Planning An Eco Home

Few of us ever get the chance to analyse the orientation of a house before it is built. Usually we have simply to make the best of what’s already there. Architects and builders rarely seem to consider the way the sun moves round a building, how much extra heat could be gained through the windows or whether the building will be constantly exposed to the cooling effect of the wind. It is common to see rows of new houses with the same large picture windows facing both north and south on either side of a road. Because of this lack of foresight you will have to take a careful look at the orientation of your home.

Windows

To start with, you will need a compass to find out which side of the building faces south. Once you know, you can see if there are any south-facing windows that might be made into collectors of solar energy. A single sheet of glass is a very poor insulator, even when covered with curtains at night; windows are more usually considered to be a major cause of heat loss from the warm house to the cold outside than a way of adding heat to the inside. In winter, more heat is lost from the house through a single-glazed, south-facing window than is gained to the house from the sun shining through the glass. But if the same window were double-glazed and fitted with insulated shutters, which were kept closed during the hours of darkness, it would act as a solar collector and useful energy could be gained over the winter months.

A house could be designed to obtain a large part of its space heating through the windows alone. The house would need a very massive structure, insulated on the outside, to store heat and even out variations in outside temperature. For this reason the effect of energy gains through large windows may not be so apparent in an existing house, and you would need to be sure that your house was suitable before deciding to increase the number or size of south-facing windows.

Any glass facing north, or indeed anything other than almost due south, will cause net heat loss even if the sun shines through it during part of the day. If there are large areas of glass facing north, such as floor to ceiling windows, it would be worth reducing them. A floor to ceiling window could be boarded up to a more normal sill height and the new solid area insulated to a high standard.

Any reduction in the number or surface area of windows will have to comply with local building regulations. In the UK, these require that the area of glass in the window or windows of a room shall be not less than one-tenth of the floor area of that room, to ensure adequate daylighting, and that the total area of openable window shall be not less than one-twentieth of the floor area of the room, to ensure adequate natural ventilation. It should also be noted that any north-facing window in the roof will lose more heat than a similar window in the vertical wall as the former is completely exposed to the sky. At the same time a south-facing window in the roof will fail to collect as much energy in winter as a vertical south-facing window: the low winter sun will strike the vertical window directly whereas it will be reflected off the sloping glass of the roof window.

Rooms

Having examined the outside of your house, it would then be worth considering the layout inside. If there are any rooms which are to be heated to a lower temperature, only heated occasionally or even left unheated (such as spare bedrooms, larders, storerooms and workshops), it would be best to try and place these against the north wall of the building. These rooms would then form a buffer between the heated part of the building and the cold outside. Although the buffer zone is unheated, its temperature is higher than the outside temperature: hence the temperature difference between the heated space and the buffer zone is less than that between the heated space and the outside — this also happens with the unheated attic space of a house.

Conservatories

It might now be worth looking round to see if there are any south, south-west or south-east walls against which a lean-to conservatory could be built. A conservatory will form a buffer zone and act as a collector of energy. Even on an overcast day the temperature inside the unheated conservatory rises and thus reduces the heat loss through the walls of the building.

To build a conservatory less than 50m3 in volume you will need to comply with the building regulations, but you will not need planning permission. Traditionally the conservatory has always been added on to the back of the house where this faces approximately south, although its role as a buffer zone has perhaps been less appreciated than its use for growing seedlings and storing wellington boots; but there is no reason why a conservatory built over a south-facing front door should not act as a useful entrance porch.

Roofs

The compass will also enable you to find out in which direction the roof slopes. A solar device will collect energy not only from the sunlight striking it directly but also quite considerably from the indirect radiation reflected from the whole sky. This split in the directions from which energy is received has some bearing on the orientation of solar collectors. If your roof faces south it is the obvious site for a collector, although a wall facing south would also be suitable. However, a roof system collects more energy than a wall one, even in winter when the rays of the sun shine almost straight into the vertical wall-mounted collector. This is because the roof collector is visible to a greater expanse of open sky and hence receives more scattered indirect radiation.

This has another bearing on orientation when considering a roof slope that does not face south. Very little difference in the amount of energy collected by a roof-mounted collector will be found if the roof points up to 300 either side of due south and even at 50° either side of south only a 10 per cent reduction will occur. However, a wall collector will be far less effective if it does not face due south, as it will be cut off from the direct rays of the sun for a greater part of the day as well as being sheltered from indirect radiation.

Because the arc made by the sun across the sky changes from summer to winter, solar panels required for hot water heating in the summer only (the hot water coming from another source in winter) could be placed on a vertical wall facing east or west where the rays of the sun will be more nearly perpendicular to the collector. Such an orientation would, however, be of little use in winter. If the roof slopes east or west and a collector is required for winter operation, some other site will have to be found, such as the roof of a back extension. You might add some kind of structure to the roof on to which a well orientated collector could be built; but this might be expensive as well as worrying to local planning authorities and building inspectors. A better alternative is to use a free-standing fixture in the garden.

Vegetation

Even in a cool climate, if your house has large areas of glass facing south, overheating can be a problem in the summer. This can be controlled by well placed vegetation, which brings us to another way of modifying the climate around the building. Deciduous trees will provide shade in summer, as will trellises with roses and honeysuckle growing over them, whilst the same trees and trellises, free of leaves in winter, will let the sunlight through to the house. Careful planting can modify the exposure of the building to the wind. As the wind blows across the building it increases the amount of heat lost from it. This is demonstrated by the relative heat losses of windows in sheltered and exposed sites.

In coastal areas and on hill tops, where the prevailing wind directions are easy to work out, planting shelter-belts or windbreak hedges upwind of the building may help. A solid barrier would not be as effective as is a hedge or a row of trees which let some wind through: it would produce eddy currents on the downwind side rather than a reduction in wind speed. The sheltered zone can extend to thirty times the height of the belt but for practical purposes it will be between five and ten times the height of the barrier.

A true shelter-belt, the most effective windbreak, consists of two or three rows of trees planted in a staggered pattern; it is perhaps usefully applied only to a large building or to a group of houses. For a single house a suitable hedge, which will grow faster than the trees, will provide protection from the wind sooner. Care must be taken that the windbreak hedge doesn’t grow to overshadow a carefully positioned solar collector, although in some cases the overshadowing may be useful. For example, the prevailing wind direction in most parts of the UK is south-westerly, so a screen of deciduous plants might provide useful shade in the summer; in the evening it will also prevent the low summer sun entering rooms facing south-west or west and overheating them. Another possible use of vegetation is to grow a dense evergreen creeper up the walls: this will trap a layer of still and therefore warmer air against the side of the building.

Leave a reply

Share On Facebook
Share On Twitter
Share On Google Plus