We all know the basic statistics about the role of the built environment in global warming and carbon dioxide emissions.
Buildings are responsible for 50 per cent of energy use in the European Union. Housing is responsible for 60 per cent of this and heating energy typically accounts for up to 60 per cent of building energy use. Energy efficiency and renewable energy improvements to buildings, particularly housing, can bring great dividends in cutting CO 2, and can also help us deal with fuel poverty and raise living standards - a classic win-win situation.
How to insulate?
Modern buildings built to the best of current knowledge can achieve extremely low energy demands. How? Well, once you get into the detail, you realise that there are three principal aspects of building fabric design that have to be optimised to reduce energy use: thermal insulation; ventilation heat loss; and glazing heat loss.
Our energy modelling study (for a typical terraced house) shows this - illustrating the effects of gradually improving specification.
At what we call the 2000 Standard (actually the same as 2002 Part L1, but we think at least two years late), the heat loss is 50 per cent in ventilation, 30 per cent through glazing and 20 per cent through the opaque surfaces. In going further it is obvious that we need to increase airtightness, consider heat-recovery ventilation and improve the glazing specification, and the modelling shows how the energy use is gradually reduced by a series of improvements in these areas.
But what is an optimum level of insulation? Clearly a balance has to be achieved between whole-system performance, capital cost and buildability. We think a reasonable balance is offered in our 'LowHeat Standard', which reduces heating energy use by around 75 per cent compared to 2002 regulations, and 93-98 per cent compared to a typical existing UK dwelling.
This standard can, we believe, be achieved at low cost and without compromising living quality. And if it was to be applied at a large scale, perhaps replacing inefficient and poorly performing existing housing, then we could reduce total heating demand by 15-50 per cent (and cut fuel poverty and increase living standards).
Which insulation material?
It is a common reaction, when designing 'green' buildings, to think about the materials first. We can touch materials, they are in front of us, whereas energy is invisible (and, for some, incomprehensible). For example, embodied energy is a common criterion for rating materials on environmental impact.
It is a factor, but there are two problems with focusing on it. First is that most of a building's energy use in its life will be energy-in-use, namely heating, lighting, cooking, etc. Of course, the figures will vary considerably, but on average for a 2002 standard building and a 70-year life, the embodied energy is about 15 per cent of the total. Increase the life to a more realistic 100 years, and the embodied percentage is closer to 10 per cent. Now it is clear that to reduce energy use you start by worrying about the 90 per cent, not the 10 per cent. Once you've made the 90 per cent nearly disappear, you can start worrying about the materials again, although don't make a materials switch if it will affect in-use performance!
The second issue with embodied energy is that it is particularly misleading in relation to energy-efficiency materials. The embodied energy of an insulation material is about 2-5 per cent of the energy it saves over its lifetime. More importantly, if you look at the total building embodied energy, keeping all other assumptions constant, changing the insulation material makes very little difference.
There are two key issues in choosing insulation material. The obvious factor is achieving the required thermal performance, but achieving longevity of performance is also important. A house or other building may last 50-150 years, and the insulation will probably not be upgraded in that time. If the insulation does not perform, then the heat loss will increase - in some cases by up to 100 per cent.
There is little knowledge in this area and more research is required (the widespread use of insulation is relatively recent). However, it is clear that the 'risk factors' affecting potential failure are something that designers must increasingly consider, and something the industry should research and report on.
Case Study - LightWeight AirTight LowHeat Finally, let's discuss a case study example which shows one route to achieving the LowHeat Standard within normal (indeed social-housing) budgets. It may be fair to say that the most innovative approaches to lowemissions housing in the past few years have tended to follow a thermally heavyweight, wet construction approach. But it's not the only approach. In our LightWeight AirTight LowHeat project we are building social housing to standard budgets, while optimising the building and the energy systems.
Working with Longhurst Group, a residential social landlord in Nottinghamshire, and contractor Robert Woodhead, we are building two houses using the TekHaus system of structural insulated panels (SIPs) by Kingspan, which achieves very good U-values (0.21 W/m 2K) and excellent airtightness. Predictive energy modelling using dynamic thermal analysis has been used to predict the energy performance under a range of different occupancy conditions, and for different specifications, and allowed us to consider different servicing options to look at the best for energy use and carbon-dioxide emissions.
We found that the building is so airtight that you can eliminate the radiator system and replace it with a heat-recovery ventilation system with a heating coil in the air-supply duct - and eliminating one pays for the other. The result is a house with heating energy use of 16-32 kWh/m 2yr(compared with 40-71 kWh/m 2yr for a 2002 regulations building, and 150-241 kWh/m 2yrfor an existing building to the same dimensions and orientation), and gas costs of around £30 a year (compared with £90 and £340 respectively).
And by the way, this house can easily become carbon-neutral through the addition of biomass heating, small wind turbines, or photovoltaics in a number of different combinations - the lowest cost option is achievable at about £7,500.
Robert Webb is managing director of XCO2 Conisbee