Although makes me wonder why rigid insulation that's used going under a screed nearly always has a reflective surface.
It's so rare to find non-foiled insulation (due to lack of demand/numbers) that buying in foiled is pretty much the only realistic option unless you opt for seconds quality boards - which are great to use under screed and pads because the screed takes out any unevenness, warp, misshapenness etc that makes them second quality in the first place
The foiling doesn't contribute significantly to the cost of the board in the volumes with which it is used, added to the fact that it's one of the most abundant elements in the earth's crust (just a pig to extract)
Heat is transmitted in one of 3 ways: convection, conduction or radiation. Ignoring convection, radiant heat crosses space/void/gases but upon reaching a solid object transfers through it via conduction (the surface is warmed, then the heat spreads through the solid by making molecules vibrate, knocking into each other more and transferring the heat energy like balls on a snooker table cause each other to move when striking). As the heat reaches the other side of the solid and again encounters a void it converts back to radiant heat to cross the space. Transfer by radiation is rather ineffective and this is how insulations work; foams that inhibit air movement - reducing convection, having bubbles with thin membranes - inhibiting conduction and providing lots of tiny voids that (ineffective) radiation must traverse
If a foiled board has an air gap, the radiant heat is reflected back. If it has no void then it's just part of the solid - the concrete molecules vibrate the aluminium molecules directly by conduction
As noted earlier, radiation is rather ineffective. Those radiant halogen heaters heat you and make you feel warm rather than heat the room. Because radiant heat is ineffective if someone offered me 25mm foiled kingspan or 50mm nonfoiled kingspan for the same price I'd go for the 50 because there's no way the foil would compensate for the loss of half the insulating material. It's also why those space foil insulations were a jar of snake oil