised more frequently.

The size and depth of the floor plate will have a significant impact on both the building’s ability for passive daylighting and natural ventilation. Designs should therefore ideally be configured with this in mind, but this is not always possible. In these instances, mechanical ventilation and artificial lighting will required all year around in the internal zones, whilst the perimeter is able to benefit from passive measures.

An area for careful consideration is the use of thermal mass to regulate internal temperatures, which when used correctly can achieve amazing results. But it needs careful management to recharge the mass using night cooling. It also needs to be considered as part of the wider whole life carbon assessment, because concrete, which is typically used to provide the mass, is a high embodied carbon material. Less intensive solutions like phased change material can be used but still need careful consideration.

The building envelope itself is an important element for controlling thermal heat loss and infiltration, and when considered as a whole these elements can reduce the need for active heating and cooling. Where appropriate the Passivhaus standards should be considered, i.e. heat lead buildings like houses. For other types of buildings, the general principles should be applied where appropriate. This should include how Passivhaus treats things like repeat thermal bridges in their u-values calculations and only use worst-case performance for insulation material not the average as is done in the Building Regulations.

Non-repeating thermal bridges: where there is a one-off thermal bridge, for example, a structural column in an insulated wall should be designed out where possible or improved upon if they can’t.

Current treatment of passive design is largely overlooked or grossly oversimplified; only considering fabric efficiency (i.e. U-values, G-values, air permeability) not the wider performance issues.

One area of the industry that seems to have a much greater understanding of passive design optimisation is the educational sector. Cundall advised the Education Funding Agency in 2014 on the design principles for their baseline schools and are currently working with the Welsh Government on their standards. As part of this project Cundall defined the daylighting, acoustic, ventilation and thermal comfort standards. These originally required classrooms to be lit from two sides to achieve daylight and uniformity standards, in addition to having minimum ventilation opening areas and cross ventilation requirements. All these lead to higher quality low energy teaching spaces.

It is essential that the wider industry start to put passive design optimisation at the heart of the design process. This is the first and arguably the most important step on the path to net zero carbon. If we set off in the wrong direct at the start, then the chances of achieving net zero will be lost.

We'll be going into more detail about each of the steps identified above over the coming weeks. For more information in the meantime, head over to our website to see how we can make your project more sustainable.

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