I think you'll find that a greenhouse in SD will get very warm in the summer and will need some sort of cooling strategy. You can just vent the hot air but then it is replaced by dry air. That's why you'll find greenhouses with swamp coolers blowing cool wet air. Don't worry it will still be quite warm in there.

Generally people try to humidify the garden in SoCal to cool the area. Lol

I get you. UC Berkeley has a small 12 x12 tropical humid greenhouse for the smaller potted tropical jewels like Ferns. I could see a chair and small round table,to read of what ever. Put in a little waterfall.

But now matter what- air circulation is the key. Plants develop thicker leaves with strong air movement in a greenhouse. More resistant to pests.

So how can I keep temperature regulated, humidity up, but also support good airflow?

Temperature regulation: shade, evaporative cooler, AC

Humidity: evaporative cooler, misters, ultrasonic fogger

Airflow: evaporative cooler, fans.

We tend to have liquid rock for water in SoCal, so an RO filter is very useful on the water supply for evaporative coolers, misters, and foggers.

You won't need the evaporative cooler in the winter but you will still need humidity and air circulation. So there should be some redundancy there.

I think you are going to be surprised at how hard it is to keep the GHS temps from going way above ambient air temps. I have 50% shade cloth on the roof and have 30% of the walls as open ventilation during the day and its still barely enough.

A swamp cooler or a misting system will be your best bets for keeping temps reasonable and humid with some airflow along with the shade cloth.

Unless you live near the coast I think your plants could easily fry in a greenhouse unless it was very carefully designed for our climate. Would you consider a lath house instead? Have you seen the Botanical Building at Balboa Park? It is a giant lath house and has a definite tropical feel inside, although perhaps not as humid as you would like.

There is a hot, humid greenhouse at the San Diego Zoo Safari Park, the Hidden Jungle. I believe there are a lot of trees around it which keep the most direct sun off of it. I don't know if they use it all year, I've only been in it in the spring when they have a butterfly exhibit.

There is a company near San Diego that sells a lot of tropical plants, Kartuz Greenhouses, you might go check out their greenhouses to see how they are constructed/ventilated.

Having half of the greenhouse underground can also help regulate temp.

Temperature could be controlled with shade/shade cloth. I would think maintaining humidity with airflow is a bigger challenge in dry San Diego summers.

Just visited Andy's Orchids in Encinitas. I asked Andy that very question--aren't GHs too hot here? He said heat was not a problem. Vents in the GH ceilings that open automatically, and a gravel floor that stays cool and moist. He did have fans for air circulation. He maintains cool, moderate, and hot greenhouses for the different species of orchids that require those temperature ranges.

How close are you to the ocean? San Diego (like Los Angeles) has several micro climates.

I am in the ridgeview-webster neighborhood. North of 94 and east of 805.

In that neighborhood you should mainly need to add humidity in the summer - you would not need much heat in the winter.

When you say "tropical rain forest", what plants do you specifically have in mind? A lot of them will grow without much adjustment. I have a large collection of bromeliads/tillandsia, and I live in a micro climate very similar to yours, although I am in Los Angeles, about 2-1/2 miles from the beach. I also grow orchids outdoors and protect them from too much sun, and I do bring them inside from December - February. To get a rain forest look, you will typically need to use quite a bit of water - less so if you provide shaded areas. I keep a lot of my bromeliads inside a pergola that has a 90% covered roof. I also have a lot of epiphyllum, most of which are on a sprinkler system, but the bromeliads and tillandsia I mostly water by hand. The bromeliads remind me of Costa Rica, Chiapas, and Guatemala.

Are you planning on planting tropical trees? I've seen papaya trees doing well in San Diego, as well as banana, and I myself had a large mango tree when I lived in Venice.