Floor Slab Insulation or Not?

[DanDan]

I have very little build experience and have encountered a problem in the past.
Green field new build, my function was to 'steer' an existing plan acoustically.
I was not on site. The builder did a fabulous job on the 6x4Metre Concrete Slab/Floor, which was smooth enough to paint.
However he did something involving insulation which allowed the slab to bounce a little.
This is audible under footfall, but it also resonates with Kick Drum and Bass, both live and on playback.
I am pretty sure he used one of the phenolic insulations which are popular here. They have very high thermal performance but afaik no acoustic absorption or damping.
https://www.kingspan.com/irl/en-ie/prod ... ion-boards
It could also have been Aeroboard, the light white Polysterene which used to be common here. I am trying to find out which. but for now let's assume PUR.

So, another similar advisory job has come in. In this case the Plans are specifying
150mm Concrete Slab on 100mm Aeroboard on a Damp Proofing Membrane on 50mm Sand on 150mm HardCore.

Over time on these International Fora, I have always seen 'On Grade' stated. I presume that means on Hardcore and Sand? Never insulation.
So I am inclined to lose the insulation or at least suggest the much more rigid PUR. That is if our codes allow.
I would greatly appreciate experienced build advice on this.

[Paulus87]

Hi Dan,

It is advisable to use Kingspan or celotex or whatever variant you want to call it in new builds to reduce thermal bridging and to stop damp rising up.

Usually you would dig the footings, put the hardcore down, then the chippings, then the sand, then a dpm then the insulation (I used 100mm celotex) then you need to put another membrane over the insulation as the foil facing reacts badly with the concrete, finally the concrete.

I also thought about this resonance issue or bounce as you call it, however my slab is 9” thick. It is on top of the celotex which is on top of a sub slab which is on grade. Round the perimeter and on top of the sub slab is a row of concrete blocks with a layer of celotex on the inner face. The new 9” slab was poured inside of that.

No doubt infinite damping from Mother Earth is the best way to go, but I had to do it this way for numerous reasons, but it is so dense and thick that the resonant frequency should be way below 20hz.

I am not sure about codes but you could probably compromise and put the insulation in the center leaving the edges of the slab damped by the earth.

Paul

[DanDan]

Thanks so much Paul. Others please do join in. I realise PIR is thermally the best, but has anyone experience of using sound absorbent insulation under the floor slab?
Could one assume this would acoustically dampen the slab?

[Paulus87]

Thanks so much Paul. Others please do join in. I realise PIR is thermally the best, but has anyone experience of using sound absorbent insulation under the floor slab?
Could one assume this would acoustically dampen the slab?

Me again...

I do not have experience of using rigid Rockwool or the like under a slab and am happy to be wrong, but my gut would say it wouldn't be able to effectively dampen it as I think it would end up too compressed under the weight of the concrete. Perhaps if there were a cavity under the slab, or if it were a block and beam floor with a cavity underneath then I could see it being quite effective.

Paul

[DanDan]

I think the very high density stuff intended for flooring doesn't really compress much. But perhaps this stuff is for 'interior' floors?
https://www.knaufinsulation.co.uk/produ ... floor-slab