The purpose of a greenhouse is to trap the heat of the sun and create a microclimate that is warmer than the surrounding environment, allowing gardeners to extend their growing season. The best way to maximise solar gain is to orient the greenhouse toward the south in the northern hemisphere and toward the north in the southern hemisphere.
A greenhouse that is oriented directly to the south (or directly to the north for locations south of the equator) will receive the maximum amount of solar exposure. This factor is more important in colder climates and may even be less than ideal in tropical environments where overheating can be a problem. Before building or orienting your greenhouse, talk to people in your area who have experience with greenhouses to get advice about the ideal siting for your conditions.
Greenhouses that are exposed to cold winds will have more trouble retaining heat. Building your greenhouse onto the south side of a larger building is one of the best ways to shelter it from northerly winds. Building windbreaks or planting hedges on the side of the greenhouse that is most exposed to cold can also be effective. A south-facing gradual slope is ideal for placement of a greenhouse, because the surrounding land will absorb more solar heat and create a microclimate that will help the greenhouse to stay warm.
The door to the greenhouse should be placed on the side that is away from prevailing winds. This will make entry and exit less problematic during bad weather and will minimise heat loss from any gaps that are around the edges of the door. A small greenhouse that is attached to a larger building can be accessed from inside the larger building, avoiding the need for an exterior entry.
Many greenhouses are built entirely of glass, supported by frameworks of wood or metal. You can increase the efficiency of a greenhouse by building only its south wall (or its north wall in the southern hemisphere) out of glass and its opposite wall out of brick or concrete. This will allow the greenhouse to absorb heat into its thermal mass during the day and release it during the night, providing a more even temperature for plants around the clock.