How to address Phd dissertation committee members in email?
At my place, they are called SRC (Student Research Committee) members, so that makes the acronym more usable. I always started my emails by writing:


  Respected SRC members, 


The SRC members part additionally makes the context clear (in addition to the subject of the mail). I prefer Respected to Dear, and while that may (or may not) be culture specific, it never hurts to shower respects on these people, unless of course, they explicitly ask you not to do that. 

Additionally, I deliberately avoided Professors. Even though these committee members could generically be addressed as professors, one of my SRC members was technically still not a Professor, but an Associate Professor. While the distinction might be a bit over-strict in this context, I preferred being technically correct too. (That might be a nerdy, personal choice.)

Hope that helps :)