Stale air is bad news, not just for phrags, but for most orchids. A moisture-retaining media that provides air circulation is also important. There is no single media that is best. Generally growers use a combination of medium size bark chips, perlite, and charcoal. Some growers use rock wool in the mix to help retain water.
Some growers have great success with pro-mix.
Some growers recommend repotting phrags twice a year. Keeping the media fresh prevents many problems, including root rot, and encourages a strong and healthy root system. Please read it.