since you have a greenhouse, you can open windows and let out the heat.
one way is to use a pipe and floats.
if you have a large pipe that is 1/2 meter tall, an outlet on the bottom and an open top to allow it to fill.
put one under each window. in the pipe you have a smaller pipe that will float and be connected to the window.
when the pipe is filled, the float rises and the window opens. the power comes from the pump.
to close the window, drain the water.
a relay is the best way to connect to higher voltages.