PVC greenhouses hold a lot of advantages. They are lightweight, affordable and anyone with medium DIY skills can build one from scratch. They also function quite well and can be moved if needed. One disadvantage of PVC greenhouses is that they are poor insulators. The temperature rises, but heat is lost quite rapidly. But it is possible to maintain a warm temperature in a PVC greenhouse.
Choose the right location. Your PVC greenhouse should be placed in an area that will receive plenty of sun. In the summer months, loosing heat won't be such an issue, and quite often you may have to ventilate. When fall and winter set in is when conserving heat becomes crucial. Getting the most sun possible will help the temperature rise during the day.
Use a passive solar system to preserve heat. When the temperatures start to fall and the hours of daylight decrease, these will help maintain the temperature during the chilly night hours. Take a large container that is painted or colored black. Fill it with water and leave it in the greenhouse during the day. The water temperature will rise with that of the greenhouse during the day. At night it will release the stored heat, keeping the temperature constant.
Install a trombe wall. Trombe walls are made of masonry. They are low in height and range from about 8 to 16 inches thick. The walls are coated with heat absorbing material. They work by placing a layer of glass about 3 to 6 inches away from the wall. The heat is reflected through the glass and absorbed by the wall. During the day the heat is stored in the wall and at night it is released, keeping the greenhouse warm.
The final step one can take is to install a heater. There are several types to choose from, with gas, oil and wood being the most popular choices. Consider the types of plants you have before you choose a heater, as some plants are better suited to a particular type.