There are several ways we can do to reduce temperature in an area. I'm going to assume this is not a huge greenhouse we are talking about. If it's a non-budgeted greenhouse there would be many other ways like misting and installing temperature regulators.
Here are some of the things you could do to reduce temperature:
1) Create more shade
Build a manual retractable shade? Awnings? Especially when there's plastic or clear glass sheeting that allows sunlight penetration. This is for obvious reasons - because sun makes it warm! Maybe plant a tree outside to block the rays.
2) Increase ventilation As we all should have experienced, if you close your window for a long time, your room will feel stuffy and warm. This is because once the air flow stops, it allows the air droplets to become very saturated, resulting in high humidity. Installing exhaust fans can help too! I think we are able to use kinetic energy to create natural fans that don't require electricity.
3) Plant many plants inside! Haha, yeah well i mean, plants do help to reduce temperature! Simply because they help to absorb carbon dioxide in the day and release moisture while transpiring. When the moisture evaporates, it helps to reduce the temperature around significantly. If you didn't know, CO2 is a greenhouse gas which traps heat.
4) Installing insulation As we all learn in school, the idea of insulation is to transfer the energy somewhere else. In this case, we would create a barrier between the hot and cold object which will reduce heat transfer by either reflecting thermal radiation or decreasing thermal conduction and convection from one object to the other. It would be subjective to determine the most effective material.
Some examples of insulation material that is found on the web link below:
1. Fiberglass "Fiberglass is cheap, but requires careful handling."
2. Mineral Wool "Mineral wool is effective, but not fire resistant." 3. Cellulose "Cellulose is fire resistant, eco-friendly, and effective, but hard to apply.." 4. Polyurethane Foam
"Polyurethane is an all around good insulation product, though not particularly eco-friendly." 5. Polystyrene
"Polystyrene is a diverse insulation material, but its safety is debated."
OF course there are other material that we can find!
But yeah, I guess that's about it!
Fun Fact: We actually have a full module on Horticulture Engineering in our poly days.