Loft Insulation versus 150mm Concrete floor

[fordyjohn]

In my loft I have a 150mm concrete floor and the roof rafters are insulated with 150mm insulation.
will this configuration stop heat loss and thus save me money on heating bills.

Or does the heat just pass through the concrete ceiling/floor and then stay in the loft space.

 

[mointainwalker]

Or does the heat just pass through the concrete ceiling/floor and then stay in the loft space.

That one.

You need to stop heat getting into the loft if you want a warmer home/lower bills.