insulation thickness under concrete

[mcmook]

i'm laying a new concrete floor we dug out 9 inches. i had planned to lay 3" insulation, 4" concrete followed by 2" granolithic screed. i've since decided against the screed so have an extra 2" to make up. my first thoughts were to lay an extra 2" insulation, so 2 layers a

75mm layer and a 50mm layer then 4" concrete.

would this be OK... or am i better laying 6" concrete on the existing

3" of insulation.

can you have too much insulation and could there be any movement causing the concrete to crack?

thanks

[Phil L]

It depends what kind of insulation you are using - the cheapest is polystyrene and regs now insist on 150mm, although for years and years it was only 100mm and thousands of buildings have 100mm under them. Kingspan is thinner but almost treble the price, the choice is yours - just make sure you have 100mm (4 inch) of concrete (preferably readymixed) on top, you'll also need 1200 gauge plastic membrane on top of your insulation prior to pouring - some people put the plastic at the bottom, then the polystyrene and then the cocnrete, but I've found that the concrete gets under the slabs and lifts it up as it's being tamped.

[Roger Mills]

I think I'd go for 100mm (4") of rigid foam insulation and 5" of concrete. That would give a good combination of thermal efficiency and structural integrity.

[calums]

I would agree that extra insulation is fine, and would recommend Kingspan. If in SW Scotland then I could do you a good price on 100mm Kingspan.

The other thing to consider is what the surface is like beneath all this. It is normal to put sub base down prior to the insulation to give a stable platform to start with. Also, you can get away with less than 100mm concrete if the mix is good, You can also get away with

150mm... you just need to advise the supplier so they can give you the right strength of concrete for your job.

Calum Sabey NewArk Traditional Kitchens 01556 690544

[mcmook]

Thanks for your replies. I had already laid 75mm polystyrene boards, on top of 1200 visqueen. Base is solid, already compacted earth and rocks. So I think I will just go ahead and lay more 50mm sheets on top, giving a total of 125mm. Then 100mm of concrete. Checked this out with BC this morning and they say its fine.

[The Natural Philosopher]

Id lay some rebar in the concrete tho..just in case the compacted sub base isn't..IYSWIM.

[Weatherlawyer]

All he needs is a screeding mix on top of the foam. It can be bought ready mixed. Make sure it is not full of hard lumps and that the stuff hasn't separated during delivery.