As air-conditioning units use lots of electricity, they are both expensive to run and lead to high levels of carbon emissions. Climate change is likely to lead to hotter spells during UK summers, but there are ways to keep cool without resorting to the expenditure and emissions associated with air-conditioning. The measures you can take include:
Avoiding Internal Heat Gains
There will be many sources of heat within the house, and addressing these will help to reduce the need for air conditioning. Conventional incandescent light bulbs generate light quite inefficiently, giving off waste heat in the process. Low energy fluorescent lamps (CFLs) convert electricity to light much more efficiently, so putting in these will help reduce overheating as well as saving money.
Most electrical appliances will give off heat, whether directly as part of their operation (e.g. cookers, fridges) or indirectly from transformers on the plug or in the appliance. Switching off appliances when not in use will prevent indirect heat gains. Make sure that fridges or freezers are well ventilated at the back (by the heat sink), and if possible put a freezer out in a garage or utility room so the heat given off does not go into the main house. Avoiding the use of the oven on your cooker during very hot weather will also help.
If you have a hot water cylinder, make sure it is well-insulated and that all hot water pipes are insulated against heat loss. Uninsulated pipes can easily lead to more heat loss than a modern, efficient cylinder.
Blinds can be used to reflect incoming short wave solar radiation back out as long wave radiation. However, modern windows that have 'low-e' glass (standard in most new windows) will stop long wave solar radiation going through the glass - so the blinds will be less effective. They will still reflect some of the incoming sunlight as short wave radiation, so will still have some effect.
Otherwise, external shading is the most effective. This can be window shutters or an awning or other type of overhang to prevent sunlight coming through the windows. Shelter from vegetation can be planned in advance, by growing deciduous trees, or having plants on a trellis over windows. Shutters can be difficult on modern outward opening windows, but are easier for traditional sash or sliding windows. An awning is often the simplest - it can be retracted or removed outside of summer.
A permanent overhang is another option – it needs to be carefully sized to allow in low winter sun but not high summer sun. The following table is taken from CAT's 'Energy Saving House' book, and gives advice on calculating the optimum roof overhang. You may need to check with local planners before adding external shading, especially in designated areas like National Parks.
Insulate and Ventilate
As well as keeping a house warm in cold weather, decent levels of insulation will also keep it cool in the summer. It will complement the other measures outlined here, as once you have avoided internal heat gains and brought in cool air overnight, a well-insulated building will keep the this cool air inside the building.
Opening windows during the day will bring refreshing air in, but this incoming warm air will gradually heat up the building. It is best not to leave windows open if rooms are empty - but this needs to be in conjunction with some shading to prevent heat gain through the glass. In an occupied room, opening a window for just a few minutes will bring some fresh air in.
This works best in houses that have lots of 'thermal mass' - which means thick stone or brick walls, or exposed solid floors. These materials will take a while to either heat up or cool down, so in summer you want to keep them cool as much as possible. So use the above methods to minimise gains during the day, and ventilate at night to allow the mass of the buildings to cool down as much as possible. For optimum night cooling you would need to open about 25% of the total window area, while opening only 5 to 15% during the day. Roof windows are a very good way to provide secure ventilation overnight.