Broke up a board in the attic floor, and as feared, there's nothing between the beam and the sawdust. Planning to break up another and remove the sawdust, then slide in a sheathing board with building plastic. More tips on how to make it look neat are welcome. Another question: do you think 17mm sheathing is sufficient as a floor under kitchen frames, or should I use 22? The reason I ask is because the measurements are perfect with 17, but it requires some tricks to fit 22.