Is a green house warm

Yes, sometimes you do. It really depends on what your weather is like, and what you plan to grow. Some plants do not require a lot of warmth, while others, like peppers or tomatoes, etc. must be kept warm if you expect them to produce. It should also be noted that some plants require (or at least benefit from) different temperatures at different growing stages.

You simply need to determine what you wish to grow, what temperature those plants require, when you wish to grow them, and what your weather is going to be like at that time. Keep a thermometer in the greenhouse around the same level the plants will be kept, and take note of what it reads, especially at night, when temp's will be at their lowest. Then take appropriate measures, add a heater or heat lamp, only if you need to raise the temperature, or expect to.