Conservatories can be a major source of heat loss in winter and unwanted heat gain in summer unless certain basic principles are followed. These principles apply equally to porches:

1. Make it a separate room
Never have a conservatory that opens directly onto the heated rooms of the house without well sealed adjoining doors or windows. In the worst case it could double your annual heating bills. If you want to be able to open up your living space to the conservatory, a reasonable compromise is external quality folding doors that can be closed throughout winter. Insulate all doors and windows connecting the house to the conservatory as though they faced directly onto the outside.

2. Minimise forced cold air
To prevent cold air being forced into the house when the outer door is opened, you need to encourage people to close one door before opening the other. If you have a large enough conservatory, you can do this by placing the outer doors some way from the inner doors. If you have a small conservatory where the doors face each other, hang both doors so that they both open into the conservatory. Another option is to fit an automatic closer to the external door.

3. Plan using solar design principles
Consider the solar orientation in conservatory design. It is probably best not to build a conservatory unless at least one wall faces South-East, South, or South-West- otherwise it will get no sun in winter and will tend to overheat in summer. Concentrate glazing on the South inclined walls. Place solid unglazed walls on all sides facing North-East, North and North-West. This will make no significant difference to internal light levels in winter when very little light comes from these directions. Similar principles apply to the roof.

• For a double pitched roof: The side of the pitched roof that faces North-East, North or North-West, should be solid and insulated with a relatively low angle of pitch. The side of the roof facing South-East or South-West should be glazed.

• For a single pitched roof: When the roof projects perpendicular to the wall of the house, the optimum solar design is a solid insulated roof with a backwards pitch, sloping down from a higher glazed wall facing the sun (South-East, South, South-West) to a lower wall on the darker side (North-East, North, North-West). Such a roof will allow all the low winter light into the conservatory and progressively shade out the higher summer light. The roof should have a low angle of pitch between 9 and 20 degrees. When the roof runs parallel to the wall of the house, a backward pitch is difficult to drain, and a lean-to roof makes more sense. However, there can be a risk of overheating in summer, especially for roofs which face due South. There are two possibilities. One option is to have a solid insulated roof with a low angle of pitch - maybe with skylights that can be opened in summer instead of full glazing. You should then concentrate on maximising glazing on a vertical South facing wall. If there is shading or high level ventilation, a glazed roof will maximise light and solar heating. The best design would be a roof that is as possible to best receive the low level summer sun. Many solar houses have conservatories which are triangular in section, with a single South facing roof/wall that extends to the ground.

4. Design the floor as a heat store
On sunny days in spring and autumn, the sun will directly warm the floor. It is worth planning around the potential of the floor to act as a heat store by paving it with dark stone or tiles with insulation behind. Most purpose built solar houses have slate or terracotta tiled floors. Similar use can be made of walls that directly face the sun.

5. Pull warm air into the house
To encourage warm air to enter the house, place adjustable vents between the conservatory and the house. Air flows are hard to predict in advance and you may find on windy days that air is more interested in flowing out into the conservatory through the vent- the last thing you want! You can monitor the air flow and close the vent if this happens, or install some form of flow control (solar pioneers used sheets of the very light plastic used in dry cleaner bags hung over the vent on the room side). If you have a continuing problem it may be worth installing a small electric fan and swiching it on on sunny days. If you have a high performing South facing solar conservatory it will certainly be worth installing an electric fan to pull warm air into the house on good solar days. This could easily be linked to a thermostat to run the fan only when the air is warm.

THE REAR SUN PORCH OF THE YELLOW HOUSE
The narrow sun porch at the rear of the Yellow House was added to meet several needs a draft lobby to pre-warm air and prevent cold air entering directly into the house; a storage space for garden tools and dirty boots; a potting shed and greenhouse; and a space for drying clothes. Although it is not a real conservatory, it is a nice cosy place to sit on sunny days in Spring and Autumn.

Although it receives only short bursts of sun, the sun porch still performs well. In mid winter the air fed into the extension is typically 5°C warmer that the ouside air temperature. On sunny days in spring and autumn the air in the porch will often be warmer than the air inside the house.

At each end of the sun porch are three doors- one set of double doors and single door. All six doors were purpose built by a local joiner from plantation pine, with doorsteps of local oak.

The doors on either side are each designed to fold back, allowing up to nine different combinations. In the winter only one of the doors at either end is used for entry and exit. At this time the porch functions as insulation for the large area of glass in the inner doors which would otherwise be a major source of heat loss. In the late spring and early autumn the inner doors are opened and the sun porch becomes an extension of the living space in the extension. In the summer the doors at either or both ends can be fully folded back, opening up a large part of the extension directly into the garden.

The design of the sun porch follows the general conservatory design principles principles above:

1. Make it a Separate Room - the porch is self contained and the interior doors are sealed with draught strips. They are double glazed to the same high specifications as all the other windows in the house. In summer they fold back to open up the sunporch space. The outermost doors are single glazed.

2. Minimise Forced Cold Air - the doors are hung so both open into the conservatory. This reduces the temptation to open on door after another.

3. Plan Using Solar Design Principles - The rear of the house faces North-East. The three external walls of the sun porch therefore face, North-West, North-East and South East. The optimal design is therefore to glaze the South-East side, place the doors on the North-East side opposite the internal doors and place a solid wall on the North-West side. The bottom two feet of the South-East side, which is always shaded, is a low concrete block wall. It is insulated by the bermed flowerbed and supports a large single sheet of glass. The North-West facing side is built of standard concrete blocks. The wall is painted dark blue on the inside to absorb as much of that sun as possible (black would have performed better but would have been oppressive). If the orientation of the sunporch had inclined more to the South it might have been worth insulating the back of the North facing wall to improve its heat storage.

The roof is pitched either side of a roof ridge. The side facing South-East is glazed with a large single sheet of toughened glass from an old patio door bought for £15 from a salvage yard . The side of the roof facing North-West is unglazed- The top level of marine plywood is painted with weather resistant paint (not very environmental!), with insulation offcuts from elsewhere in the house behind it. It is pitched at 20°, the angle of the South-East sun in late March. At this time, the sun strikes all of the wall but as it rises higher through the summer the roof increasingly shades the wall.

The interior surface is panelled with pine cladding removed from the old kitchen. One side leads directly into the house- this is double glazed and only opened for access and in good weather. The other three sides face North-West, North-East and South-East. Only the South-East facing side receives sun in spring and autumn. This side is therefore glazed as much as possible.

4. Design the Floor as a Heat Store - The floor is insulated with more offcuts of insulation left from elsewhere in the house, covered in render and tiled with slate tiles (cheap from Wickes).

5. Pull Warm Air into the House - An adjustable vent controls the flow of the air into the house. We adapted one of the round vents which is normally fitted into a window pane because it has revolving fins inside. It is therefore easy to spot when the air is flowing the wrong way into the sun porch (the fins are revolving anticlockwise), and close the vent. This is a rare occurrence, though. Under most conditions air is pulled into the house, helped by the draw of the bathroom extractor fans and the stack effect of the roof trickle vents.

THE FRONT SUN PORCH OF THE YELLOW HOUSE
The front porch will be built in 2003/4. Like the rear porch it projects from the house with one solid concrete block wall facing North East. The two other walls (the fourth being the front of the house) are glazed. The plan of the porch is distorted to make the most of the sun with the main wall, which also contains the front door, turned to face due South and so set at 45° to the house. The other glazed wall which faces South West is far smaller. A monopitch roof slopes back from this high glazed wall to the gutter on the back solid wall. To be continued!