Hi TomekK,
I have had hundreds of people tramp through my greenhouse because they had exactly your questions. My setup is 700 square feet. It is actually two greenhouses butted end to end with a door between. This is because when I built the first greenhouse it wasn't big enough!
The tropical house is a kit with tubular steel frames and the poly H channels that Brian was grumbling about.
The orangerie is stick built by me out of 2x4 lumber.
So I have personally made almost EVERY mistake in the book!
First - the H channels - if you got them from a greenhouse supply place they should be UV treated. Yes, they get gross and slimy. I solved the problem by not looking at them
That greenhouse is 12 years old. Two years past the polycarbonate warranty and going strong.
Using lumber - don't spend money on pressure treated except where it comes in contact with the foundation. Lay down foam strips under the wood of the sill plate. The foam comes in rolls at Home Depot. It keeps the moisture from wicking up through the concrete (my foundation is cinder blocks. In my area, that equals "temporary" which equals "no building permit", but ALWAYS check your area. They CAN make you rip it out.).
For the frame, PAINT THE HELL OUT OF IT. I have at least four coats of "porch and patio" paint on mine. The oldest part of the frame (I have enlarged it a couple of times) is good as new at 10 years old. Paint the lumber before nailing it together. Get paint on EVERYTHING. Ends, etc. I used gray paint because I felt it "sort of" matched the steel tube frame in the tropical house. Don't let any wood come in direct contact with concrete or earth.
Humidity - I made my own fogger out of a Home Depot tub and an ultrasonic mister. You can buy fogger parts here -
https://thehouseofhydro.comThere are way cheaper places, but this stuff is industrial and is holding up REALLY well for me. Since running the fogger, my cacao have never looked so good!! They LOVE it. I am going to make a second fogger for the orangerie. I also have misters I run in the summer. But the tap water here is 55 degrees, so I don't use it in the winter!
Water - I water all my plants and fill the ultrasonic fogger with tap water. Our water here is very good. HOWEVER, I have a descaler to prevent mineral buildup. This makes the minerals in the water "slippery", so they don't leave a residue on plant leaves, clog up the misters, etc. I pump water into the pond in the greenhouse and let it circulate to remove chlorine. I have a big aquarium heater in the pond that warms it to 75-80 degrees (I have orchids also...). We get 13 inches of rain each year and I have no room for a storage tank so I do not collect rain water. I have thought about capturing it from the roof of the greenhouse, but I go through 100 gallons of water a week, and like I said, no room for a tank...
Ventilation - Airflow has major impact on cooling. I have little parrots loose in my greenhouse, so I have shuttered vents, plus a big sliding window on one end that has screen covering the opening part. For your size of greenhouse, just one intake vent and one exhaust vent fan should be plenty. Mine are not in the roof. They are at the very peak of the end walls. I also have an intake fan. The intake and exhaust are 40 ft apart and I couldn't move the air enough without an intake fan. They are wired together and on a thermostat. Along the ridge I have five cheap metal 9 inch fans. In the summer, they point towards the exhaust vent and blow hot air horizontally along the roof peak. In the winter, I rotate them so they point down, and they gently blow the hot air back down onto the plants. Super cheap fans that I got at Big Lots for $10 each, and they last years and years. Weirdly, my birds love to sit on them. Don't know if it is the vibration or downdraft or both.
Happy building!!!
Carolynn