The only reason I can think of for venting is to decrease the temperature. I don't think humidity would be a problem.

As long as your heat source isn't powered by natural gas or propane (or wood burning, but that's pretty unlikely!) you don't need to vent. (Would produce carbon monoxide)

But you might find that it might begin overheat when the sun is out, as the season progresses. THe smaller the GH, the more difficult it is to regulate the temperature. I have a small window in the ridge of the roof that has an automatic opening device on it, that opens when it gets too hot. I cut a hole in the solar pool cover to accomodate it and taped the edges. Eventually, I installed a small exhaust fan, set on a thermostat in the wall.

I have a 12" exhaust fan on a thermo plug (that's unplugged) along with two roof vents w/solar openers (that are currently uninstalled)..... I haven't cut any openings in the pool cover to accommodate either roof vent, but I might have to do that in the near future.... My mobile alert system recorded a temp of 91.2F just this afternoon when the outside temp was only 33F.

Your likely to not need an exhaust fan in winters 98% of the time. However, you should be planning for Spring and Fall. Come end of March when sun is high in the sky and outside temps are fluctuating around 40*F the internal temp could easily soar over 100*F on a sunny day. For your smaller structure you could blow in outside air through the open door but your plants won't like the draft. An exhaust fan allows you to "Baffle " incoming air through a cracked doorway. I would highly suggest one. For that size structure I would even be looking for a two speed exhaust fan.

wyndacre and Barrie, you make good suggestions.

I have a question for you two: I have plants in my 8X12 ft greenhouse now that can not take temps below 32F. I do not want temps to get above 95F in the spring when outside temps get into the 50's. Temps in the GH have reached 100 when outside temps were in the high 40's outside even with automatic opening roof vents. I plan on installing an automatic opening louver window when it gets warm enough. Will I also need an exhaust fan?

I believe you should. The louver will allow some of the heat to escape but it relies on passive air movement.

In my one high tunnel I have a 2 stage thermostat where the louver will open on the first stage and about 4*F higher the exhaust fan will come on. There are rare days that the louver will open and close without the exhaust fan starting. That was one reason why I suggested a two stage fan for you. A low speed exhaust may be all you need on many days. I have larger structures so I have different setup. Currently 2 structures have 1 exhaust fan, one has 2, one has 3. I rarely use the third large fan because if it gets that hot I roll up the sides every morning.

An exhaust fan allows you to "Baffle" incoming air through a cracked doorway. I would highly suggest one.

I have a two speed, 12" exhaust fan on a thermo plug (that's unplugged) along with two roof vents w/solar openers (that are currently uninstalled)..... I haven't cut any openings in the pool cover to accommodate either roof vent, but I might have to do that in the near future....

The automatic roof vent plus opening/closing doors worked ok for a few years...the roof vent was a real life saver when I couldn't be home to babysit the greenhouse. But it was a huge relief to be able to install the exhaust fan on a thermostat too and that controlled the temperature much better and is one of those things that you wished you just had done to begin with!!

Before cutting any holes to accomodate vents, make sure you have sorted out the best way to install the solar pool cover and that it is something you can replicate every year, so that the holes land in the same place each time. Then use some strong duct tape or gorilla tape around the edges of the hole to help reinforce the pool cover.

Thanks folks! Yeah, the whole hole (no pun) cutting alignment thing applies to shade cloths as well... My 30% shade cloth has holes (flaps) cut for the roof vents...... I'll also need to cut a third "flap" in the cover for my exhaust fan......