Orchids are unlike other houseplants most of us are used to growing. In nature the parents of majority of the orchids grown for their beautiful flowers are epiphytes which means they grow on trees. They are not parasites, but simply use the tree as a support on which to grow.
When we put orchids in pots, we are trying to grow them in an unnatural environment just to make it easy for us. We need to try to come as close as possible in this new growing system to keep them happy. Over the years many different media have been used to grow orchids in pots, the most common at the moment is tree bark, specifically fir bark. Orchid roots need plenty of air circulation in order to keep from rotting and good sized pieces of bark provide the open environment that they need. Another common and available media is coconut husk chips (CHC). This media is becoming more and more accepted due to its availability and the fact that it is a sustainable and renewable resource.
What is the best media for you? is of course a difficult question. Ask ten orchid people and you will get about twenty different answers. Each grower is different. Each one grows different kinds of orchids under different conditions. I recommend that you talk with others who grow in your area, especially if you can find someone whose growing environment is similar to yours. Then find out what media they use, what their watering schedule is and what kind of plants they grow. Then find a good supplier for that media. It is much easier to care for plants which are all in the same kind of media – with exceptions for specific plants such as Paphs and Phrags.
My media is a combination of fir bark, charcoal and perlite which is special ordered from a local nursery. It is important to determine that the media you choose will be available over time, or you will find that once you finish the initial bag you are back to different potting media. You can add sphagnum moss for those that need a little more moisture and more porous material for big plants in bit pots that dry out more slowly than other pots. A consistent potting medium will allow you to set up a good repotting schedule for all your plants.
In conclusion, the media you use should be based on your growing requirements and environment as well as the kind of plants you have. Indoor growers will need different types than outdoor growers. Sphagnum moss will provide more humidity for indoor growing while it may rot in hot humid environments outside.