roof/ceiling framing question
I have a 50s house with a room with a cathedral ceiling with exposed rafters and roof decking. The rafters are 4x6 placed at about 3' apart. The roof decking is tongue and groove boards of about 1.5-2" thick. Everything is fine now, but we want to insulate (for thermal and acoustic reasons) and put 5/8" drywall on the ceiling. Here in the earthquake-prone area of San Jose, CA (and probably everywhere) drywall can't be secured on a ceiling to structure with > 24" spacing. My thought is to attach 2x4s flat across the rafters at 16" or 24" spacing and attach the drywall to that. This also gives us an extra 1.5" of insulation space. (I prefer to lay the 2x4s flat because 3.5" is a lot of head room lost at the low end of the cathedral ceiling... 1.5" is more tolerable). Does this seem like a good plan? What would you do differently?! Is there any risk of the 2x4 laid flat sagging over time since they are only held every 3'? Thank you.