A heat-reflective can function by itself as a shelter from cold, wind, and rain. However, you can still lose critical heat through conduction (the transfer of heat to colder surfaces around you). Thickly pad the area underneath your bivvy with as much dry debris as possible to add insulation between you and th ecold ground. Find a safe spot for your shelter that is free from risk of falling trees or rocks, running water if it rains, high winds, or other natural hazards.