Polyester Fibre is mainly used as a comfort layer on top of foam, or by itself, as stuffing in furniture backs. The most common and default thickness, when you order fibre wrap, is 1.25″ thick. It is cut to the size of the cushion and joined, using foam adhesive, to the top, bottom and front of a seat cushion.
The fibre fills in all the gaps in the fabric and gives the finished cushion a more rounded look. Without the fibre the foam alone will look flat and the fabric may appear loose and saggy.
You can add one layer 1.25″ thick, use a different thickness of fibre or double it up for extra comfort in back cushions. You can also use thicker fibre to give you a crown look in the centre of the cushion.
Since fibre compresses easily, you do not need to make an allowance for the fibre when deciding on your foam size or thickness. For more information see fibre wrap video.