what's the difference foil/non foil kingspan

[Amy]

This is what I particularly LOVE about this forum, everything can & will be broken down to the Nth degree with no intended malice, (ok occasionally tight twisted knickers) but in the main a facility to compare, measure, advise & work around.

Meaning?

[kris]

the foil acts as vapour control layer which is important as moisture affects the efficiency of the insulation.  You could also use a vapour control membrane which can act as a airtightness layer (latest in energy efficient houses).  Don't forget to tape where 2 boards meet.  As you can't cut foam insulation to completely fit between the rafters, you need PU foam (the stuff in canisters) where you have gaps.  Make sure you maintain a 50mm gap between the insulation and the tiles (cut some 50mm wooden strips which you can use as spacers).  Ideally, you should use 50mm between the rafters and 50mm below.  The ones between the rafters can then be without the foil.  for practical reasons you may want to use insulation backed plasterboard (which will incorporate vapour control) - don't forget the tape where boards meet.
I have to point out I made some assumptions about your roof.