Sustainable costs – the future is passive

Home energy use is responsible for over 25% of UK carbon dioxide emissions. Energy costs are rocketing, but there is a greater awareness of environmental sustainability. We already have the revised part L in force and the EU Energy Performance of Building Directive will soon be upon us, requiring new buildings to have a certificate detailing their energy usage.

Energy-efficient homes can be built according to one of two approaches: passive solar emphasises orienting the house to the sun while super-insulation emphasises improving the thermal efficiency of the structure.

Passive solar

Some homes in Europe have been designed and built without the need for radiators, fan heaters, stoves or air conditioners. Heat is generated from sunlight coming through windows, from people who live there and from household appliances. There is evidence that these ‘passive’ homes can achieve internal temperatures of around 21 degrees centigrade in winter. This is actually warmer than the average British home. Solar domestic hot water systems added to the roof or even the south-facing wall can also cut the requirements for heating water.

Energy efficiency can be improved by building homes that are airtight and have no thermal bridges. Glazed areas should be carefully designed, with an eye on the areas and volume of the house and its orientation. Given the reduced requirement for space heating, much of the cost of design and specification of super-insulation might be offset by the cost of the heating system.

Some companies build passive homes at prices comparable to standard construction but in general the construction cost is higher. The price premium is estimated to be in the region of 10%. Energy savings are estimated to be between £350 and £450, saving about 80%. Assuming an extra over cost of £6,000 (10% of the £60,000 house), we can see that simple paybacks can be achieved within about 15-25 years, allowing for regular maintenance of the passive components. This calculation ignores tax savings and grants that are available for energy efficient construction and fit-out.

Ventilation in a passive house is controlled through a passive fresh air system. Adequate ventilation is needed to prevent indoor air pollution problems. Pollution indoors can be from cigarette smoke, radon gas, formaldehyde gas, combustion gases from heating systems and cooking odours. The recommended air exchange in a house is 0.5 air changes per hour. A passive fresh air system can achieve this rate of air change, and a filter system can improve internal air quality by keeping out dirt and pollen. Bacteria growth is not a problem because the same air is not re-circulating. Valve noises may need to be reduced by specifying acoustic control measures.

Payback time

Significantly improving fabric insulation to the roof, loft and walls will also improve energy efficiency – as much as a third of space-heating costs can escape through the roof. It’s also important to improve wall insulation – both external and internal, plus floors and gaps between the floor and skirting boards. All can offer paybacks for the occupier of around two to five years through lower heating costs.

Solar domestic hot water can provide 50% of a typical house’s hot water requirement. A solar-thermal water heating system would cost between £1,500 and £5,000. Heated solar water is topped up by a back-up source of heat, either gas or electricity. Solar collectors are best mounted at an incline with a southerly orientation, although southeast or southwest both work and with a larger solar collector east and west orientations also do the job.