The Building Research Establishment has estimated that up to 50 per cent of the total primary energy consumption of the UK is used to service buildings: for heating, lighting, providing hot water, cooking, and running appliances. Of this energy use about half is required to keep the buildings warm. It seems obvious that the reduction in energy used for space heating is the most important area to tackle first if the aim is to reduce a building’s energy consumption.
At the individual level the bills for heating a house always seem to be the largest fuel bills, with those for cooking, heating water and lighting, if you can distinguish between them, being far smaller. This supposition is borne out by the BRE whose figures show that in a typical centrally heated house the energy use is divided as follows:
- space heating 71%
- water heating 18%
- cooking 8%
- TV, lights, etc. 3%
One way of using less energy is to turn down the heating; but this method is unlikely to appeal to most people whose houses are at present under-heated. The best way is to reduce the rate at which the heat in a house is lost to the outside air. When you heat your home, the heat that you put in continually passes from the warm interior to the cold outdoors and you keep putting in more heat to make up this loss. A typical 1930s semidetached house loses heat as follows:
- through the walls 35%
- through the roof 25%
- through the ground floor 15%
- draughts 15%
- through the windows 10%
A more recently built house with very large windows might lose more heat through the windows, an old house might be draughtier. But all buildings will lose heat because heat will always flow to a cooler place; if it was not cold outside you would not need the heating and there would be no flow.
Although this flow of heat cannot be stopped it can be slowed down. This means that less heat will be lost in a given period, so less energy will be needed to replace it; and less energy means lower fuel bills. The way to slow down the rate of heat loss is to use insulation materials in the fabric of the building. It is clear ‘tom the table that the walls and roof are likely to be the first parts to be insulated, as they make up the largest part of the total heat loss. Draughts however can be stopped up cheaply, so they will also be a high priority simply on the grounds of cost effectiveness.
Where to insulate
The position of insulation in a building is quite important. A house with internally insulated masonry walls, with a timber frame or built with other lightweight materials, is said to be thermally ‘light’ and to respond quickly to heating. The insulated surfaces of the rooms have little mass and therefore require little heating to make the building warm. But when the heating is turned off the building will cool quickly because the materials do not store heat.
A thermally ‘heavy’ house, made of dense masonry materials, insulated on the outside, will take a long time to heat up because the massive materials have a high thermal capacity and will need a lot of heat to bring them up to the required temperature of the building. The house will store casual energy such as the heat given off by the occupants, by electric lights and appliances and by the sun shining through the windows. When the heat source, whether intentional or casual, is removed, the heat that has been stored in the heavy walls is released slowly to the rooms as they cool down, just as an electric storage heater gives off heat during the day after it has been heated up at night.
The reason people say that old stone cottages with walls half a metre thick are warm in winter and pleasantly cool in summer has nothing to do with the insulation value of stone, which is very poor; it has to do with thermal mass. The thick walls store the heat put into the building and even out variations in the internal temperature. If the weather becomes very cold the stored heat will delay the time when the effect of the lower temperature is felt indoors. Conversely in summer the building heats up very slowly and will therefore tend to feel cooler than a building with a quicker response.
The position of the insulation depends to some extent on the way the house is to be used. For example, if you are out all day and need heat only in the morning and evening, and at weekends, a structure with a quick response may be more appropriate as the house could be heated quickly when you come home. On the other hand a thermally heavy structure is more appropriate for a building that is to be heated more or less continuously.
In the real world nothing is straightforward and in the conversion of an existing building it may well be that other factors than the intended occupancy will determine the position of the insulation. Do not worry overmuch about this; it is perfectly possible to heat a lightweight structure continuously, although less easy to heat a heavy one intermittently. However, a lightweight structure will tend to overheat if you design it to absorb solar energy
External insulation clad with boardin through the windows, while a heavy structure will soak up this extra energy to be given out later. If you hope to make use of casual solar gains it is a good idea to try to achieve as much thermal mass as possible, using exposed quarry tiled or concrete floors with edge insulation, masonry internal partitions and external wall insulation.
Wherever the insulation is it must be protected from condensation. Cooking, washing clothes, running a bath, using a gas cooker, and simply breathing, produce water vapour. The vapour produced by an average household each day is equivalent to about twelve litres of water. It diffuses through the building and as long as it remains as vapour there is no problem. The difficulty arises when the water vapour diffuses through the structure of the building to the outside, where the vapour pressure is lower. Warm air can hold a lot more water vapour than cold air, and when warm moist air is cooled there comes a point where the air is saturated with vapour and will not hold any more. This is called the dewpoint. Any cooling below the dewpoint will cause the vapour to condense into water.
In winter the outside temperature may well be below the dewpoint for the warm moist air within the building. As the water vapour seeks to balance the difference in vapour pressure between inside and outside by diffusing through the structure, it will reach a point where the temperature in the walls is cold enough for the vapour to condense into water. If this point occurs within a material that may be damaged by water the result can be disastrous. Insulating materials work by trapping layers of still, dry air within them; and if they get full of water they cease to insulate, as water is a good conductor of heat.
To overcome the problem of water in the insulation it is essential to put a vapour barrier on the warm side of any insulating materials used in a building. It is also a good idea to arrange a ventilated cavity behind the insulation wherever possible, so that any water that passes through is evaporated by the air. A vapour barrier is any material that prevents the passage of water vapour, for example aluminium foil, heavy gauge polythene sheet with joints folded and taped, or even two coats of oil-based gloss paint. The gloss paint is not reckoned to be very effective as a vapour barrier and under no circumstances must it be applied to expanded polystyrene ceiling tiles, where it will become a fire hazard.
When a building is insulated the temperatures of the surfaces of walls, ceilings and other elements are raised, and if the temperature is above the dewpoint there will be no condensation on the surface. However, if there is no vapour barrier, condensation will occur within the wall (interstitial condensation), and once the insulation becomes saturated the condensation will spread to the inside wall, and may cause mould to grow. Insulation will therefore relieve the problem of condensation in your house, but there must be a vapour barrier and it must be in the right place, if the insulation is to reduce heat loss and condensation.
The advantages of insulation
Insulation is definitely the most effective way of saving energy in your home; the more exciting solutions, such as wind or solar power, are useful only when the demand for space heating and hot water have been minimised by the use of insulating materials. People sometimes worry about the amount of energy used to manufacture these materials, but, even if a relatively high technology material such as glass fibre is used, the energy that went into its manufacture will be paid for by the energy it saves in one winter, or at most two. For the rest of the building’s life, provided that the insulation has been installed correctly it will be saving energy.
If a building is thoroughly insulated it will need a much smaller, simpler and cheaper heating system than the same building when not insulated. If you move into an old house, it is much better to spend money on insulating it to the point where you can heat it with a single stove rather than spend the same money on a conventional central heating system. The running costs of the insulated house will be very low compared with a centrally heated house with no insulation; yet the house will be kept just as warm. If you decide to put in central heating as well as insulation you will find that you can use a very much smaller boiler and smaller radiators; so you will pay less for the central heating system than if the house were not insulated.
Finally, it is worth pointing out that, in our view, a programme of complete insulation of all existing houses in the UK combined with the abandonment of electric space heating could reduce the national primary energy demand by about 10 per cent.