A stuffy greenhouse is a death trap for most plants. Lack of ventilation causes damping off, wilting, drying out, and an ideal breeding ground for infection, pests and disease!
Efficient ventilation is essential to avoid these problems. When you purchase a greenhouse ensure you have sufficient roof vents, side louvres and extractor fans. This combination ensures that there is a regular change of air in the greenhouse, blowing cool air in at the bottom and pushing the hot air out of the top. Of course the temperature will vary considerably from day to night and through the seasons. It can even change dramatically in a single day.
To help maintain a steady temperature you can fit automatic openers which can be set to open and close as the heat from the sun waxes and wanes. these are very useful, especially if you are not around during the working week, or find reaching to the roof of the greenhouse a bit of a stretch.
If you have draught-hating plants that require steady heat then an extractor fan can be useful to boost a poor ventilation system.