The Design Review Panel is very pleased to bring you the excellent blog article below, written by Paul Copper. In 2013 Paul designed his first building to the passivhaus standard and became a Certified Passivhaus Designer. Paul is an Architect and Certified Passivhaus Designer with Perigrine Mears Architects.
"When considering a new build project, the first thoughts are usually around where it's going to be sited, what needs to be included in the building and what might it look like. Today, more clients, particularly self-builders, are looking further ahead and considering running costs and energy use.
As an architect, I don't think low energy use and running costs are features we should reserve for those who ask, it should be something we offer to all clients, whatever their project. I would maybe go a step further and say that given we are in a privileged position to influence and design our built environment, we have an obligation to make all our buildings low energy, to ensure new buildings avoid unnecessary energy use. Regardless of your thoughts on global warming, most of the energy we currently use is from finite resources. Low energy buildings will not only make better use of those resources, they will also limit the effects of escalating costs as those resources diminish and make better use of renewable energy.
While there are other factors which affect running costs, for example maintenance, cleaning and redecoration, delivering a low energy building will result in a significant reduction in running costs. However, there is more to sustainable design than just reducing energy use, we need to consider the environment we are creating. At its simplest, building is about shelter and protection, from the sun, rain, wind outside, and of your worldly belongings within. Given we spend most of our time indoors, it is also about creating environments which are comfortable and healthy.
The energy use of a building could be reduced simply by lowering the internal temperatures to 16ºc, but this would produce an environment which most people would consider uncomfortable and would certainly result in unhealthy side effects, such as mould growth. So, energy use needs to be considered alongside the criteria necessary to deliver buildings which are comfortable and healthy.
With these considerations in mind, in the late 1980s Dr Wolfgang Feist used research data into what people perceive to be comfortable and building science to establish building performance targets for the delivery of comfortable, healthy and low energy buildings. The Passivhaus Standard was the result of this research.
What makes a building a Passivhaus?
For a building to be a passivhaus, there are building performance criteria to be met and proven through modelling, for example limits on annual energy use, airtightness and frequency of overheating. There are passivhaus design principles which can be followed to assist with meeting these criteria. The principles follow the ‘fabric first’ approach, a strategy I had already adopted prior to learning about the Passivhaus Standard. Rather than using the budget on eco-bling (the visibly ‘green’ additions like solar thermal panels, photovoltaics etc.), the passivhaus principles focus first on the building fabric, using budget to optimise the unseen materials and construction details to deliver high quality, low energy buildings. Improving the basic performance of a building can be very difficult to do retrospectively, so it’s best to get this right during construction. On the other hand, renewables are advancing quickly and are usually relatively easy to add to buildings, so buildings be designed to receive future installations.
The Passivhaus principles
1. siting, orientation and form
First considerations relate to the topography and setting which are unique to every site. Where can you locate the building within your site to maximise the daylight and sunlight received and minimise the effect of prevailing wind. Part of your site may be overshadowed by neighbouring buildings, so if planning and other constraints permit, it would be desirable to locate your building elsewhere on the site. The benefits are obvious, access to sunlight will provide useful solar gains and more daylight will mean less reliance on artificial lighting, both of which mean less energy use.
The orientation of a building, particularly the aspect of the windows, will affect the ability of a building to benefit from useful solar gains. Buildings lose more heat through windows than they do through walls, but windows do also provide solar gain which can be a benefit during the heating season. The gains through windows to south facing elevations more than compensate for the heat losses, whereas the losses from north facing windows far exceed any gains. Therefore, it is preferable to have larger windows oriented to the south with smaller windows to the north.
While considering window positions, careful consideration needs to be given to solar shading. While heat gains during the heating season help reduce energy, summer gains will result in overheating if not managed. The only way to do this effectively is to provide external shading (brise soleil, balconies, blinds, shutters).
Windows facing east and west will also benefit from useful solar gain, but owing to lower sun angles to these elevations (morning and evening), shading of windows to these is harder to achieve and options more limited (blinds, shutters).
The form of the building also affects heat loss. By keeping a building compact you will have less surface area through which heat can be lost for the same floor area. For example a single storey U-shaped building could have as much as 40% more surface area of a two storey, compact building of the same floor area. There are obvious cost benefits for a client too if you’re using less building envelope to create the required area.
