Aha! Ok thanks! So I made sure the relevant insulation fill had the "Fit to Skin" orientation selected. But this made no change as I had the slab modelled as a basic slab... So then I created a new composite slab type just for the insulation, and set the insulation thickness to what is required. I then modelled the insulation as a composite slab with the new composite slab type, and it looks like I need it to. Thanks for your help!