....Whitestone (above) did the right thing and stitched-in some extra depth to the ceiling joists....
It gives you more depth for insulation but in addition it increases both strength and stiffness.
The increases in strength and stiffness that can be acheived this way are remarkable; just adding 1" to a 3" joist as per whitestone's description means the joists could be ~x2.37 improved in both strength and stiffness.
When I increased (approximately doubled) the depth of my joists they became nearly x8 stiffer and x8 stronger. In fact it was better than that because the noggings transfer local loading better to neighbouring joists than the original arrangement (a cross-nailed spreader timber over the top of the joists at right angles).
Just for fun I had an assistant measure the ceiling deflection in the rooms below under my bodyweight. It has been awhile but IIRC it was roughly;
a) original ceiling joists, with cross-nailed spreader in place; static bodyweight caused 3/8" deflection, flexing ones knees gently produced over 1/2" deflection.
b) improved joists, with noggings. Maybe 1/16" deflection static, 1/8" by flexing knees.
c) original joists with spreader removed; 3/4" static deflection. Flexing one's knees caused well over 1" deflection and loud cries of 'please stop it looks like you are going to come through" from below....
cheers