Building to reduce energy demand

Materials World magazine
,
1 Mar 2017

Peter Wilson looks at the role of the built environment in reducing our energy needs.

Debates about energy generation and supply, whether from nuclear, fossil fuels or renewables, and the vexed issue of government subsidies or not – and for which options – have been current for so long that we seem to have almost forgotten the reasons why more and more power is required and whether simply producing more of it is the correct strategy to pursue. A quick analysis of those areas of our economy and daily life makes clear that one of the largest consumers of energy is our built environment. But, more importantly, it indicates what an inefficient user of heat and electrical power it is. This can in part be attributed to the large proportion of the UK’s building stock that was constructed before the environmental and financial cost of producing energy made us aware of the need for good thermal insulation and the massive reductions required in heat loss through floors, walls and roofs.

Given the many variations in architectural design and size involved, it is notoriously difficult – and invariably expensive – to effectively retrofit our traditional buildings to achieve modern standards of insulation and airtightness, with consequent reductions in heat energy required. But, while many ingenious, project-specific solutions continue to emerge as well as increasing numbers of innovative, replicable systems that are appearing on the market, all too often it is simply not financially viable to follow this path. This is not the place to discuss the iniquities of a taxation system that levies a substantial penalty on work to old buildings while largely absolving the new of this burden, but there is no doubt more could be done to make our existing building stock substantially more energy efficient. 

The UK market

Our property market too, in preferencing the upfront capital cost of construction over the life cycle cost of a building (its construction, maintenance, energy use and, ultimately, deconstruction), has undoubtedly contributed to a lack of concern for long-term running costs, responsibility for these being passed on to the purchaser. Caveat emptor (let the buyer beware) has long been the mantra, but consumer protection measures have increasingly led developers to not only address the issue of wasted energy and concomitant (and ever-rising) cost, but also to recognise the commercial advantage to be gained in offering highly energy-efficient buildings to the market. 

It would be fair to say that the housebuilding sector has long been something of a guilty party in its often indifferent approach to environmental standards, but slowly, progressively, this is changing. This is attributable to new – and more onerous – building standards that have to be met but also, more recently, to the introduction of substantial Government incentives – the “carrot” designed to encourage house building. Presuming the industry can deliver on targets of 250,000 houses per year in the UK over the next four years (a radical industrial upscaling from the 100,000 or so homes currently being built), these need to meet significantly higher levels of airtightness and thermal insulation if we are not to create major additional pressures on our energy generation and supply networks. 

Not all new houses are produced by volume housebuilders, however, and the Government’s recent Self-Build and Custom Housebuilding Act (2015), Housing and Planning Act (2016) and forthcoming Housing Bill (2017) all recognise value in encouraging more individual approaches to the construction of new homes, combining to stimulate the release of publicly-owned land for this purpose. These initiatives complement a noticeable increase in interest in self-building – a broad term that embraces both the commissioning of a house from an architect and the direct employment of a building contractor to carry out the work, as well as individuals carrying out all of the work themselves. With house prices being what they are, the latter is undoubtedly a growing (and sizeable) market but the former has increased exponentially, no doubt encouraged by the plethora of television programmes that have emerged in recent years to demystify the design and building process.

Yet, the rising standards mentioned above still have some way to go to meet the best of what has been achieved in Europe over the past three decades. The passivhaus (German for passive house) concept, for example, was developed in Germany in as 1988 and, with more than 65,000 buildings now completed to its standards, is becoming the increasingly accepted gold parameters for reducing the energy requirements of new buildings, particularly houses. Generally speaking, buildings constructed to meet its certification requirements achieve around a 75% reduction in space heating compared with UK standard practice for new-build. So what are these standards, and are they difficult to achieve? In answer to the latter, it is fair to say that individually their delivery is relatively straightforward – it is in their combination that the passivehaus bar becomes much harder to reach, but the goal is worth stretching for. Essentially, there are eight standards (see right) that, in conventional housebuilding terms, may appear expensive to deliver, but over the life of the building significant savings in maintenance and energy costs can be demonstrated.

Passivhaus construction

The fundamental principle of passivhaus is simple – reduce the heat loss of a building to such an extent that it hardly needs heating at all. According to the UK-based Passivhaus Trust, the sun, human occupants, household appliances and the warmth from extracted air covers the majority of a house’s heat demands. Passivhaus construction requirements include enhanced levels of insulation with minimal thermal bridges and well-considered use of solar and internal heat gains. As these houses are highly airtight, whole house mechanical heat and ventilation recovery systems are necessary to provide highly efficient heat recovery and excellent internal air quality. Any shortfall in heating is usually met by a small unit, such as a woodburning stove. 

