I installed a thermostat controlled 1250CFM power roof fan with 6sqft of louvre intake vents, and it works great so far. Your venting needs depend on several factors - size of greenhouse, intake vent availability, budget, shading, and etc. Maybe if you gave some more info we could point you in the right direction.

Ok I have a 6x8 greenhouse. The covering is fiberglass. recently we replaced the roof with polycarbonate. We live in an area that doesn't get extremely hot but it does get too hot in the greenhouse for optimum growing. My venting has been 2 small louvered closable vents--that are not in the best of shape after 15 years-- and leaving the door open and that usually keeps it at a reasonable temperature. I don't necessarily need the vents to be automatic. I have a mini-max thermometer that can be read from the house but I'm just looking at alternatives. That roof fan sounds good but my greenhouse is not wired for electricity. My heater is run with an extension cord from the house

Where are those two louvered vents? I would hope they are high in the gable ends. Is your heater gas or electric? If the heater is electric, my suggestion may require you to beef up your electric supply.

I used an evaporative cooler (swamp cooler, or some other name depending on where you are located). Get a self contained cooler room size...about 18 inches deep and wide, and about 2 feet tall. they have Aspen, or some other semi absorbent material around three sides, and a water pan at the bottom with a float valve and pump to pump water through a perforated pipe to keep the pads wet. An internal fan pulled air through the wet pads and blew it into the greenhouse. The cooler blows cooled moist air into the bottom of the house, and forces the warm air out the vents at the top. That with good shade cloth served me well for years in multiple locations around the country where I grew orchids while in the service.

Your house is a bit small for optimum climate control, but you can do it, especially since you are not mobile, and can adapt to your conditions easily. One other benefit from this kind of ventilation system is that it keeps the greenhouse under a positive pressure, which discourages critters from entering.

Bill

Since your greenhouse is relatively small and has no electricity, a solar-powered exhaust attic fan may provide enough heat reduction to help. The lower output fans are not too expensive, but the higher output ones get more expensive. With these type of fans, you would not need automatic vent openers. Here is one example:
http://www.homedepot.com/p/Active-Ventilation-5-Watt-Solar-Powered-Exhaust-Attic-Fan-RBSF-8-WT/204203001?cm_mmc=Shopping|Base&gclid=CMLGgNeWmsUCFY-TfgodTikAkg&gclsrc=aw.ds

cuestaroble--That solar powered fan sounds very interesting and something I'm going to look into. Actually in my zone 3 heating is more of a problem than excessive heat and I my system for heating works just fine. I only use the greenhouse for 2 months in spring to start garden plants and flowers. I also have a light set up in my basement and a pop up put up and take down cold frame for my excess hardy plants. The lights come into play early in the season and the cold frame late in the season. My greenhouse is small because I designed it that way to save on heating. I only wanted to heat the space for plants not excess space. It has a slanted roof that is 6 1/2 foot tall at the back sloping to 4 1/2 feet in the front. It has a 30 inch shelf running around the 3 sunny sides and a pull out shelf low to the ground under the east side shelf for shady flowers like impatiens. This only leaves me a 3 x 3 1/2 space in the middle which is enough space for me and only me to work.. The sun does heat it up fast which is what I wanted but such a small space requires some venting to cool it off on warm sunny days. Right now I have a small oscillating fan on the south side blowing towards the door on the north that I will have to start using soon.

Hi Dowlingram - Our customers love the Superior Autovent made by Bayliss. It is an adjustable automatic vent opener powered with a wax cylinder that can be adjusted to open from approximately 60 to 80 degrees, with a full 12" opening when it is extended all the way. You can find them at www.superiorautovents.com.

I used this automatic heat activated window opener from Lee Valley in my 10x16 greenhouse.

http://www.leevalley.com/en/garden/page.aspx?p=10543&cat=2,43224

I attached it to a small window installed near the top ridge of the roof. I picked up the window inexpensively from the Habitate Restore and it has worked well for me for more than a dozen years. Eventually, I also added a thermostatically controlled exhaust fan but the automatic vent in the roof has remained a useful tool in keeping the greenhouse cool during my zone 6b summers.