Orchid Boy, maintaining high humidity is beneficial for many orchids. Here is the (simplified) reason.
During the photosynthesis, H2O get broken down, and O2 is generated as the byproduct. The problem is that an important enzyme for photosynthesis (rubisco) can bind O2 or CO2. Carbon gets fixed if it binds to CO2 (good thing for plant growth). However, if the concentration of O2 is high in the leaf, rubisco binds to O2 and produces an useless product. Subsequently, to convert the useless product to more useful product, plants have to spend a lot of energy, and also it will lose a carbon (bad thing for plant growth). This problematic reaction is called photo respiration. In order to reduce the O2 concentration in the leave, plants need to open stomata. But if the relative humidity is low, the plant leaves loose too much moisture, so they have to close the stomata (therefore, carbon fixation rate goes lower due to photo respiration). Plants which has adapted to humid environments grow slower if you can't maintain high humidity.
Most orchids can tolerate lower/moderate humidity, but they may not be growing at the optimal speed.
I try to target 70-80%RH in the grow tent (mostly species Phals and Paphs, including P. sanderianum), but it could dip down to 60% during the day time. As a side note, for Phals, night time humidity is most relevant (they are CAM plants which open stomata at night).
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