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Phil L

wrote:
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

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.