Most greenhouse kits will come with roof vents already as part of the structure. They are simply vents at the ridge that will open and close either manually or automatically. Most of the hobby houses have several individual vents and will have an appropriate number for the size of the greenhouse. Some of the larger or more commercial greenhouses will have a continuous ridge vent. This is typically on a motorized system, although I have installed a continuous ridge vent once with a manual system for opening and closing. It was opened using a chain and had pulleys to open and close the vents.
The purpose of the roof vents is to allow warm air to escape. We all learned in school that warm air rises, so just open your roof vent and get rid of the unwanted heat. Roof vents should be closed in the winter when you want to keep your heat in the greenhouse.
Some people will chose to leave their roof vents manual but I prefer automatic openers. You do not have to have an elaborate electrical system. There are solar powered automatic roof vent openers available that work very well. They are a piston type arm with a wax cylinder and work on contraction and expansion. When the wax gets warm it expands and pushes the roof vent open. When the wax cools it contracts and pulls the vent shut. No electricity required.
There are several manufacturers of these, but most of them install pretty much the same. It is just a matter of putting a few bolts or screws into the mechanism and into the frame of your greenhouse. There is a thumbscrew at the end of these that will allow for adjustment of the temperature. Usually I will take several days when starting to use the openers for adjustment. Most manufacturers recommend taking the wax cylinder out during the winter months. These cylinders will wear out eventually, but replacement cylinders are readily available.
I have been asked before about the vents being open when it starts to rain. They will not shut due to rain. The opening and closing is only based on temperature. So, if you are trying to use these on a sunroom rather than a greenhouse, you may want to consider this.
Roof vents are a necessary feature for a greenhouse. Automatic openers are an option, but they will definitely make life much easier. And, they are not all that expensive. They are usually found for under $100.00 each.
Tammy Wylie has been selling and installing greenhouses since 1993. She currently owns and operates two greenhouses, an 8 x 12 is used to overwinter plants and an 18 x 24 greenhouse is used to start vegetables, herbs and annual flowers. She also tests new products in this greenhouse and evaluates them for efficiency and user friendliness. She is the owner of Advance Greenhouses AdvanceGreenhouses.com.