Dense materials like wood fibre provide additional benefits when insulating a roof, unlike the, often default use of Polyurethane boards - PIR, wood fibre is much more dense up to 275kg's per cubic metre. The boards are tongue and groove and simple and clean to fit. Most have a protective layer of natural wax to allow exposure to the elements during construction. These boards are vapour open which means they will enable the passage of trapped water vapour through the structure preventing damage to the structure caused by damp. these types of insulation are often used commercially in schools and hospitals for example where they are valued for their performance.
Over the past few years many self builders and house builders have recognised the value and long term benefits to building health of using wood to insulate roofs, with ever increasing demand on performance this usage will grow significantly as we move away form judging an insulations suitability solely on price and U value