Just a quick question: what's the difference between general purpose and cavity slab insulation?

Trying to improve soundproofing and we're priced out of some of the more expensive options we were hoping to do. Wickes have slab insulation of the above 2 kinds, but the cavity slab is a lot less expensive than the other. Have checked and they are both 45kg/m3, both 100mm, but there must be some difference? Is the cavity slab still ok for acoustic insulation?

Ta, Hannah

Heavier the better there are sound deadening rockwool batts available for the purpose.

Having read other threads about soundproofing walls etc I wonder how much sound is transfered through the stud work and how much through p/b and void. I really like the split framing method to disconnect the frame. Anyone got any figures?

MDF is a wood fibre board, but wood fibre board in this context is not MDF (if that makes sense).

The wood fibre board in this context is likely to be the high density wood fibre batt insulation.

As for mineral wool, the Acoustic stuff is special - in-so-far-as it has acoustic on the label. In every other way it is just mineral wool like any other. Standard 'thermal' mineral wool insulation is totally fine and will work just as well. 100 mm batt in 45kg/m4 sounds totally fine for this purpose.

As for tony comment about the heavier the better - that is not really the case. Anything between about 10kg/m3 and 60odd kg/m3 will perform similarly. Remember mineral wool insulation in a floor or wall is not there to 'stop' sound passing though. It is predimantly there to absorb sound that is reverberating within the stud or joist voids.

In a standard floor or wall construction, the addition of mineral wool insulation into the voids will at most get you a 3dB improvement in sound reduction. FYI a change in 3dB is the smallest change in sound pressure level that the human hear can detect, so taking a wall or floor of a set construction, if you add insulation into the construction you would only just notice the difference.

With acoustics you need to many things at once to make a big difference. Mass and insolation being the two vital factors. These should always be done together, if possible.

Pulling the existing ceiling down and putting back two layers of sound rated board fixed via resilient bars is THE SINGLE best thing to do for upgrading the performance of any floor (subject to sorting out obvious things like gaps between floor boards etc).

A second set of joists to hold the ceiling is better at reducing impact noise than "resilient bars" are in my experience. More joists is not often practical tho'.

The acoustic rated rockwool IS a bit better than the standard "cavity bats" (I believe the weave is different, and the density is a bit higher and/or variable), but not actually very much better. Both are significantly better than fibreglass insulation.

Half-filling the void with rockwool is nearly as good full-filling (for acoustic purposes only, not thermal!).

Introducing a floating floor on top of the existing is also an option (I've done this - 18mm T+G chipboard on top of a rubber sheet made of shredded car tyres - I think it was called "R10"). Ceilings are generally easier to move tho (doors, skirting etc!).

Are there any differences between the performance of regular joists, I-beam joists and metal web joists in this regard?