insulating loft, do I do the loft floor or the roof?

[buzzer]

I want to insulate the loft of my house soon. I went around to a mates house a while ago and he has just done his with the foil covered insulation which looks to be very good quality. However, he has used it on the underside of the roof as opposed to on the loft floor.

While this may be a bit easier for me (I have some stuff stored up there and part of the loft is already boarded)I am concerned that it might not be as efficient as putting the insulation on the loft floor. My rational is that if its on the underside of the roof, I am effectively heating the loft space!

So, in efficiency terms, whats my best option?

1. Just insulate the whole underside of the roof with foil layer type insulation.

2. Use foil layer insulation on the loft floor (this would be over the existing fiber type
insulation that is already there, and about 4 inches thick and up to the top of the joists)

3. Just cover the existing fiber insulation with another layer of fiber.


Any advice guys?