Adding mass between closely spaced joists

Is there a preferred method for adding mass when there is only a 4cm space between the ceiling joists??

Why would you add mass between the ceiling joists? Adding it to the bottom side via several techniques is common, but to add the mass between the joists suggests a peculiar situation. Do you have any pictures of this ceiling?

Sorry I wasnt clear. up against the deck between the existing ceiling joists I am adding two layers of 5/8 as shown in Build It like the pros, however two sets of existing joists are spaced at 4cm instead of 16" oc as the rest are. I would like to add mass up against the deck in this 4cm space. Is there a preferred method to do this??

Really small pieces of drywall? :mrgreen:

lol, yeah then what?

Is there a preferred method to do this??

Is there any reason why you can't just cut a piece of wood a bit less than 4 cm wide, and as deep and long as necessary to fill that gap, then glue it in there tightly, and caulk the edges?

gonna give that a try. sounds good to me.