There are 3 main types of floor insulation available for solid ground floor construction.
Foil Faced PIR Insulation
This is the most popular product used in solid ground floor construction (commonly known as Kingspan).
PIR insulation has the best insulation value / thickness ratio of all the commonly used floor insulation products.
Expanded Polystyrene (EPS) Insulation
EPS insulation comes in 2 main types.
White EPS is the one which everyone will recognise, as it is used extensively in packaging, as well as for building insulation.
Grey Low K EPS offers a better insulation performance than white EPS (although not as good as PIR), so would be used where there is a restriction on thickness.
The standard grade for all EPS products in domestic buildings is EPS70, with higher densities (EPS100 or EPS150) being used where higher floor loadings are expected.
Extruded Polystyrene (XPS)
XPS is generally used in commercial flooring situations, as it has a higher compressive strength.
Its other main advantage is that it is waterproof, and is used in situations (e.g. basements) where there is a risk of dampness.