As you highlight, the purpose of the deflection limits is not because

that is where a joist will fail catastrophically, but that is where any

more movement would become unacceptable. L&P ceilings can move a fair

amount, but don't expect one that is supported by a floor structure that

deflects 5 inches every time someone walks over it to last long!



Not to mention the possibility of seasickness every time anyone bounces around up there !




Re noise transmission, the gap between the 2 joist layers has more

effect than 2x8s. And I don't expect anyone lives in your loft

anyway.



Loft floors (once strengthened) are actually very good for noise

insulation since they typically have two separate sets of joists

carrying the floor and ceiling loads.


I'm hoping this one will be good for heat insulation too, with a full load of insulation between the ceiling and the new floor.