Thanks for the information Daintree. I will hold off on an evaporative cooling system until it proves to be necessary. I am hoping shade cloth + ventilation will be enough.
However, I think you are mistaken about the CO2 emissions of burning natural gas. This absolutely produces significant amounts of CO2. You may be thinking about Hydrogen which only produces water vapor. I am also thinking about putting birds in my greenhouse. How does it work out for you? I'd worry about them destroying smaller plants. And if you are running an unvented heater with birds living inside and have no issues that is encouraging in terms of emissions safety.
Well, that just shows you how much I know about chemistry!
Of course, my greenhouse is not a tight construction so I don't have to have a vented heater. I do know that the blue flame heater DOES make a lot of water vapor, which is great with our low humidity. Mine is 20,000 BTUs.
As for the birds, they do great out there (some have lived out there for 7 years). I have nets under their perches (poop-catchers!) for MY comfort, and my seedlings live on racks that I have covered with cheap, green plastic fencing. The poles that my vanilla orchids and Nigerian walnuts are on are also fenced in. The green plastic fencing is easy to work with and blends in pretty well with the general "decor" of the place.
The birds do read Latin, and can count, however! They only go for plants that have "edulis" or "deliciosa" on the tag, or that were particularly expensive to purchase…
Some bird species are more destructive than others, so shop carefully. Giving them something they are allowed to eat helps a lot. Mine LOVE chia pets!
You can see the furnace on the back wall in the picture. Notice what looks like bbq racks on the front of the heater - that keeps the birds from exploring during the non-heating season. They are cavity nesters, and are in to every nook and cranny in the place! I am trying to build a fake tree for nesting, but I am on about failure #6 right now. They have to put up with unrealistic-looking bird houses right now.
It's an ecosystem that is tremendous fun, but always teetering on the brink of chaos.
I don't know why the shade cloth directly on the greenhouse held in heat, but it really made a HUGE difference to put it on a frame. A friend here in town with a greenhouse had the same problem. My guess is the size of our backyard greenhouses, compared to a commercial type of setup. My ceiling height is only a little over 10 ft. Also, I don't have roof-top vents. I have a 3 ft window on one end, and an exhaust fan at the top of the opposite end wall.
Here is my granddaughter with one of my Bourke's grasskeets (anything she loves, she kisses. My plants are also well-kissed!)
Cheers!
Carolyn