Linda, here's a couple of things to consider:
- Orchid-growing success is determined by your entire culture - air, water, light, temperature, humidity, air movement, nutrition, and your personal watering tendencies.
- "Semi-hydroponics" does not define the whole picture. It is merely an inert medium and watering method that simplifies the balancing of moisture and air flow to the roots.
- It is possible to grow orchids in almost anything, but you must pay attention to how all of those aspects interact.
If "drying out too fast" is a real issue, and you don't want to water more frequently, you really have no choice but to increase the humidity. If you go to more traditional culture, the potting medium will dry out just as fast, and if you formulate it so that it holds water longer, you will risk suffocating the roots.
Water retention in media has two aspects - the liquid that is
absorbed by the components of the medium, and the droplets
held in-between the particles by surface tension, bridging the air spaces. It is that "bridging water" that can suffocate the roots, so we try to keep it to a minimum.
Then, when you consider the liquid absorbed by the particles, you have to consider how well they release what they aborbed - and for orgainic components - how fast they decompose.
Many medium components absorb really well, but don't release the absorbed solutions as quickly, so surface evaporation outstrips the speed with which the liquid is fed out from the center of the particle, which leads to fairly rapid mineral buildup. The only way to improve that balance is to slow the evaporation rate, which means
increasing the humidity.