By Mr. John Brennan; Architecture & Landscape Architecture, Edinburgh College of Art
Introduction by Dr Faye Wade, Science, Technology and Innovation Studies, University of Edinburgh
With 74% of Scotland’s household energy use occurring through space heating, and another 13% through water heating, the built environment has to be part of any strategy for a more sustainable future. Indeed, the Scottish Government’s Energy Strategy seeks to make Scotland’s buildings near zero carbon by 2050, delivering this through Scotland’s Energy Efficiency Programme (SEEP), which includes funding for the retrofitting of existing homes, and seeks to increase the adoption of low carbon heating solutions. However, in order to achieve the Energy Strategy’s ambitious target to achieve a ‘low carbon transformation’ in the built environment by 2050, it is important to think about the realities of delivering low carbon buildings. John Brennan, Programme Director for an MSc in Advanced Sustainable Design, considers some of the challenges:
Over the years, we’ve witnessed a transformation in the regulation of energy consumption in the housing market. In this period I designed and delivered a series of low carbon homes and now reflect that we must be careful what we wish for. Governments have an obligation to deliver on agreed national and transnational targets and in Scotland a vehicle for this has been the various iterations of The Sullivan Report. In its current guise its ambition is to enforce ’nearly zero new buildings’ by 2019.
A significant influence is the Passivhaus Institut. Independent of any government, it advocates a relentless focus on heat loss until predicted energy loads are negligible. This is achieved through compact shape, orientation for passive heat gain, superinsulation, and near total air tightness harnessed to mechanical ventilation and heat recovery. Metabolic gains from occupants, incidental gains from the equipment they use and heat collected from solar gain through windows should be enough to keep the house comfortable even in winter. Intentionally, there is no obligation to employ renewables. What is attractive to regulators is a design method defined as a series of quantitative thresholds. A passive house is only a passive house when it achieves a verifiable dwelling air change rate of 0.6/hr@50pa and a projected heating load of 15kWh/m2/year
In theory, this seems inherently virtuous but in practice can deliver new housing stock in danger of being neither energy efficient nor fully fit for habitation. The nub of the problem lies in the nature of the construction industry. Achieving a quantitative standard requires careful design and obsessive quality control. This is possible in bespoke operations or sophisticated manufacturing techniques but neither of these traits are characteristics familiar to the UK construction industry. To achieve air tightness approaching passive house standards requires a level of fit otherwise reserved for the car industry. [fig.1] Passive house is an achievable but inherently fragile standard.
As of now, the national baseline for air tightness is around 5 times less onerous than the passive house standard, but past experience suggests a progressive uplift over time to deliver the promises of Sullivan. Over forty years, insulation standards have increased five-fold with little in the way of adverse effects for the householder. Seeking to replicate this process for air tightness will be more troublesome. Increasing air tightness can degrade air quality in our homes. Recent research shows that current standards for air tightness without effective mechanical ventilation either put occupants’ health at risk or make a mockery of energy conservation in the heating season through intermittent episodes of uncontrolled exfiltration (otherwise known as opening windows) . However as passive house has gifted the means to define clear air tightness targets to our regulators, the likelihood is that the baseline standard will become progressively more onerous.
The nub is that environmental regulation in building must be based on quantitative standards given the sheer volume of individual projects that require oversight and approval. Governments therefore only manage what can be measured; an approach that tends the certainties of measurement methodologies as found in Passive House. However, the realities of housing design are more complex. It can be explained by Dean Hawkes who identified two fundamental strands in environmental design, those of exclusive and selective environments. [fig.2]
Exclusive Mode buildings operate through separation of internal and external environmental conditions and regulation of thermal comfort is through mechanical means. Except in relation to collecting solar gain, Passive Houses are Exclusive Mode buildings.
Selective Mode buildings embraces surrounding climatic and environmental contexts. They maximise solar gain and encourage unassisted ventilation flow. They have much more freedom in shape and form, and are not compelled to seek out the most efficient way to enclose a particular volume. They remain low energy buildings but recognise that depending on time, season and location, some form of heat is required to maintain comfort in the home. The selective mode approach to building services are that ‘mechanical systems for heating, cooling and ventilation and lighting should be regarded as supplementary to the primary control provided by the selective built form’. 
On the face of it, housing standards that reduce heating demand to a negligible level are praiseworthy. However, it comes with a susceptibility to poor air quality and dependency on mechanical plant, all of which delivered by industry with an entrenched inability to deliver decent levels of fit and finish. Selective Mode building accepts that from time to time, heating and cooling is required to maintain comfort. In this context, the question is then to consider those forms of generation that are most benign in their impact.
I would suggest it is inadvisable for building regulation to focus on ever higher standards of air tightness as a means to progressively reduce heating demand in our homes. To use a medical analogy, such medication can be accompanied by unpleasant side effects. Our legislators should continue to recognise that renewable energy only plays a part in reducing our carbon footprint, but also is more likely to keep our households healthy.
We don’t usually value our homes in terms of their measurable performance. Some of us enjoy the comfort, control and running costs that come with a finely calibrated passive house which embodies the best of the exclusive tradition in environmental design. Many others wish to have homes that have greater freedom in their design, a closer relationship to external worlds and the ability to stand up to robust treatment by the householder. These are the key characteristics of the selective tradition and ideally, both selective and exclusive modes should be accommodated for by our regulators. It may seem counter intuitive, but now I tend not to define the benefits of renewable energy technologies in terms of resource use and carbon impact. Instead, we should advocate their use to enable healthy, forgiving and long-lasting homes that help us all along the way to having the freedom to live as each of us wish.
 Howieson, S.G., Sharpe, T. & Farren, P., 2014. Building tight – ventilating right? How are new air tightness standards affecting indoor air quality in dwellings? Building Services Engineering Research & Technology, 35(5), pp.475–487.
 Hawkes, D., McDonald, J. & Steemers, K., 2002. The Selective Environment, London: Spon Press.
Passive House Construction
These photographs of a passive house designed and built by our practice shows the disconnect between the domestic interior and the materials, technology and precision required to reach required levels of air tightness. [photographs Julie Wilson]
Exclusive and Selective Mode Design
Both are entirely valid approaches to designing sustainable homes. The illustration shows a cross section through a house identical in its rooms and shape. The top illustration shows how Exclusive Mode buildings are fully sealed and rely on mechanical ventilation. Below this, we see how Selective Mode buildings have a more porous relationship to the outside often using buffer spaces to collect heat and mediate temperature. [illustration John Brennan]