The passivhaus standard does not necessitate the use of repetitive designs or serially produced construction processes and systems, leaving the individual layout and look of a house to its owners and architects. It is not only suitable for detached houses in rural situations – increasingly, passivhaus projects are emerging in high-density urban areas.

An example of the former is Kirsty Maguire Architect Ltd’s design for the Hayshed Farmhouse in East Ayrshire, UK. The house was conceived to complement an existing farm cluster and thereby root it to its agricultural context, while improving and refining the Scottish tradition of timber construction to exceed passivhaus standards. The structure is a combination of timber frame walls made with I-joists to give the additional depth necessary to accommodate increased levels of thermal insulation, with glulam (glued laminated timber) beams used to create the roof’s curved agricultural barn form. The whole building is clad with recyclable seamless zinc sheet coloured to match the local red earth. 

The overall savings are impressive, with U-values for the floor, walls, roof and windows of 0.12, 0.11, 0.11 and 0.84W/m2K, respectively, a primary heat demand of 101kWh/m2/year, a space heating load of 9W/m2, an air-to-air heat pump for hot water and an air change rate of 0.22 at 50Pa. There are even plans to add a domestic turbine to harness the local wind and enable the house to be powered entirely from the site itself. With a total floor area of 162.5m2, and a construction cost of £1,750/m2 (including internal fit-out, garage and landscaping), the house is reasonably priced, but its energy performance is remarkable – better than originally modelled, the energy bills are more than 90% less than those of the old farmhouse, with an overall aim for the building to generate significantly more energy than it uses, also making it a ‘PlusEnergy’ and ‘zero carbon’ house.

By contrast, the single detached house in the London Borough of Hackney’s Lansdowne Drive is decidedly urban in its design, its infill position in a conservation area being shaded by a four-storey Victorian terrace, preventing direct sunlight reaching it from the south. This two-storey house has an in-situ reinforced concrete lower level, the superstructure above which has been constructed from prefabricated cross laminated timber (CLT) panels, the material of choice in a borough where large numbers of apartment buildings are currently being erected using this environmentally-effective product. With a total floor area of 94m2, it is slightly smaller than the Farmhouse but – because of very high building costs in the capital – has been more expensive to construct, at £3,000/m2. Nevertheless, this is still relatively cheap for a detached house in London, albeit that there will likely have been fairly high land purchase costs to include in the total bill. 

For passivhaus, though, energy performance is everything and this being the first such certified house in Hackney, the Lansdowne Road property delivers outstanding credentials. Primary energy use has been measured at 83.73kWh/m2/per year, with a space heating load of 8W/m2 and U-values for floor, walls, roof and windows of 0.103, 0.113, 0.074 and 0.06 W/m2K, respectively. The main construction components have been left exposed internally (the hygroscopic properties of the CLT adding to the air quality), with the exterior given a cladding of zinc sheet. On the face of it, a simple, unobtrusive design and construction, but one that has been recognised as an urban exemplar by its success in the UK Passivhaus Awards 2016. 

Initiatives like passivhaus are unquestionably driving construction quality forward and, with it, a different perspective on how the energy demands of our built environment can be progressively reduced. Passive technology – undoubtedly. Pro-active measures to reduce energy consumption – certainly. 

Passivhaus standards:

  • Total energy for space heating and cooling of less than 15kWh per m2 per year for treated floor areas.
  • Total primary energy use for all appliances, hot water, space heating and cooling of less than 120kWh per m2 per year (low energy household appliances are a must).
  • The exterior shell of the building must be insulate to a U-value not exceeding 0.15W per m2K.
  • Houses to have a southerly orientation for solar gain.
  • Construction must be free of thermal bridges.
  • Windows with U-values not exceeding 0.8W per m2K for glazing and frames. At least 50% solar heat gain co-efficient through glazing. 
  • An air change rate of no more than 0.6 air changes per hour at 50Pa.
  • A mechanical ventilation system with highly efficient (more than 80%) heat recovery with ground heat exchanger. 

 Peter Wilson is an architect and managing director of Timber Design Initiatives Ltd, a UK-based company delivering Europe-wide approaches to education, innovation and demonstration of best practice in the use of wood and advanced timber technologies in architecture, design and construction.