Greenhouse Ventilation

Have 10ftx8ft greenhouse gas heater propane realise that to burn it will need fresh air any ideas on how much ventilation it will need. Have louvre on one long side 4 roof vents
Thank You

I would not expect your greenhouse to be air tight. So ypu will have advantageous air coming in though all the leaks(for cubustion). Plants like CO2, you dont so put the heater off when you are in there. Or leave the door just ceacked open. If its a purpose built greenhouse heater it will only be a couple of KW's. So unless you intend to move in it will not be life threating.