Insulation levels in a building affect energy use and comfort. The more insulation you wrap around the building, the less heat loss you'll have, resulting in lower energy use to replace the lost heat.
Higher insulation levels also result in warmer internal surfaces, providing greater levels of comfort and eliminating mould growth. Careful attention must be paid to junctions to ensure insulation is continuous (no thermal bridges) to avoid cold spots.
Passivhaus doors and windows are generally tripled glazed to meet the required performance criteria, which also provide great sound reduction which may be considered a bonus in urban environments.
The thermal performance of a passivhaus is significantly better than new buildings meeting the minimum standards set down in the Building Regulations. Rather than arbitrary U-value targets incrementally improved every few years, as is the case with Building Regulations, the minimum U-value targets for passivhaus have been set/optimised to deliver comfortable and healthy buildings, based on research data and scientific study. As such, passivhaus performance targets won't be changing.
Another significant cause of heat loss from existing and many new buildings is air leakage. Trying to heat leaky buildings is like trying to fill a bucket with a hole in it. It will take longer to warm a leaky building and will require constant heat input to maintain a set temperature when it's reached it.
Passivhaus sets a high minimum airtightness target. As a comparison, Building Regulations have a minimum performance equivalent to a hole the size of a 20 pence piece in every 1m2 of external envelope, while the minimum Passivhaus target is equivalent to a hole the size of a 5 pence piece in every 5m2 of external envelope.
All buildings need a fresh air supply, otherwise comfort levels will decrease as CO2 and humidity levels increase. Given the high levels of airtightness in a passivhaus and the eradication of unmanaged an air supply is required is ventilated however the high levels of airtightness in a passivhaus enable the incoming fresh air to be managed.
A common misconception is that a Passivhaus is a sealed box, but this isn't the case. During the non-heating season, ventilation of a passivhaus is generally achieved by opening windows.
During the heating season however, windows are kept shut. While most other buildings will receive 'fresh' air by virtue of uncontrolled draughts, a Passivhaus is provided with a controlled supply of fresh air via a ventilation system.
This is the passivhaus criteria I struggled with most when learning about the standard, as a ventilation system doesn't seem passive, however when you understand the system, it makes perfect sense.
All new homes have extract ventilation from bathrooms and kitchens, it's a Building Regulations requirement. Whereas a normal building will extract the warm, stale air and discharge it straight to outside (throwing away useful heat at the same time), a passivhaus uses Mechanical Ventilation with Heat Recovery (MVHR). This system extracts the heat from the exhaust air and uses it to heat the incoming air supply. The result is you don't waste heat and you have a warm fresh air supply rather than cold draughts.
5. low energy systems
With the energy use associated with heating reduced to a minimum in a passivhaus, other forms of energy use become more significant, for example lighting and appliances. The passivhaus standard encourages the use of the best rated appliances, systems and lighting. While generally there are no minimum standards set, the energy use of key appliances and systems do have a bearing on the passivhaus modelling. There is a minimum efficiency and energy use set for the MVHR system.
6. renewable energy
The final principle, after all the passive measures have been addressed, consideration is given to renewable energy. With budgets usually limited, it is better to focus available funds on the building fabric and passive elements which can positively contribute to a buildings performance, before including renewable energy. Of course, even if renewable energy systems aren’t to be included at the outset, consideration should be given to how these can be incorporated at a later date, for example south facing roof slopes for solar (PV or thermal) panels.
Once you know the rules…
As with all rules, once you know them, you can bend them. The passivhaus principles are a good framework for designing buildings which will comply with standard. However, there are often other factors which influence the design of a building, for example planning restrictions or views from the site. These can be accommodated with the passivhaus standard, but where you stray from some principles, compensation must be made elsewhere.
The first passivhaus project I was involved with was on a site next to an RAF base and the views from the site were to the north across rolling countryside. So with the form of the building restricted to single storey and the larger windows oriented to the north to capture the views, the designs went against the passivhaus principles. In this instance, these characteristics were accommodated by significantly improving the thermal performance of the walls, floor and roof."
Finding out more
Further information about Passivhaus is available from the following organisations:
The Passivhaus Trust (www.pasivhaustrust.org.uk)
International Passive House Association (www.passivehouse-international.org)
The UK Passivhaus Conference 2017 takes place on 24th October at the Business Design Centre, London.
Published by The Design Review Panel
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