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Questions and answers on energy efficent home extensions

Extension shape and orientation

Q What is the best shape for my extension?

A Ideally, extensions should be compact, to reduce heat losses. Two storeys are better than one, and complicated or elongated shapes should be avoided.

Q I would like to have large windows. Is this a good idea?

A Windows contribute significantly to heat losses, and thus to fuel use, fuel costs and carbon dioxide emissions. Windows should only be large enough to admit adequate daylight, and the area of north-facing windows should be minimised. Aim for the area of windows to be 15 to 20 per cent of the floor area.

Q But what if my extension is north-facing?

A If you cannot avoid north-facing windows, it is appropriate to compensate for the extra heat loss by including more insulation elsewhere, e.g. in the walls or roof.

Q What about south-facing windows?

A South-facing windows do trap some useful solar gains, but they also contribute to summer overheating. South-facing glazing should be shaded from high-angle summer sun, and highly-glazed south-facing rooms must be well ventilated.

Conservatories for a house extension

Q Should I include a conservatory?

A Conservatories contribute significantly to heat losses, and thus to fuel use, fuel costs and carbon dioxide emissions. They are rarely comfortable in winter unless they are heated, and they tend to overheat dramatically in summer. If you wish to maximise your year round use of the extension it is better to build a ‘sun room’ (i.e. a conventional extension with an opaque well insulated roof, perhaps some roof windows and well-shaded south-facing windows).


Q How much insulation should I include?

A Adopt the Best Practice insulation standards. This will reduce heat losses, and thus reduce fuel use, fuel costs and carbon dioxide emissions. The cost of the extra insulation can be offset by having a smaller heating system, and it may be possible to retain the existing boiler.

Q Is insulation always effective?

A Yes, if it is properly installed. Make sure that your architect and builder understand the need to eliminate ‘thermal bridges’ and to achieve a good standard of airtightness. There should be no gaps in the insulation at the junctions of walls, roofs and floors, or around openings. Windows and doors should be properly sealed into the walls, and the places where services (pipes and wires) penetrate through walls and floors should also be sealed.

Glazed openings

Q Should I specify high-performance glazing?

A Yes, double- or triple-glazing with wide gaps, a low emissivity coating and gas filling will reduce heat losses, and thus reduce fuel use, fuel costs and carbon dioxide emissions. It will also improve comfort by reducing down-draughts and the risk of internal surface condensation.


Q Does the extension have to be ventilated?

A Yes, there must be provision for background, rapid and (in ‘wet areas’) extract ventilation. Trickle ventilators and openable windows meet most of this requirement. For wet areas, there are several controlled ventilation options, including energy efficient extract fans, heat recovery room ventilators and (for some extensions) passive stack ventilation.


Q Will the existing heating boiler have to be replaced?

A Not necessarily. If the extension is compact, well insulated and airtight there may be little or no overall increase in overall heat loss, and the spare capacity in the existing boiler may be sufficient. This can be confirmed by calculation.

Q What if the existing boiler is not adequate?

A The existing boiler should be replaced by a new condensing boiler, of appropriate output and seasonal efficiency grade A or B. If you install an efficient, condensing boiler, the improved efficiency will offset the additional heat demand, so fuel costs will not necessarily increase significantly.

Q Will I have to upgrade my heating controls?

A If you don’t already have good controls, and you replace the boiler, yes, you will have to upgrade. The upgraded system must be fullypumped, and include a programmer, room thermostat and hot water cylinder thermostat. The room thermostat must be interlocked to the boiler so that the boiler does not fire when there is no demand for heat.

Q Should I include a room heater in the extension?

A An efficient gas-fired room heater or a wood-burning stove is sometimes a good alternative or supplement to extending or installing central heating. Electric room heaters are efficient, but they are also expensive to run and have high carbon dioxide emissions. Woodburning room heaters have no associated carbon dioxide emissions.


Q What type of lighting should I install?

A Energy efficient lighting with compact fluorescent lamps (CFLs) is much less expensive (over the life of the lamps) than conventional tungsten lighting, even though the energy efficient lamps are more expensive initially. Energy efficient lighting significantly reduces electricity use and the associated carbon dioxide emissions.

Energy efficient domestic extensions

Q How can I achieve the desired lighting effect with CFLs?

A A large range of CFL lamp types is available, including spot lamps, candle lamps, and coloured lamps of every description. The multitube lamps light up instantly, and quickly reach their full brightness.

Special dimmer switches are available for use with CFLs.

Professional assistance

Q Who can help me with all this?

A Choose your architect and builder carefully. Ask them if they know how to design energy efficient domestic extensions, and whether they have completed any. Ask them if they are familiar with this guide, and with the other guides listed below. If in doubt, contact your local Energy Efficiency Advice Centre (EEAC) via the Energy Saving Trust’s

Energy Efficiency Helpline on 0845 727 7200.



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