From the OPs point of view, that's someone else's responsibility. I can understand why he would want to wear braces when it's someone else's job to put on the belt!
Not sure where that knowledge came from but if I lived in an upstairs flat with bare boards and the downstairs flat set on fire I think I'd prefer to know sooner rather than later so I could exit. Absent any effective smoke alarm in their flat alerting me it might not be such a bad thing if a bit of smoke drifted up through the floorboards!
I can see there being some advantage to sealing up the floorboards to reduce smoke damage from the property below in the event of a fire though it won't really add any additional fire protection to the floor. The lath and plaster ceiling may be in great condition or it could be rubbish and soon fail in the event of a proper fire and the joists will be completely unprotected, very little testing has been done to see how well it performs in a fire, not only that it's condition and quality can be so varied that it can make any test for one ceiling irrelevant for many others. Ordinarily where a ceiling is L&P the void should have a 100mm or so layer of mineral wool insulation sat level with the bottom of the joists, it being held in place with chicken wire or similar so that it remains in place after the L&P has fallen off. Or the L&P should be over-boarded with PB, though not an option here obviously, anyway went off on a bit of a tangent there.
Getting back to the original question I don't think it really matters what board you use, any continuous board under the original floor boards will help a little, seal any gaps in the boards or around the perimeter with intumescent sealant, though there will still be plenty of ways for the smoke to enter, through the walls/cavity's gaps behind/under skirtings etc etc. Smoke's a bit like water, it will find a path if there is one. Perhaps stick an interlinked smoke alarm in the floor void!
6 or 12mm cement board...adds mass, thermal insulation and sound reducing properties...oh yeah, and they are very difficult to set a light...very difficult
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