why should underfloor insulation be *suspended* betweeen the joists?

[Duncan]

Can't I just lay in flat on the plasterboard ceiling below?

The instructions I've seen say that mineral wool insulation should be suspended in hammocks of netting between joists. But i don't understand why this is. I forgot to get the netting - can I just lay it between the joists on the plasterboard below and replace the flooring (plywood) on top? The insulation is feather-light, so I don't think weight can be the reason, but perhaps there has to be an air gap between the insulation and the plasterboard?

This is in a second floor flat, and the primary purpose is to insulate our sound from our neighbours in the flat below.

Thanks,

Duncan

[Piers James]

suspending it generally only refers to the ground floor where there is no plaster board beneath.

However you should consider the consequences of your downstairs neighbour's lighting circuit getting too hot, so maybe suspending it is a good idea after all.

[John Rumm]

This is often done when the insulation is being installed as a fire retardant rather than for its insulation properties. It ensure it stays in place even when the ceiling below fails.

[Nick]

So it should therefore be suspended by some fireproof mechanism like steel wires ? Just so they don't burn and drop down with the burning ceiling below it ?

Nick

[John Rumm]

A roll of chicken wire is the usual solution. The idea being that it might add a few mins extra protection containing a fire in one storey of a property.

[The Natural Philosopher]

Well it will barely make any difference to that, but install away..

[Duncan]

Really? Is that because most of the noise is transmitted through the joists? Is there anything else I could do to reduce footfall noise? The final surface will be slate tiles (laid onto that 18mm ply with BAL singlepart flexible).

Thanks for the advice re the insulation fitting.

Duncan

[The Natural Philosopher]

Most noise goes either via the structure or the air. Rockwool Insulation is full of air, and whilst it damps out the high frequencies, it does little for the low and medium ones.

What you NEED is sand, and lots of it, to weight the structure down, then hermetic sealing to get rid of any stray air paths, and then layer of something rubbery, on top, with ply on that and tiles on top.

At the least strip the flooring, and lay fibre board 0 or maybe rubber sheet (old car tyres?) over the joists before re-applying chip or ply. That will reduce the structural borne sounds a bit.

The tiles themselves will add mass anyway, which is good. and provide good sealing..with grout.

If the timbers will take it, making containers between the joists and sand filling them would be ideal.

Or you might be able to get heavy sound deadening material from a car type suppliers to stick to the floor between the joists as well.

[John Rumm]

Partly... also the insulation lacks the density required to attenuate low frequencies like footfall noise.

Decent underlay and carpet will make the biggest difference!

That makes it somewhat harder. Decoupling the ceiling from the floor is another way, but also not very practical in this circumstance.

[Ben Blaukopf]

If you neighbours ever want to take their ceiling down, they may be rather surprised to find a load of insulation falling on them.