Two Layer Off-set Two layers of synthetic insulation are used and offset so each covers the other's seams for increased insulation. Isofill Isofill: Siliconised hollow fibre that gives optimum warmth and insulation. Left Zip Option The zipper is placed at the left side of the sleeping bag. Heavy Duty Zips Our own specification heavy duty zips ensure ease with little wear over extended periods. 2-way open. Liner Attachment Points Sleeping bag attachment loop and liner snap closure fastening. Square Sack Square Sack: For excellent storage. 2 seasons 1 season: for use in the summer months only. 2 seasons: For use from late spring through to early autumn. 3 seasons: For use from spring right through autumn. 4 seasons: For use throughout all seasons but more suited to colder conditions. PFC free Safe finish without PFC.