boarded out loft now looking to board rafters , i understand i need some insulation up there , but i have found i am very allergic to fiber glass while boarding joists and not sure i want to buy that silver blanket stuff a staple gun and then batons which i can't screw in and will lose some head rood, so i was wondering...i have seen great big thick sheet of polstyreen in b&q , could i cut this and put it between the rafters????, would it have to be an absolute snug fit or a general fit