Loft conversion floor insulation?

21 Oct 2014
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West Midlands
United Kingdom
Just had my first building inspection on the structure. Inspector said it was a good job! Next is insulation. Going 100mm celotex between rafters and 50mm over the top. Bloody expensive.....

Although not a requirement (I believe), Im considering insulating between the floor joists for 2 reasons. 1 heat transfer between floors and 2 is that I have a ton of mineral wool insulation lifted from the original loft I want to "loose"!

I've watched some YouTube videos where they lay chicken wire between joists apparently due to regs suggesting that if there was a fire and the ceilings fell down, the insulation would be held in place. Now to me this seems ridiculous. My house ceings are lath and plaster overboarded with 12.5mm plaster board. Should there be a fire that was so severe the ceings fall, a bit of mineral wool insulation gently wafting on your head is the least of your problems...... You'd be burned by fire and knocked unconscious by the plaster board and plaster long before the insulation came into play.

Can someone clarify this for me.....
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The point is if it were lathe and plaster on it's own then that may fail in the event of a fire and bring the mineral wool with it, thus defeating the object of it. But if you have over-boarded and fixed into the joists then theoretically you've eliminated that problem so no need for the additional chicken wire. But generally (depending on the new joist depths and deck material) an un-insulated floor will achieve the fire regs anyway with no rockwool.
Cheers for that. That's what confused me. Fire regs are met with no insulation so by adding some mineral they maybe aren't.... But your logic that overboarding the lath and plaster screwed into joists seems far more appropriate.
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Sorry, just to be clear before someone pulls me up on it, in my last sentence I should have written

But generally with a layer of 12mm plasterboard fixed into the joists (depending on the new joist depths and deck material) an un-insulated floor will achieve the fire regs anyway with no rockwool.

A floor with just a lathe and plaster ceiling will always need the mineral wool.

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