Loft Insulation

RaspberryFool

(Also posted on the Motley Fool site. Apologies for those that have already seen it)

I have been considering topping up the depth of loft insulation in our house from the current approx. 4 inches to the recommended depth of 10 inches. I, therefore, have been following the previous threads on the subject within this board with some interest.

However, our upstairs floor has coombed ceilings. Thus there are sloping bits which currently have some sort of solid polystyreme foam blocks between the joists and the walls (which back onto the eaves) have got glass fibre wedged between the vertical posts.

The questions I have are:

1) Is it sensible/feasible to lay any more insuation on top of the polystyrene blocks? If so, at right angles to them?
1a) As an incidental, some of the blocks appear to have warped slightly. Need I worry about this?

2) What is the sensible thing to do to try to put more insulation (at right angles?) over the glass fibre on the vertical walls? What would be the best way of attaching it? Would it also have to be 10 inches thick?

3) I have seen the very thin, reflective insulation that Screwfix sells. This would, obviously, be easier to fit in both instances. However, it is VERY expensive. We would need a good few rolls to do the whole house. Would it do the trick? Whilst I have seen figures for it being equivalent to so many millimaeters of polystyrene, it is not clear how many millimetres of glass fibre it would be equivalent to. Anyone know?

Thanks in advance for any answers/advice.